[/
 / Copyright (c) 2003-2023 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:allocation 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 an Boost.Asio library function. The implementation guarantees,
for the asynchronous operations included with the library, that within the
context of an individual operation 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 an asynchronous operation need to call the allocation functions from
different threads. (Note: If the same allocator is shared across multiple
concurrent asynchronous operations, this can result in concurrent calls to the
memory allocation functions. Use of a [link boost_asio.overview.core.strands strand]
does not prevent these concurrent calls, as an operation may need to allocate
memory from outside the strand. In this case, the shared allocator is
responsible for providing the necessary thread safety guarantees.)

[heading See Also]

[link boost_asio.reference.associated_allocator associated_allocator],
[link boost_asio.reference.get_associated_allocator get_associated_allocator],
[link boost_asio.examples.cpp03_examples.allocation custom memory allocation example (C++03)],
[link boost_asio.examples.cpp11_examples.allocation custom memory allocation example (C++11)].

[endsect]
