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### Handlers

A handler must meet the requirements of `MoveConstructible` types (C++Std [moveconstructible]).

In the table below, `X` denotes a handler class, `h` denotes a value of `X`, `p` denotes a pointer to a block of allocated memory of type `void*`, `s` denotes the size for a block of allocated memory, and `f` denotes a function object taking no arguments.

Table 18. Handler requirements

expression | return type | assertion/note pre/post-conditions
--- | --- | ---

    using boost::asio::asio_handler_allocate;
    asio_handler_allocate(s, &h);

`void*`

Returns a pointer to a block of memory of size `s`. The pointer must satisfy the same alignment requirements as a pointer returned by `::operator new()`. Throws `bad_alloc` on failure.
The `asio_handler_allocate()` function is located using argument-dependent lookup. The function `boost::asio::asio_handler_allocate()` serves as a default if no user-supplied function is available.

    using boost::asio::asio_handler_deallocate;
    asio_handler_deallocate(p, s, &h);

Frees a block of memory associated with a pointer `p`, of at least size `s`, that was previously allocated using `asio_handler_allocate()`.
The `asio_handler_deallocate()` function is located using argument-dependent lookup. The function `boost::asio::asio_handler_deallocate()` serves as a default if no user-supplied function is available.

    using boost::asio::asio_handler_invoke;
    asio_handler_invoke(f, &h);

Causes the function object `f` to be executed as if by calling `f()`.
The `asio_handler_invoke()` function is located using argument-dependent lookup. The function `boost::asio::asio_handler_invoke()` serves as a default if no user-supplied function is available.