Connections and simple queries

Connecting to the database

The session class encapsulates the database connection and other backend-related details, which are common to all the statements that will be later executed. It has a couple of overloaded constructors.

The most basic one expects two parameters: the requested backend factory object and the generic connection string, which meaning is backend-dependent.


session sql(oracle, "service=orcl user=scott password=tiger");

Another example might be:

session sql(postgresql, "dbname=mydb");

Above, the sql object is a local (automatic) object that encapsulates the connection.

This session constructor either connects successfully, or throws an exception.

Another constructor allows to name backends at run-time and supports the dynamically loadable backends, which have to be compiled as shared libraries. The usage is similar to the above, but instead of providing the factory object, the backend name is expected:

session sql("postgresql", "dbname=mydb");

For convenience, the URL-like form that combines both the backend name with connection parameters is supported as well:

session sql("postgresql://dbname=mydb");

The last two constructors described above try to locate the shared library with the name (or libsoci_ABC.dll on Windows), where ABC is the backend name. In the above examples, the expected library name will be for Unix-like systems.

Environment configuration:

The SOCI_BACKENDS_PATH environment variable defines the set of paths where the shared libraries will be searched for. There can be many paths, separated by colons, and they are used from left to right until the library with the appropriate name is found. If this variable is not set or is empty, the current directory is used as a default path for dynamically loaded backends.

The run-time selection of backends is also supported with libraries linked statically. Each backend provides a separate function of the form register_factory_name, where name is a backend name. Thus:

extern "C" void register_factory_postgresql();
// ...
session sql("postgresql://dbname=mydb");

The above example registers the backend for PostgreSQL and later creates the session object for that backend. This form is provided for those projects that prefer static linking but still wish to benefit from run-time backend selection.

An alternative way to set up the session is to create it in the disconnected state and connect later:

session sql;

// some time later:, "dbname=mydb");

// or:"postgresql://dbname=mydb");

The rules for backend naming are the same as with the constructors described above.

The session can be also explicitly closed and reconnected, which can help with basic session error recovery. The reconnect function has no parameters and attempts to use the same values as those provided with earlier constructor or open calls.

See also the page devoted to multithreading for a detailed description of connection pools.

It is possible to have many active sessions at the same time, even using different backends.

Portability note:

The following backend factories are currently (as of 3.1.0 release) available:

The following backends are also available, with various levels of completeness:

Simple SQL statements

In many cases, the SQL query is intended to be executed only once, which means that statement parsing and execution can go together. The session class provides a special once member, which triggers parsing and execution of such one-time statements:

sql.once << "drop table persons";

For shorter syntax, the following form is also allowed:

sql << "drop table persons";

The IOStream-like interface is exactly what it looks like, so that the statement text can be composed of many parts, involving anything that is streamable (including custom classes, if they have appropriate operator<<):

string tableName = "persons";
sql << "drop table " << tableName;

int id = 123;
sql << "delete from companies where id = " << id;