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#### Custom Memory Allocation

Many asynchronous operations need to allocate an object to store state associated with the operation. For example, a Win32 implementation needs `OVERLAPPED`-derived objects to pass to Win32 API functions.

Furthermore, programs typically contain easily identifiable chains of asynchronous operations. A half duplex protocol implementation (e.g. an HTTP server) would have a single chain of operations per client (receives followed by sends). A full duplex protocol implementation would have two chains executing in parallel. Programs should be able to leverage this knowledge to reuse memory for all asynchronous operations in a chain.

Given a copy of a user-defined `Handler` object `h`, if the implementation needs to allocate memory associated with that handler it will obtain an allocator using the `get_associated_allocator` function. For example:

    boost::asio::associated_allocator_t<Handler> a = boost::asio::get_associated_allocator(h);

The associated allocator must satisfy the standard Allocator requirements.

By default, handlers use the standard allocator (which is implemented in terms of `::operator new()` and `::operator delete()`). The allocator may be customised for a particular handler type by specifying a nested type `allocator_type` and member function `get_allocator()`:

    class my_handler
    {
    public:
      // Custom implementation of Allocator type requirements.
      typedef my_allocator allocator_type;

      // Return a custom allocator implementation.
      allocator_type get_allocator() const noexcept
      {
        return my_allocator();
      }

      void operator()() { ... }
    };

In more complex cases, the `associated_allocator` template may be partially specialised directly:

    namespace boost { namespace asio {

      template <typename Allocator>
      struct associated_allocator<my_handler, Allocator>
      {
        // Custom implementation of Allocator type requirements.
        typedef my_allocator type;

        // Return a custom allocator implementation.
        static type get(const my_handler&,
            const Allocator& a = Allocator()) noexcept
        {
          return my_allocator();
        }
      };

    } } // namespace boost::asio

The implementation guarantees that the deallocation will occur before the associated handler is invoked, which means the memory is ready to be reused for any new asynchronous operations started by the handler.

The custom memory allocation functions may be called from any user-created thread that is calling a library function. The implementation guarantees that, for the asynchronous operations included the library, the implementation will not make concurrent calls to the memory allocation functions for that handler. The implementation will insert appropriate memory barriers to ensure correct memory visibility should allocation functions need to be called from different threads.

###### See Also

associated_allocator, get_associated_allocator, custom memory allocation example (C++03), custom memory allocation example (C++11).