Store a completion handler in the container.

template<
    class Handler,
    class Allocator>
void
emplace(
    Handler&& handler,
    Allocator const& alloc,
    net::cancellation_type cancel_type = net::cancellation_type::terminal);
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template<
    class Handler>
void
emplace(
    Handler&& handler,
    net::cancellation_type cancel_type = net::cancellation_type::terminal);
  » more...