All of the examples are complete running programs. If you built the library from source, the examples should have been built as well. If you use RPMs instead, the example programs’ source code and a simplified Makefile are in the mysql++-devel package. They are typically installed in /usr/share/doc/mysql++-devel-*/examples, but it can vary on different Linuxes.

Before you get started, please read through any of the README*.txt files included with the MySQL++ distribution that are relevant to your platform. We won’t repeat all of that here.

Most of the examples require a test database, created by resetdb. You can run it like so:

resetdb [-s server_addr] [-u user] [-p password]

Actually, there’s a problem with that. It assumes that the MySQL++ library is already installed in a directory that the operating system’s dynamic linker can find. (MySQL++ is almost never built statically.) Unless you’re installing from RPMs, you’ve had to build the library from source, and you should run at least a few of the examples before installing the library to be sure it’s working correctly. Since your operating system’s dynamic linkage system can’t find the MySQL++ libraries without help until they’re installed, we’ve created a few helper scripts to help run the examples.

MySQL++ comes with the exrun shell script for Unixy systems, and the exrun.bat batch file for Windows. You pass the example program and its arguments to the exrun helper, which sets up the library search path so that it will use the as-yet uninstalled version of the MySQL++ library in preference to any other on your system:

./exrun resetdb [-s server_addr] [-u user] [-p password]

That’s the typical form for a Unixy system. You leave off the ./ bit on Windows. You can leave it off on a Unixy system, too, if you have . in your PATH. (Not a recommendation, just an observation.)

All of the program arguments are optional.

If you don’t give -s, the underlying MySQL C API (a.k.a. Connector/C) assumes the server is on the local machine. It chooses one of several different IPC options based on the platform configuration. There are many different forms you can give as server_addr with -s to override this default behavior:

For the TCP forms, you can mix names and numbers for the host and port/service parts in any combination. If the server name doesn’t contain a colon, it uses the default port, 3306.

If you don’t give -u, it assumes your user name on the database server is the same as your login name on the local machine.

If you don’t give -p, it will assume the MySQL user doesn’t have a password. (One hopes this isn’t the case...)

When running resetdb, the user name needs to be for an account with permission to create the test database. Once the database is created, you can use any account when running the other examples that has DELETE, INSERT, SELECT and UPDATE permissions for the test database. The MySQL root user can do all this, of course, but you might want to set up a separate user, having only the permissions necessary to work with the test database:

CREATE USER mysqlpp_test@'%' IDENTIFIED BY ’nunyabinness';
GRANT ALL PRIVILEGES ON mysql_cpp_data.* TO mysqlpp_test@'%';

You could then create the sample database with the following command:

./exrun resetdb -u mysqlpp_test -p nunyabinness

(Again, leave off the ./ bit on Windows.)

You may have to re-run resetdb after running some of the other examples, as they change the database.

See README-examples.txt for more details on running the examples.

The following example demonstrates how to open a connection, execute a simple query, and display the results. This is examples/simple1.cpp:

#include "cmdline.h"
#include "printdata.h"

#include <mysql++.h>

#include <iostream>
#include <iomanip>

using namespace std;

int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }

    // Connect to the sample database.
    mysqlpp::Connection conn(false);
    if (conn.connect(mysqlpp::examples::db_name, cmdline.server(),
            cmdline.user(), cmdline.pass())) {
        // Retrieve a subset of the sample stock table set up by resetdb
        // and display it.
        mysqlpp::Query query = conn.query("select item from stock");
        if (mysqlpp::StoreQueryResult res = query.store()) {
            cout << "We have:" << endl;
            mysqlpp::StoreQueryResult::const_iterator it;
            for (it = res.begin(); it != res.end(); ++it) {
                mysqlpp::Row row = *it;
                cout << '\t' << row[0] << endl;
            }
        }
        else {
            cerr << "Failed to get item list: " << query.error() << endl;
            return 1;
        }

        return 0;
    }
    else {
        cerr << "DB connection failed: " << conn.error() << endl;
        return 1;
    }
}

This example simply gets the entire "item" column from the example table, and prints those values out.

Notice that MySQL++’s StoreQueryResult derives from std::vector, and Row provides an interface that makes it a vector work-alike. This means you can access elements with subscript notation, walk through them with iterators, run STL algorithms on them, etc.

Row provides a little more in this area than a plain old vector: you can also access fields by name using subscript notation.

The only thing that isn’t explicit in the code above is that we delegate command line argument parsing to parse_command_line() in the excommon module. This function exists to give the examples a consistent interface, not to hide important details. You can treat it like a black box: it takes argc and argv as inputs and sends back database connection parameters.

The simple1 example above was pretty trivial. Let’s get a little deeper. Here is examples/simple2.cpp:

#include "cmdline.h"
#include "printdata.h"

#include <mysql++.h>

#include <iostream>
#include <iomanip>

using namespace std;

int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }

    // Connect to the sample database.
    mysqlpp::Connection conn(false);
    if (conn.connect(mysqlpp::examples::db_name, cmdline.server(),
            cmdline.user(), cmdline.pass())) {
        // Retrieve the sample stock table set up by resetdb
        mysqlpp::Query query = conn.query("select * from stock");
        mysqlpp::StoreQueryResult res = query.store();

        // Display results
        if (res) {
            // Display header
            cout.setf(ios::left);
            cout << setw(31) << "Item" <<
                    setw(10) << "Num" <<
                    setw(10) << "Weight" <<
                    setw(10) << "Price" <<
                    "Date" << endl << endl;

            // Get each row in result set, and print its contents
            for (size_t i = 0; i < res.num_rows(); ++i) {
                cout << setw(30) << res[i]["item"] << ' ' <<
                        setw(9) << res[i]["num"] << ' ' <<
                        setw(9) << res[i]["weight"] << ' ' <<
                        setw(9) << res[i]["price"] << ' ' <<
                        setw(9) << res[i]["sdate"] <<
                        endl;
            }
        }
        else {
            cerr << "Failed to get stock table: " << query.error() << endl;
            return 1;
        }

        return 0;
    }
    else {
        cerr << "DB connection failed: " << conn.error() << endl;
        return 1;
    }
}

The main point of this example is that we’re accessing fields in the row objects by name, instead of index. This is slower, but obviously clearer. We’re also printing out the entire table, not just one column.

By default, MySQL++ uses exceptions to signal errors. We’ve been suppressing this in all the examples so far by passing false to Connection’s constructor. This kept these early examples simple at the cost of some flexibility and power in error handling. In a real program, we recommend that you leave exceptions enabled. You do this by either using the default Connection constructor, or by using the create-and-connect constructor.

All of MySQL++’s custom exceptions derive from a common base class, Exception. That in turn derives from Standard C++’s std::exception class. Since the library can indirectly cause exceptions to come from the Standard C++ Library, it’s possible to catch all exceptions from MySQL++ by just catching std::exception. However, it’s better to have individual catch blocks for each of the concrete exception types that you expect, and add a handler for either Exception or std::exception to act as a “catch-all” for unexpected exceptions.

When exceptions are suppressed, MySQL++ signals errors by returning either an error code or an object that tests as false, or by setting an error flag on the object. Classes that allow you to suppress exceptions derive from the OptionalExceptions interface. When an OptionalExceptions derivative creates another object that also derives from this interface, it passes on its exception flag. Since everything flows from the Connection object, disabling exceptions on it at the start of the program disables all optional exceptions. This is why passing false for the Connection constructor’s “throw exceptions” parameter suppresses all optional exceptions in the simple[1-3] examples. It keeps them, well, simple.

This exception suppression mechanism is quite granular. It’s possible to leave exceptions enabled most of the time, but suppress them in sections of the code where they aren’t helpful. To do this, put the section of code that you want to not throw exceptions inside a block, and create a NoExceptions object at the top of that block. When created, it saves the exception flag of the OptionalExceptions derivative you pass to it, and then disables exceptions on it. When the NoExceptions object goes out of scope at the end of the block, it restores the exceptions flag to its previous state:

mysqlpp::Connection con; // default ctor, so exceptions enabled

{
  mysqlpp::NoExceptions ne(con);
  if (!con.select_db("a_db_that_might_not_exist_yet")) {
    // Our DB doesn’t exist yet, so create and select it here; no need
    // to push handling of this case way off in an exception handler.
  }
}

When one OptionalExceptions derivative passes its exceptions flag to another such object, it is only passing a copy; the two objects’ flags operate independently. There’s no way to globally enable or disable this flag on existing objects in a single call. If you’re using the NoExceptions feature and you’re still seeing optional exceptions thrown, you disabled exceptions on the wrong object. The exception thrower could be unrelated to the object you disabled exceptions on, it could be its parent, or it could be a child created before you disabled optional exceptions.

MySQL++ throws some exceptions unconditionally:

It’s educational to modify the examples to force exceptions. For instance, misspell a field name, use an out-of-range index, or change a type to force a String conversion error.

SQL syntax often requires certain data to be quoted. Consider this query:

SELECT * FROM stock WHERE item = 'Hotdog Buns' 

Because the string “Hotdog Buns” contains a space, it must be quoted. With MySQL++, you don’t have to add these quote marks manually:

string s = "Hotdog Buns";
query << "SELECT * FROM stock WHERE item = " << quote_only << s; 

That code produces the same query string as in the previous example. We used the MySQL++ quote_only manipulator, which causes single quotes to be added around the next item inserted into the stream. This works for any type of data that can be converted to MySQL++’s SQLTypeAdapter type, plus the Set template. SSQLS also uses these manipulators internally.

Quoting is pretty simple, but SQL syntax also often requires that certain characters be “escaped”. Imagine if the string in the previous example was “Frank’s Brand Hotdog Buns” instead. The resulting query would be:

SELECT * FROM stock WHERE item = 'Frank's Brand Hotdog Buns' 

That’s not valid SQL syntax. The correct syntax is:

SELECT * FROM stock WHERE item = 'Frank''s Brand Hotdog Buns' 

As you might expect, MySQL++ provides that feature, too, through its escape manipulator. But here, we want both quoting and escaping. That brings us to the most widely useful manipulator:

string s = "Frank’s Brand Hotdog Buns";
query << "SELECT * FROM stock WHERE item = " << quote << s; 

The quote manipulator both quotes strings and escapes any characters that are special in SQL.

MySQL++ provides other manipulators as well. See the manip.h page in the reference manual.

It’s important to realize that MySQL++’s quoting and escaping mechanism is type-aware. Manipulators have no effect unless you insert the manipulator into a Query or SQLQueryParms stream. ^[2] Also, values are only quoted and/or escaped if they are of a data type that may need it. For example, Date must be quoted but never needs to be escaped, and integer types need neither quoting nor escaping. Manipulators are suggestions to the library, not commands: MySQL++ will ignore these suggestions if it knows it won’t result in syntactically-incorrect SQL.

It’s also important to realize that quoting and escaping in Query streams and template queries is never implicit.^[3] You must use manipulators and template query flags as necessary to tell MySQL++ where quoting and escaping is necessary. It would be nice if MySQL++ could do quoting and escaping implicitly based on data type, but this isn’t possible in all cases.^[4] Since MySQL++ can’t reliably guess when quoting and escaping is appropriate, and the programmer doesn’t need to^[5], MySQL++ makes you tell it.

The C++ and SQL data type systems have several differences that can cause problems when using MySQL++, or any other SQL based system, for that matter.

Most of the data types you can store in a SQL database are either numbers or text strings. If you’re only looking at the data going between the database server and your application, there aren’t even numbers: SQL is a textual language, so numbers and everything else gets transferred between the client and the database server in text string form.^[6] Consequently, MySQL++ has a lot of special support for text strings, and can translate to several C++ numeric data types transparently.

Some people worry that this translation via an intermediate string form will cause data loss. Obviously the text string data types are immune from problems in this regard. We’re also confident that MySQL++ translates BLOB and integer data types losslessly.

The biggest worry is with floating-point numbers. (The FLOAT and DOUBLE SQL data types.) We did have a problem with this in older versions of MySQL++, but we believe we fixed it completely in v3.0.2. No one has since proven data loss via this path. There is still a known problem ^[7] with the SQL DECIMAL type, which is somewhat related to the floating-point issue, but it’s apparently rarely encountered, which is why it hasn’t been fixed yet.

The best way to avoid problems with data translation is to always use the special MySQL++ data types defined in lib/sql_types.h corresponding to your SQL schema. These typedefs begin with sql_ and end with a lowercase version of the standard SQL type name, with spaces replaced by underscores. There are variants ending in _null that wrap these base types so they’re compatible with SQL null. For instance, the SQL type TINYINT UNSIGNED NOT NULL is represented in MySQL++ by mysqlpp::sql_tinyint_unsigned. If you drop the NOT NULL part, the corresponding C++ type is mysqlpp::sql_tinyint_unsigned_null.

MySQL++ doesn’t force you to use these typedefs. It tries to be flexible with regard to data conversions, so you could probably use int anywhere you use mysqlpp::sql_tinyint_unsigned, for example. That said, the MySQL++ typedefs give several advantages:

Most of these typedefs use standard C++ data types, but a few are aliases for a MySQL++ specific type. For instance, the SQL type DATETIME is mirrored in MySQL++ by mysqlpp::DateTime. For consistency, sql_types.h includes a typedef alias for DateTime called mysqlpp::sql_datetime.

MySQL++ doesn’t have typedefs for the most exotic data types, like those for the geospatial types. Patches to correct this will be thoughtfully considered.

Both C++ and SQL have things in them called NULL, but they differ in several ways. Consequently, MySQL++ has to provide special support for this, rather than just wrap native C++ facilities as it can with most data type issues.

mysqlpp::Null<mysqlpp::sql_tinyint_unsigned> myfield;

mysqlpp::sql_tinyint_unsigned_null myfield;

myfield = mysqlpp::null;

mysqlpp::Null<unsigned char, mysqlpp::NullIsZero> myfield(mysqlpp::null);
cout << myfield << endl;
cout << int(myfield) << endl;

MySQL++ has two classes that work like std::string to some degree: String and SQLTypeAdapter. These classes exist to provide functionality that std::string doesn’t provide, but they are neither derivatives of nor complete supersets of std::string. As a result, end-user code generally doesn’t deal with these classes directly, because std::string is a better general-purpose string type. In fact, MySQL++ itself uses std::string most of the time, too. But, the places these specialized stringish types do get used are so important to the way MySQL++ works that it’s well worth taking the time to understand them.

int x = row["x"];

Historically, there was no way to hold arbitrary-sized blocks of raw binary data in an SQL database. There was resistance to adding such a feature to SQL for a long time because it’s better, where possible, to decompose blocks of raw binary data into a series of numbers and text strings that can be stored in the database. This lets you query, address and manipulate elements of the data block individually.

A classic SQL newbie mistake is trying to treat the database server as a file system. Some embedded platforms use a database engine as a file system, but MySQL doesn’t typically live in that world. When your platform already has a perfectly good file system, you should use it for big, nondecomposable blocks of binary data in most cases.

A common example people use when discussing this is images in database-backed web applications. If you store the image in the database, you have to write code to retrieve the image from the database and send it to the client; there’s more overhead, and less efficient use of the system’s I/O caching system. If you store the image in the filesystem, all you have to do is point the web server to the directory where the images live, and put a URL for that image in your generated HTML. Because you’re giving the web server a direct path to a file on disk, operation is far more efficient. Web servers are very good at slurping whole files off of disk and sending them out to the network, and operating systems are very good at caching file accesses. Plus, you avoid the overhead of pushing the data through the high-level language your web app is written in, which is typically an interpreted language, not C++. Some people still hold out on this, claiming that database engines have superior security features, but I call bunk on that, too. Operating systems and web servers are capable of building access control systems every bit as granular and secure as a database system.

Occasionally you really do need to store a nondecomposable block of binary data in the database. For such cases, modern SQL database servers support BLOB data types, for Binary Large OBject. This is often just called binary data, though of course all data in a modern computer is binary at some level.

The tricky part about dealing with binary data in MySQL++ is to ensure that you don’t ever treat the data as a C string, which is really easy to do accidentally. C strings treat zero bytes as special end-of-string characters, but they’re not special at all in binary data. We’ve made a lot of improvements to the way MySQL++ handles string data to avoid this problem, but it’s still possible to bypass these features, wrecking your BLOBs. These examples demonstrate correct techniques.

#include "cmdline.h"
#include "images.h"
#include "printdata.h"

#include <fstream>

using namespace std;
using namespace mysqlpp;


// This is just an implementation detail for the example.  Skip down to
// main() for the concept this example is trying to demonstrate.  You
// can simply assume that, given a BLOB containing a valid JPEG, it
// returns true.
static bool
is_jpeg(const mysqlpp::sql_blob& img, const char** whynot)
{
    // See http://stackoverflow.com/questions/2253404/ for
    // justification for the various tests.
    const unsigned char* idp =
            reinterpret_cast<const unsigned char*>(img.data());
    if (img.size() < 125) {
        *whynot = "a valid JPEG must be at least 125 bytes";
    }
    else if ((idp[0] != 0xFF) || (idp[1] != 0xD8)) {
        *whynot = "file does not begin with JPEG sigil bytes";
    }
    else if ((memcmp(idp + 6, "JFIF", 4) != 0) &&
             (memcmp(idp + 6, "Exif", 4) != 0)) {
        *whynot = "file does not contain JPEG type word";
    }
    else {
        *whynot = 0;
        return true;
    }

    return false;
}


// Skip to main() before studying this.  This is a little too
// low-level to bother with on your first pass thru the code.
static bool
load_jpeg_file(const mysqlpp::examples::CommandLine& cmdline,
        images& img, string& img_name)
{
    if (cmdline.extra_args().size() == 0) {
        // Nothing for us to do here.  Caller will insert NULL BLOB.
        return true;
    }

    // Got a file's name on the command line, so open it.
    img_name = cmdline.extra_args()[0];
    ifstream img_file(img_name.c_str(), ios::binary);
    if (img_file) {
        // Slurp file contents into RAM with minimum copying.  (Idiom
        // explained here: http://stackoverflow.com/questions/116038/)
        //
        // By loading the file into a C++ string (stringstream::str())
        // and assigning that directly to a mysqlpp::sql_blob, we avoid
        // truncating the binary data at the first null character.
        img.data.data = static_cast<const stringstream*>(
                &(stringstream() << img_file.rdbuf()))->str();

        // Check JPEG data for sanity.
        const char* error;
        if (is_jpeg(img.data.data, &error)) {
            return true;
        }
        else {
            cerr << '"' << img_name << "\" isn't a JPEG: " <<
                    error << '!' << endl;
        }
    }

    cmdline.print_usage("[jpeg_file]");
    return false;
}


int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }

    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());

        // Load the file named on the command line
        images img(mysqlpp::null, mysqlpp::null);
        string img_name("NULL");
        if (load_jpeg_file(cmdline, img, img_name)) {
            // Insert image data or SQL NULL into the images.data BLOB
            // column.  The key here is that we're holding the raw
            // binary data in a mysqlpp::sql_blob, which avoids data
            // conversion problems that can lead to treating BLOB data
            // as C strings, thus causing null-truncation.  The fact
            // that we're using SSQLS here is a side issue, simply
            // demonstrating that mysqlpp::Null<mysqlpp::sql_blob> is
            // now legal in SSQLS, as of MySQL++ 3.0.7.
            Query query = con.query();
            query.insert(img);
            SimpleResult res = query.execute();

            // Report successful insertion
            cout << "Inserted \"" << img_name <<
                    "\" into images table, " << img.data.data.size() <<
                    " bytes, ID " << res.insert_id() << endl;
        }
    }
    catch (const BadQuery& er) {
        // Handle any query errors
        cerr << "Query error: " << er.what() << endl;
        return -1;
    }
    catch (const BadConversion& er) {
        // Handle bad conversions
        cerr << "Conversion error: " << er.what() << endl <<
                "\tretrieved data size: " << er.retrieved <<
                ", actual size: " << er.actual_size << endl;
        return -1;
    }
    catch (const Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return -1;
    }

    return 0;
}

#include "cmdline.h"
#include "images.h"

#define CRLF            "\r\n"
#define CRLF2           "\r\n\r\n"

int
main(int argc, char* argv[])
{
    // Get database access parameters from command line if present, else
    // use hard-coded values for true CGI case.
    mysqlpp::examples::CommandLine cmdline(argc, argv, "root",
            "nunyabinness");
    if (!cmdline) {
        return 1;
    }

    // Parse CGI query string environment variable to get image ID
    unsigned int img_id = 0;
    char* cgi_query = getenv("QUERY_STRING");
    if (cgi_query) {
        if ((strlen(cgi_query) < 4) || memcmp(cgi_query, "id=", 3)) {
            std::cout << "Content-type: text/plain" << std::endl << std::endl;
            std::cout << "ERROR: Bad query string" << std::endl;
            return 1;
        }
        else {
            img_id = atoi(cgi_query + 3);
        }
    }
    else {
        std::cerr << "Put this program into a web server's cgi-bin "
                "directory, then" << std::endl;
        std::cerr << "invoke it with a URL like this:" << std::endl;
        std::cerr << std::endl;
        std::cerr << "    http://server.name.com/cgi-bin/cgi_jpeg?id=2" <<
                std::endl;
        std::cerr << std::endl;
        std::cerr << "This will retrieve the image with ID 2." << std::endl;
        std::cerr << std::endl;
        std::cerr << "You will probably have to change some of the #defines "
                "at the top of" << std::endl;
        std::cerr << "examples/cgi_jpeg.cpp to allow the lookup to work." <<
                std::endl;
        return 1;
    }

    // Retrieve image from DB by ID
    try {
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());
        mysqlpp::Query query = con.query();
        query << "SELECT * FROM images WHERE id = " << img_id;
        mysqlpp::StoreQueryResult res = query.store();
        if (res && res.num_rows()) {
            images img = res[0];
            if (img.data.is_null) {
                std::cout << "Content-type: text/plain" << CRLF2;
                std::cout << "No image content!" << CRLF;
            }
            else {
                std::cout << "X-Image-Id: " << img_id << CRLF; // for debugging
                std::cout << "Content-type: image/jpeg" << CRLF;
                std::cout << "Content-length: " <<
                        img.data.data.length() << CRLF2;
                std::cout << img.data;
            }
        }
        else {
            std::cout << "Content-type: text/plain" << CRLF2;
            std::cout << "ERROR: No image with ID " << img_id << CRLF;
        }
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        std::cout << "Content-type: text/plain" << CRLF2;
        std::cout << "QUERY ERROR: " << er.what() << CRLF;
        return 1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        std::cout << "Content-type: text/plain" << CRLF2;
        std::cout << "GENERAL ERROR: " << er.what() << CRLF;
        return 1;
    }

    return 0;
}

The Transaction class makes it easier to use SQL transactions in an exception-safe manner. Normally you create the Transaction object on the stack before you issue the queries in your transaction set. Then, when all the queries in the transaction set have been issued, you call Transaction::commit(), which commits the transaction set. If the Transaction object goes out of scope before you call commit(), the transaction set is rolled back. This ensures that if some code throws an exception after the transaction is started but before it is committed, the transaction isn’t left unresolved.

examples/transaction.cpp illustrates this:

#include "cmdline.h"
#include "printdata.h"
#include "stock.h"

#include <iostream>
#include <cstdio>

using namespace std;

int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }

    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());

        // Show initial state
        mysqlpp::Query query = con.query();
        cout << "Initial state of stock table:" << endl;
        print_stock_table(query);

        // Insert a few rows in a single transaction set
        {
            // Use a higher level of transaction isolation than MySQL
            // offers by default.  This trades some speed for more
            // predictable behavior.  We've set it to affect all
            // transactions started through this DB server connection,
            // so it affects the next block, too, even if we don't
            // commit this one.
            mysqlpp::Transaction trans(con,
                    mysqlpp::Transaction::serializable,
                    mysqlpp::Transaction::session);

            stock row("Sauerkraut", 42, 1.2, 0.75,
                    mysqlpp::sql_date("2006-03-06"), mysqlpp::null);
            query.insert(row);
            query.execute();

            cout << "\nRow inserted, but not committed." << endl;
            cout << "Verify this with another program (e.g. simple1), "
                    "then hit Enter." << endl;
            getchar();

            cout << "\nCommitting transaction gives us:" << endl;
            trans.commit();
            print_stock_table(query);
        }
            
        // Now let's test auto-rollback
        {
            // Start a new transaction, keeping the same isolation level
            // we set above, since it was set to affect the session.
            mysqlpp::Transaction trans(con);
            cout << "\nNow adding catsup to the database..." << endl;

            stock row("Catsup", 3, 3.9, 2.99,
                    mysqlpp::sql_date("2006-03-06"), mysqlpp::null);
            query.insert(row);
            query.execute();
        }
        cout << "\nNo, yuck! We don't like catsup. Rolling it back:" <<
                endl;
        print_stock_table(query);
            
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        cerr << "Query error: " << er.what() << endl;
        return -1;
    }
    catch (const mysqlpp::BadConversion& er) {  
        // Handle bad conversions
        cerr << "Conversion error: " << er.what() << endl <<
                "\tretrieved data size: " << er.retrieved <<
                ", actual size: " << er.actual_size << endl;
        return -1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return -1;
    }

    return 0;
}

One of the downsides of transactions is that the locking it requires in the database server is prone to deadlocks. The classic case where this happens is when two programs both want access to the same two rows within a single transaction each, but they modify them in opposite orders. If the timing is such that the programs interleave their lock acquisitions, the two come to an impasse: neither can get access to the other row they want to modify until the other program commits its transaction and thus release the row locks, but neither can finish the transaction because they’re waiting on row locks the database server is holding on behalf of the other program.

The MySQL server is smart enough to detect this condition, but the best it can do is abort the second transaction. This breaks the impasse, allowing the first program to complete its transaction.

The second program now has to deal with the fact that its transaction just got aborted. There’s a subtlety in detecting this situation when using MySQL++. By default, MySQL++ signals errors like these with exceptions. In the exception handler, you might expect to get ER_LOCK_DEADLOCK from Query::errnum() (or Connection::errnum(), same thing), but what you’ll almost certainly get instead is 0, meaning “no error.” Why? It’s because you’re probably using a Transaction object to get automatic roll-backs in the face of exceptions. In this case, the roll-back happens before your exception handler is called by issuing a ROLLBACK query to the database server. Thus, Query::errnum() returns the error code associated with this roll-back query, not the deadlocked transaction that caused the exception.

To avoid this problem, a few of the exception objects as of MySQL++ v3.0 include this last error number in the exception object itself. It’s populated at the point of the exception, so it can differ from the value you would get from Query::errnum() later on when the exception handler runs.

The example examples/deadlock.cpp demonstrates the problem:

#include "cmdline.h"

#include <mysql++.h>
#include <mysqld_error.h>

#include <iostream>

using namespace std;

// Bring in global holding the value given to the -m switch
extern int run_mode;


int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }

    // Check that the mode parameter was also given and it makes sense
    const int run_mode = cmdline.run_mode();
    if ((run_mode != 1) && (run_mode != 2)) {
        cerr << argv[0] << " must be run with -m1 or -m2 as one of "
                "its command-line arguments." << endl;
        return 1;
    }

    mysqlpp::Connection con;
    try {
        // Establish the connection to the database server
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());

        // Start a transaction set.  Transactions create mutex locks on
        // modified rows, so if two programs both touch the same pair of
        // rows but in opposite orders at the wrong time, one of the two
        // programs will deadlock.  The MySQL server knows how to detect
        // this situation, and its error return causes MySQL++ to throw
        // a BadQuery exception.  The point of this example is that if
        // you want to detect this problem, you would check the value of
        // BadQuery::errnum(), not Connection::errnum(), because the
        // transaction rollback process executes a query which succeeds,
        // setting the MySQL C API's "last error number" value to 0.
        // The exception object carries its own copy of the error number
        // at the point the exception was thrown for this very reason.
        mysqlpp::Query query = con.query();
        mysqlpp::Transaction trans(con);

        // Build and run the queries, with the order depending on the -m
        // flag, so that a second copy of the program will deadlock if
        // run while the first is waiting for Enter.
        char dummy[100];
        for (int i = 0; i < 2; ++i) {
            int lock = run_mode + (run_mode == 1 ? i : -i);
            cout << "Trying lock " << lock << "..." << endl;

            query << "select * from deadlock_test" << lock << 
                    " where x = " << lock << " for update";
            query.store();

            cout << "Acquired lock " << lock << ".  Press Enter to ";
            cout << (i == 0 ? "try next lock" : "exit");
            cout << ": " << flush;
            cin.getline(dummy, sizeof(dummy));
        }
    }
    catch (mysqlpp::BadQuery e) {
        if (e.errnum() == ER_LOCK_DEADLOCK) {
            cerr << "Transaction deadlock detected!" << endl;
            cerr << "Connection::errnum = " << con.errnum() <<
                    ", BadQuery::errnum = " << e.errnum() << endl;
        }
        else {
            cerr << "Unexpected query error: " << e.what() << endl;
        }
        return 1;
    }
    catch (mysqlpp::Exception e) {
        cerr << "General error: " << e.what() << endl;      
        return 1;
    }

    return 0;
}

This example works a little differently than the others. You run one copy of the example, then when it pauses waiting for you to press Enter, you run another copy. Then, depending on which one you press Enter in, one of the two will abort with the deadlock exception. You can see from the error message you get that it matters which method you call to get the error number. What you do about it is up to you as it depends on your program’s design and system architecture.

There are three major ways to execute a query in MySQL++: Query::execute(), Query::store(), and Query::use(). Which should you use, and why?

execute() is for queries that do not return data per se. For instance, CREATE INDEX. You do get back some information from the MySQL server, which execute() returns to its caller in a SimpleResult object. In addition to the obvious — a flag stating whether the query succeeded or not — this object also contains things like the number of rows that the query affected. If you only need the success status, it’s a little more efficient to call Query::exec() instead, as it simply returns bool.

If your query does pull data from the database, the simplest option is store(). (All of the examples up to this point have used this method.) This returns a StoreQueryResult object, which contains the entire result set. It’s especially convenient because StoreQueryResult derives from std::vector<mysqlpp::Row>, so it opens the whole panoply of STL operations for accessing the rows in the result set. Access rows randomly with subscript notation, iterate forwards and backwards over the result set, run STL algorithms on the set...it all works naturally.

If you like the idea of storing your results in an STL container but don’t want to use std::vector, you can call Query::storein() instead. It lets you store the results in any standard STL container (yes, both sequential and set-associative types) instead of using StoreQueryResult. You do miss out on some of the additional database information held by StoreQueryResult’s other base class, ResultBase, however.

store*() queries are convenient, but the cost of keeping the entire result set in main memory can sometimes be too high. It can be surprisingly costly, in fact. A MySQL database server stores data compactly on disk, but it returns query data to the client in a textual form. This results in a kind of data bloat that affects numeric and BLOB types the most. MySQL++ and the underlying C API library also have their own memory overheads in addition to this. So, if you happen to know that the database server stores every record of a particular table in 1 KB, pulling a million records from that table could easily take several GB of memory with a store() query, depending on what’s actually stored in that table.

For these large result sets, the superior option is a use() query. This returns a UseQueryResult object, which is similar to StoreQueryResult, but without all of the random-access features. This is because a “use” query tells the database server to send the results back one row at a time, to be processed linearly. It’s analogous to a C++ stream’s input iterator, as opposed to a random-access iterator that a container like vector offers. By accepting this limitation, you can process arbitrarily large result sets. This technique is demonstrated in examples/simple3.cpp:

#include "cmdline.h"
#include "printdata.h"

#include <mysql++.h>

#include <iostream>
#include <iomanip>

using namespace std;

int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }

    // Connect to the sample database.
    mysqlpp::Connection conn(false);
    if (conn.connect(mysqlpp::examples::db_name, cmdline.server(),
            cmdline.user(), cmdline.pass())) {
        // Ask for all rows from the sample stock table and display
        // them.  Unlike simple2 example, we retreive each row one at
        // a time instead of storing the entire result set in memory
        // and then iterating over it.
        mysqlpp::Query query = conn.query("select * from stock");
        if (mysqlpp::UseQueryResult res = query.use()) {
            // Display header
            cout.setf(ios::left);
            cout << setw(31) << "Item" <<
                    setw(10) << "Num" <<
                    setw(10) << "Weight" <<
                    setw(10) << "Price" <<
                    "Date" << endl << endl;

            // Get each row in result set, and print its contents
            while (mysqlpp::Row row = res.fetch_row()) {
                cout << setw(30) << row["item"] << ' ' <<
                        setw(9) << row["num"] << ' ' <<
                        setw(9) << row["weight"] << ' ' <<
                        setw(9) << row["price"] << ' ' <<
                        setw(9) << row["sdate"] <<
                        endl;
            }

            // Check for error: can't distinguish "end of results" and
            // error cases in return from fetch_row() otherwise.
            if (conn.errnum()) {
                cerr << "Error received in fetching a row: " <<
                        conn.error() << endl;
                return 1;
            }
            return 0;
        }
        else {
            cerr << "Failed to get stock item: " << query.error() << endl;
            return 1;
        }
    }
    else {
        cerr << "DB connection failed: " << conn.error() << endl;
        return 1;
    }
}

This example does the same thing as simple2, only with a “use” query instead of a “store” query.

Valuable as use() queries are, they should not be the first resort in solving problems of excessive memory use. It’s better if you can find a way to simply not pull as much data from the database in the first place. Maybe you’re saying SELECT * even though you don’t immedidately need all the columns from the table. Or, maybe you’re filtering the result set with C++ code after you get it from the database server. If you can do that filtering with a more restrictive WHERE clause on the SELECT, it’ll not only save memory, it’ll save bandwidth between the database server and client, and can even save CPU time. If the filtering criteria can’t be expressed in a WHERE clause, however, read on to the next section.

Sometimes you must pull more data from the database server than you actually need and filter it in memory. SQL’s WHERE clause is powerful, but not as powerful as C++. Instead of storing the full result set and then picking over it to find the rows you want to keep, use Query::store_if(). This is examples/store_if.cpp:

#include "cmdline.h"
#include "printdata.h"
#include "stock.h"

#include <mysql++.h>

#include <iostream>

#include <math.h>


// Define a functor for testing primality.
struct is_prime
{
    bool operator()(const stock& s)
    {
        if ((s.num == 2) || (s.num == 3)) {
            return true;    // 2 and 3 are trivial cases
        }
        else if ((s.num < 2) || ((s.num % 2) == 0)) {
            return false;   // can't be prime if < 2 or even
        }
        else {
            // The only possibility left is that it's divisible by an
            // odd number that's less than or equal to its square root.
            for (int i = 3; i <= sqrt(double(s.num)); i += 2) {
                if ((s.num % i) == 0) {
                    return false;
                }
            }
            return true;
        }
    }
};


int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }

    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());

        // Collect the stock items with prime quantities
        std::vector<stock> results;
        mysqlpp::Query query = con.query();
        query.store_if(results, stock(), is_prime());

        // Show the results
        print_stock_header(results.size());
        std::vector<stock>::const_iterator it;
        for (it = results.begin(); it != results.end(); ++it) {
            print_stock_row(it->item.c_str(), it->num, it->weight,
                    it->price, it->sDate);
        }
    }
    catch (const mysqlpp::BadQuery& e) {
        // Something went wrong with the SQL query.
        std::cerr << "Query failed: " << e.what() << std::endl;
        return 1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        std::cerr << "Error: " << er.what() << std::endl;
        return 1;
    }

    return 0;
}

I doubt anyone really needs to select rows from a table that have a prime number in a given field. This example is meant to be just barely more complex than SQL can manage, to avoid obscuring the point. That point being, the Query::store_if() call here gives you a container full of results meeting a criterion that you probably can’t express in SQL. You will no doubt have much more useful criteria in your own programs.

If you need a more complex query than the one store_if() knows how to build when given an SSQLS examplar, there are two overloads that let you use your own query string. One overload takes the query string directly, and the other uses the query string built with Query’s stream interface.

SQL is more than just a database query language. Modern database engines can actually do some calculations on the data on the server side. But, this isn’t always the best way to get something done. When you need to mix code and a query, MySQL++’s Query::for_each() facility might be just what you need. This is examples/for_each.cpp:

#include "cmdline.h"
#include "printdata.h"
#include "stock.h"

#include <mysql++.h>

#include <iostream>

#include <math.h>


// Define a functor to collect statistics about the stock table
class gather_stock_stats
{
public:
    gather_stock_stats() :
    items_(0),
    weight_(0),
    cost_(0)
    {
    }

    void operator()(const stock& s)
    {
        items_  += s.num;
        weight_ += (s.num * s.weight);
        cost_   += (s.num * s.price.data);
    }
    
private:
    mysqlpp::sql_bigint items_;
    mysqlpp::sql_double weight_, cost_;

    friend std::ostream& operator<<(std::ostream& os,
            const gather_stock_stats& ss);
};


// Dump the contents of gather_stock_stats to a stream in human-readable
// form.
std::ostream&
operator<<(std::ostream& os, const gather_stock_stats& ss)
{
    os << ss.items_ << " items " <<
            "weighing " << ss.weight_ << " stone and " <<
            "costing " << ss.cost_ << " cowrie shells";
    return os;
}


int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }

    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());

        // Gather and display the stats for the entire stock table
        mysqlpp::Query query = con.query();
        std::cout << "There are " << query.for_each(stock(),
                gather_stock_stats()) << '.' << std::endl;
    }
    catch (const mysqlpp::BadQuery& e) {
        // Something went wrong with the SQL query.
        std::cerr << "Query failed: " << e.what() << std::endl;
        return 1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        std::cerr << "Error: " << er.what() << std::endl;
        return 1;
    }

    return 0;
}

You only need to read the main() function to get a good idea of what the program does. The key line of code passes an SSQLS examplar and a functor to Query::for_each(). for_each() uses the SSQLS instance to build a select * from TABLE query, stock in this case. It runs that query internally, calling gather_stock_stats on each row. This is a pretty contrived example; you could actually do this in SQL, but we’re trying to prevent the complexity of the code from getting in the way of the demonstration here.

Just as with store_if(), described above, there are two other overloads for for_each() that let you use your own query string.

MySQL has a large number of options that control how it makes the connection to the database server, and how that connection behaves. The defaults are sufficient for most programs, so only one of the MySQL++ example programs make any connection option changes. Here is examples/multiquery.cpp:

#include "cmdline.h"
#include "printdata.h"

#include <mysql++.h>

#include <algorithm>
#include <iostream>
#include <iomanip>
#include <vector>

using namespace std;
using namespace mysqlpp;


typedef vector<size_t> IntVectorType;


static void
print_header(IntVectorType& widths, StoreQueryResult& res)
{
    cout << "  |" << setfill(' ');
    for (size_t i = 0; i < res.field_names()->size(); i++) {
        cout << " " << setw(widths.at(i)) << res.field_name(int(i)) << " |";
    }
    cout << endl;
}


static void
print_row(IntVectorType& widths, Row& row)
{
    cout << "  |" << setfill(' ');
    for (size_t i = 0; i < row.size(); ++i) {
        cout << " " << setw(widths.at(i)) << row[int(i)] << " |";
    }
    cout << endl;
}


static void
print_row_separator(IntVectorType& widths)
{
    cout << "  +" << setfill('-');
    for (size_t i = 0; i < widths.size(); i++) {
        cout << "-" << setw(widths.at(i)) << '-' << "-+";
    }
    cout << endl;
}


static void
print_result(StoreQueryResult& res, int index)
{
    // Show how many rows are in result, if any
    StoreQueryResult::size_type num_results = res.size();
    if (res && (num_results > 0)) {
        cout << "Result set " << index << " has " << num_results <<
                " row" << (num_results == 1 ? "" : "s") << ':' << endl;
    }
    else {
        cout << "Result set " << index << " is empty." << endl;
        return;
    }

    // Figure out the widths of the result set's columns
    IntVectorType widths;
    size_t size = res.num_fields();
    for (size_t i = 0; i < size; i++) {
        widths.push_back(max(
                res.field(i).max_length(),
                res.field_name(i).size()));
    }

    // Print result set header
    print_row_separator(widths);
    print_header(widths, res);
    print_row_separator(widths);

    // Display the result set contents
    for (StoreQueryResult::size_type i = 0; i < num_results; ++i) {
        print_row(widths, res[i]);
    }

    // Print result set footer
    print_row_separator(widths);
}


static void
print_multiple_results(Query& query)
{
    // Execute query and print all result sets
    StoreQueryResult res = query.store();
    print_result(res, 0);
    for (int i = 1; query.more_results(); ++i) {
        res = query.store_next();
        print_result(res, i);
    }
}


int
main(int argc, char *argv[])
{
    // Get connection parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }

    try {
        // Enable multi-queries.  Notice that you almost always set
        // MySQL++ connection options before establishing the server
        // connection, and options are always set using this one
        // interface.  If you're familiar with the underlying C API,
        // you know that there is poor consistency on these matters;
        // MySQL++ abstracts these differences away.
        Connection con;
        con.set_option(new MultiStatementsOption(true));

        // Connect to the database
        if (!con.connect(mysqlpp::examples::db_name, cmdline.server(),
                cmdline.user(), cmdline.pass())) {
            return 1;
        }

        // Set up query with multiple queries.
        Query query = con.query();
        query << "DROP TABLE IF EXISTS test_table; " <<
                "CREATE TABLE test_table(id INT); " <<
                "INSERT INTO test_table VALUES(10); " <<
                "UPDATE test_table SET id=20 WHERE id=10; " <<
                "SELECT * FROM test_table; " <<
                "DROP TABLE test_table";
        cout << "Multi-query: " << endl << query << endl;

        // Execute statement and display all result sets.
        print_multiple_results(query);

#if MYSQL_VERSION_ID >= 50000
        // If it's MySQL v5.0 or higher, also test stored procedures, which
        // return their results the same way multi-queries do.
        query << "DROP PROCEDURE IF EXISTS get_stock; " <<
                "CREATE PROCEDURE get_stock" <<
                "( i_item varchar(20) ) " <<
                "BEGIN " <<
                "SET i_item = concat('%', i_item, '%'); " <<
                "SELECT * FROM stock WHERE lower(item) like lower(i_item); " <<
                "END;";
        cout << "Stored procedure query: " << endl << query << endl;

        // Create the stored procedure.
        print_multiple_results(query);

        // Call the stored procedure and display its results.
        query << "CALL get_stock('relish')";
        cout << "Query: " << query << endl;
        print_multiple_results(query);
#endif

        return 0;
    }
    catch (const BadOption& err) {
        cerr << err.what() << endl;
        cerr << "This example requires MySQL 4.1.1 or later." << endl;
        return 1;
    }
    catch (const ConnectionFailed& err) {
        cerr << "Failed to connect to database server: " <<
                err.what() << endl;
        return 1;
    }
    catch (const Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return 1;
    }
}

This is a fairly complex example demonstrating the multi-query and stored procedure features in newer versions of MySQL. Because these are new features, and they change the communication between the client and server, you have to enable these features in a connection option. The key line is right up at the top of main(), where it creates a MultiStatementsOption object and passes it to Connection::set_option(). That method will take a pointer to any derivative of Option: you just create such an object on the heap and pass it in, which gives Connection the data values it needs to set the option. You don’t need to worry about releasing the memory used by the Option objects; it’s done automatically.

The only tricky thing about setting options is that only a few of them can be set after the connection is up. Most need to be set just as shown in the example above: create an unconnected Connection object, set your connection options, and only then establish the connection. The option setting mechanism takes care of applying the options at the correct time in the connection establishment sequence.

If you’re familiar with setting connection options in the MySQL C API, you’ll have to get your head around the fact that MySQL++’s connection option mechanism is a much simpler, higher-level design that doesn’t resemble the C API in any way. The C API has something like half a dozen different mechanisms for setting options that control the connection. The flexibility of the C++ type system allows us to wrap all of these up into a single high-level mechanism while actually getting greater type safety than the C API allows.

By default, current MySQL servers have an 8 hour idle timeout on connections. This is not a problem if your program never has to run for more than 8 hours or reliably queries the database more often than that. And, it’s a good thing for the database server, because even an idle connection takes up server resources.

Many programs must run continually, however, and may experience long idle periods, such as nights and weekends when no one is around to make the program issue database queries. It’s therefore common for people writing such programs to get a bug report from the field complaining that the program died overnight or over a long weekend, usually with some error message about the database server going away. They then check the DB server, find that it’s still running and never did restart and scratch their heads wondering what happened. What happened is that the server’s connection idle timeout expired, so it closed the connection to the client.

You cannot detect this condition by calling Connection::connected(). When that returns true, it just means that either the connect-on-create constructor or the connect() call succeeded and that we haven’t observed the connection to be down since then. When the database server closes an idle connection, you won’t know it until after you try to issue a query. This is simply due to the nature of network programming.

One way around this problem is to configure MySQL to have a longer idle timeout. This timeout is in seconds, so the default of 8 hours is 28,800 seconds. You would want to figure out the longest possible time that your program could be left idle, then pick a value somewhat longer than that. For instance, you might decide that the longest reasonable idle time is a long 4-day weekend — 345,600 seconds — which you could round up to 350,000 or 400,000 to allow for a little bit of additional idle time on either end of that period.

Another way around this, on a per-connection basis from the client side, would be to set the ReconnectOption connection option. This will cause MySQL++ to reconnect to the server automatically if it drops the connection. Beware that unless you’re using MySQL 5.1.6 or higher, you have to set this only after the connection is established, or it won’t take effect. This means there’s a potential race condition: it’s possible the connection could drop shortly enough after being established that you don’t have time to apply the option, so it won’t come back up automatically. MySQL 5.1.6+ fixes this by allowing this option to be set before the connection is established.

A completely different way to tackle this, if your program doesn’t block forever waiting on I/O while idle, is to periodically call Connection::ping(). ^[12] This sends the smallest possible amount of data to the database server, which will reset its idle timer and cause it to respond, so ping() returns true. If it returns false instead, you know you need to reconnect to the server. Periodic pinging is easiest to do if your program uses asynchronous I/O, threads, or some kind of event loop to ensure that you can call something periodically even while the rest of the program has nothing to do.

An interesting variant on this strategy is to ping the server before each query, or, better, before each group of queries within a larger operation. It has an advantage over pinging during idle time in that the client is about to use far more server resources to handle the query than it will take to handle the ping, so the ping time gets lost in the overhead. On the other hand, if the client issues queries frequently when not idle, it can result in a lot more pings than would happen if you just pinged every N hours while idle.

Finally, some programmers prefer to wrap the querying mechanism in an error handler that catches the “server has gone away” error and tries to reestablish the connection and reissue the query. This adds some complexity, but it makes your program more robust without taking up unnecessary resources. If you did this, you could even change the server to drop idle connections more often, thus tying up fewer TCP/IP stack resources.

An important limitation of the MySQL C API library — which MySQL++ is built atop, so it shares this limitation — is that you can only have one query in progress on each connection to the database server. If you try to issue a second query while one is still in progress, you get an obscure error message about “Commands out of sync” from the underlying C API library. (You normally get this message in a MySQL++ exception unless you have exceptions disabled, in which case you get a failure code and Connection::error() returns this message.)

There are lots of ways to run into this limitation:

The following example demonstrates how to get information about the fields in a result set, such as the name of the field and the SQL type. This is examples/fieldinf.cpp:

#include "cmdline.h"
#include "printdata.h"

#include <iostream>
#include <iomanip>

using namespace std;


int
main(int argc, char *argv[])
{
    // Get database access parameters from command line
    mysqlpp::examples::CommandLine cmdline(argc, argv);
    if (!cmdline) {
        return 1;
    }

    try {
        // Establish the connection to the database server.
        mysqlpp::Connection con(mysqlpp::examples::db_name,
                cmdline.server(), cmdline.user(), cmdline.pass());

        // Get contents of main example table
        mysqlpp::Query query = con.query("select * from stock");
        mysqlpp::StoreQueryResult res = query.store();

        // Show info about each field in that table
        char widths[] = { 12, 22, 46 };
        cout.setf(ios::left);
        cout << setw(widths[0]) << "Field" <<
                setw(widths[1]) << "SQL Type" <<
                setw(widths[2]) << "Equivalent C++ Type" <<
                endl;
        for (size_t i = 0; i < sizeof(widths) / sizeof(widths[0]); ++i) {
            cout << string(widths[i] - 1, '=') << ' ';
        }
        cout << endl;
        
        for (size_t i = 0; i < res.field_names()->size(); i++) {
            // Suppress C++ type name outputs when run under dtest,
            // as they're system-specific.
            const char* cname = res.field_type(int(i)).name();
            mysqlpp::FieldTypes::value_type ft = res.field_type(int(i));
            ostringstream os;
            os << ft.sql_name() << " (" << ft.id() << ')';
            cout << setw(widths[0]) << res.field_name(int(i)).c_str() <<
                    setw(widths[1]) << os.str() <<
                    setw(widths[2]) << cname <<
                    endl;
        }
        cout << endl;

        // Simple type check
        if (res.field_type(0) == typeid(string)) {
            cout << "SQL type of 'item' field most closely resembles "
                    "the C++ string type." << endl;
        }

        // Tricky type check: the 'if' path shouldn't happen because the
        // description field has the NULL attribute.  We need to dig a
        // little deeper if we want to ignore this in our type checks.
        if (res.field_type(5) == typeid(string)) {
            cout << "Should not happen! Type check failure." << endl;
        }
        else if (res.field_type(5) == typeid(mysqlpp::sql_blob_null)) {
            cout << "SQL type of 'description' field resembles "
                    "a nullable variant of the C++ string type." << endl;
        }
        else {
            cout << "Weird: fifth field's type is now " <<
                    res.field_type(5).name() << endl;
            cout << "Did something recently change in resetdb?" << endl;
        }
    }
    catch (const mysqlpp::BadQuery& er) {
        // Handle any query errors
        cerr << "Query error: " << er.what() << endl;
        return -1;
    }
    catch (const mysqlpp::Exception& er) {
        // Catch-all for any other MySQL++ exceptions
        cerr << "Error: " << er.what() << endl;
        return -1;
    }

    return 0;
}