MySQL++ has a lot of complexity and power to cope with the variety of ways people use databases, but at bottom it doesn’t work all that differently than other database access APIs. The usage pattern looks like this:
Open the connection
Form and execute the query
If successful, iterate through the result set
Else, deal with errors
Each of these steps corresponds to a MySQL++ class or class hierarchy. An overview of each follows.
A Connection object manages the
connection to the MySQL server. You need at least one of these
objects to do anything. Because the other MySQL++ objects your
program will use often depend (at least indirectly) on the
Connection instance, the
Connection object needs to live at least as
long as all other MySQL++ objects in your program.
MySQL supports many different types of data connection between
the client and the server: TCP/IP, Unix domain sockets, and Windows
named pipes. The generic Connection class
supports all of these, figuring out which one you mean based on the
parameters you pass to
Connection::connect(). But if you know in
advance that your program only needs one particular connection type,
there are subclasses with simpler interfaces. For example,
there’s TCPConnection if you
know your program will always use a networked database
server.
Most often, you create SQL queries using a Query object created by the
Connection object.
Query acts as a standard C++ output
stream, so you can write data to it like you would to
std::cout or
std::ostringstream. This is the most C++ish
way MySQL++ provides for building up a query string. The library
includes stream
manipulators that are type-aware so it’s easy to build
up syntactically-correct SQL.
Query also has a feature called Template Queries which work something like C’s
printf() function: you set up a fixed query
string with tags inside that indicate where to insert the variable
parts. If you have multiple queries that are structurally similar,
you simply set up one template query, and use that in the various
locations of your program.
A third method for building queries is to use
Query with SSQLS. This feature lets you create C++
structures that mirror your database schemas. These in turn give
Query the information it needs to build many
common SQL queries for you. It can INSERT,
REPLACE and UPDATE rows in a
table given the data in SSQLS form. It can also generate
SELECT * FROM SomeTable queries and store the
results as an STL collection of SSQLSes.
The field data in a result set are stored in a special
std::string-like class called String. This class has conversion operators
that let you automatically convert these objects to any of the basic
C data types. Additionally, MySQL++ defines classes like DateTime, which you can initialize from a
MySQL DATETIME string. These automatic
conversions are protected against bad conversions, and can either
set a warning flag or throw an exception, depending on how you set
the library up.
As for the result sets as a whole, MySQL++ has a number of different ways of representing them:
Not all SQL queries return data. An example is CREATE TABLE. For these types of queries, there is a special result type (SimpleResult) that simply reports the state resulting from the query: whether the query was successful, how many rows it impacted (if any), etc.
The most direct way to retrieve a result set is to use
Query::store(). This returns a StoreQueryResult object, which derives
from std::vector<mysqlpp::Row>,
making it a random-access container of Rows. In turn, each Row object is
like a std::vector of
String objects, one for each field in the
result set. Therefore, you can treat
StoreQueryResult as a two-dimensional
array: you can get the 5th field on the 2nd row by simply saying
result[1][4]. You can also access row
elements by field name, like this:
result[2]["price"].
A less direct way of working with query results is to use
Query::use(), which returns a UseQueryResult object. This class acts
like an STL input iterator rather than a
std::vector: you walk through your result
set processing one row at a time, always going forward. You
can’t seek around in the result set, and you can’t
know how many results are in the set until you find the end. In
payment for that inconvenience, you get better memory efficiency,
because the entire result set doesn’t need to be stored in
RAM. This is very useful when you need large result sets.
Accessing results through MySQL++’s data structures is a pretty low level of abstraction. It’s better than using the MySQL C API, but not by much. You can elevate things a little closer to the level of the problem space by using the SSQLS feature. This lets you define C++ structures that match the table structures in your database schema. In addition, it’s easy to use SSQLSes with regular STL containers (and thus, algorithms) so you don’t have to deal with the quirks of MySQL++’s data structures.
The advantage of this method is that your program will require very little embedded SQL code. You can simply execute a query, and receive your results as C++ data structures, which can be accessed just as you would any other structure. The results can be accessed through the Row object, or you can ask the library to dump the results into an STL container — sequential or set-associative, it doesn’t matter — for you. Consider this:
vector<stock> v;
query << "SELECT * FROM stock";
query.storein(v);
for (vector<stock>::iterator it = v.begin(); it != v.end(); ++it) {
cout << "Price: " << it->price << endl;
}Isn’t that slick?
If you don’t want to create SSQLSes to match your
table structures, as of MySQL++ v3 you can now use
Row here instead:
vector<mysqlpp::Row> v;
query << "SELECT * FROM stock";
query.storein(v);
for (vector<mysqlpp::Row>::iterator it = v.begin(); it != v.end(); ++it) {
cout << "Price: " << it->at("price") << endl;
}It lacks a certain syntactic elegance, but it has its uses.
By default, the library throws exceptions whenever it encounters an error. You can ask the library to set an error flag instead, if you like, but the exceptions carry more information. Not only do they include a string member telling you why the exception was thrown, there are several exception types, so you can distinguish between different error types within a single try block.