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# Handlers

All asynchronous operations take a function object that will be called when the operation completes. The type of the function object is a template parameter, like so:

    template <typename Handler>
    void async_wait(Handler handler);

An alternative approach might have been to make the callback parameter a boost::function object:

    void async_wait(boost::function<void(error&)> f);

This approach was not used because of the performance penalty incurred by using boost::function objects, as each function object's implementation contains a dynamically allocated object.

Some asynchronous operations must already allocate an object with a lifetime of the duration of the operation. For example, the Win32 backend creates objects that are derived from the OVERLAPPED structure. These dynamically allocated objects can contain a copy of the handler object, and if a boost::function was used it would entail two memory allocations instead of one.

The chosen design still allows the user to pass boost::function objects as their callback handler. It also permits a backend implementation that is implemented using boost::function objects, but doesn't require it.

| Copyright © 2003 - 2006 Christopher M. Kohlhoff