### Handlers

A handler must meet the requirements of `CopyConstructible` types (C++ Std, 20.1.3).

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 10. Handler requirements

expression | return type | assertion/note pre/post-conditions
--- | --- | ---
    using namespace asio;
    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 `asio::asio_handler_allocate()` serves as a default if no user-supplied function is available.
    using namespace asio;
    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 `asio::asio_handler_deallocate()` serves as a default if no user-supplied function is available.
    using namespace asio;
    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 `asio::asio_handler_invoke()` serves as a default if no user-supplied function is available.