Logging - SOCI (4.0.1)

# Logging

SOCI provides a flexible, but requiring some effort to use, way to log all queries done by the library and a more limited but very simple way to do it.

## Simple logging

The following members of the `session` class support the basic logging functionality:

- `void set_log_stream(std::ostream * s);`
- `std::ostream * get_log_stream() const;`
- `std::string get_last_query() const;`

The first two functions allow to set the user-provided output stream object for logging. The `NULL` value, which is the default, means that there is no logging.

An example use might be:

    session sql(oracle, "...");

    ofstream file("my_log.txt");
    sql.set_log_stream(&file);

    // ...

Each statement logs its query string before the preparation step (whether explicit or implicit) and therefore logging is effective whether the query succeeds or not. Note that each prepared query is logged only once, independent on how many times it is executed.

The `get_last_query` function allows to retrieve the last used query.

## Flexible logging using custom loggers

If the above is not enough, it is also possible to log the queries in exactly the way you want by deriving your own `my_log_impl` class from `soci::logger_impl` and implementing its pure virtual `start_query()` and `do_clone()` methods:

    class my_log_impl : public soci::logger_impl
    {
    public:
        virtual void start_query(std::string const & query)
        {
            ... log the given query ...
        }

    private:
        virtual logger_impl* do_clone() const
        {
            return new my_log_impl(...);
        }
    };

Then simply pass a new, heap-allocated instance of this class to the `session` object:

    soci::session sql(...);
    sql.set_logger(new my_log_impl(...));

and `start_query()` method of the logger will be called for all queries.

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