|  | 
Provides support for implementing stackless coroutines.
class coroutine
| Name | Description | 
|---|---|
| Constructs a coroutine in its initial state. | |
| Returns true if the coroutine is the child of a fork. | |
| Returns true if the coroutine has reached its terminal state. | |
| Returns true if the coroutine is the parent of a fork. | 
        The coroutine class may be
        used to implement stackless coroutines. The class itself is used to store
        the current state of the coroutine.
      
Coroutines are copy-constructible and assignable, and the space overhead is a single int. They can be used as a base class:
class session : coroutine { ... };
or as a data member:
class session { ... coroutine coro_; };
        or even bound in as a function argument using lambdas or bind(). The important thing is that as the application
        maintains a copy of the object for as long as the coroutine must be kept
        alive.
      
A coroutine is used in conjunction with certain "pseudo-keywords", which are implemented as macros. These macros are defined by a header file:
#include <asio/yield.hpp>
and may conversely be undefined as follows:
#include <asio/unyield.hpp>
reenter
        The reenter macro is used
        to define the body of a coroutine. It takes a single argument: a pointer
        or reference to a coroutine object. For example, if the base class is a coroutine
        object you may write:
      
reenter (this) { ... coroutine body ... }
and if a data member or other variable you can write:
reenter (coro_) { ... coroutine body ... }
        When reenter is executed
        at runtime, control jumps to the location of the last yield
        or fork.
      
The coroutine body may also be a single statement, such as:
reenter (this) for (;;) { ... }
        Limitation: The reenter
        macro is implemented using a switch. This means that you must take care when
        using local variables within the coroutine body. The local variable is not
        allowed in a position where reentering the coroutine could bypass the variable
        definition.
      
yield statement
        This form of the yield keyword
        is often used with asynchronous operations:
      
yield socket_->async_read_some(buffer(*buffer_), *this);
This divides into four logical steps:
yield saves the current
            state of the coroutine.
          
        When the asynchronous operation completes, the function object is invoked
        and reenter causes control
        to transfer to the resume point. It is important to remember to carry the
        coroutine state forward with the asynchronous operation. In the above snippet,
        the current class is a function object object with a coroutine object as
        base class or data member.
      
The statement may also be a compound statement, and this permits us to define local variables with limited scope:
yield { mutable_buffers_1 b = buffer(*buffer_); socket_->async_read_some(b, *this); }
yield return expression ;
        This form of yield is often
        used in generators or coroutine-based parsers. For example, the function
        object:
      
struct interleave : coroutine { istream& is1; istream& is2; char operator()(char c) { reenter (this) for (;;) { yield return is1.get(); yield return is2.get(); } } };
defines a trivial coroutine that interleaves the characters from two input streams.
        This type of yield divides
        into three logical steps:
      
yield saves the current
            state of the coroutine.
          yield ;
        This form of yield is equivalent
        to the following steps:
      
yield saves the current
            state of the coroutine.
          This form might be applied when coroutines are used for cooperative threading and scheduling is explicitly managed. For example:
struct task : coroutine { ... void operator()() { reenter (this) { while (... not finished ...) { ... do something ... yield; ... do some more ... yield; } } } ... }; ... task t1, t2; for (;;) { t1(); t2(); }
yield break ;
        The final form of yield is
        used to explicitly terminate the coroutine. This form is comprised of two
        steps:
      
yield sets the coroutine
            state to indicate termination.
          
        Once terminated, calls to is_complete() return true and the coroutine cannot be
        reentered.
      
Note that a coroutine may also be implicitly terminated if the coroutine body is exited without a yield, e.g. by return, throw or by running to the end of the body.
fork statement
        The fork pseudo-keyword is
        used when "forking" a coroutine, i.e. splitting it into two (or
        more) copies. One use of fork
        is in a server, where a new coroutine is created to handle each client connection:
      
reenter (this) { do { socket_.reset(new tcp::socket(io_service_)); yield acceptor->async_accept(*socket_, *this); fork server(*this)(); } while (is_parent()); ... client-specific handling follows ... }
        The logical steps involved in a fork
        are:
      
fork saves the current
            state of the coroutine.
          
        The functions is_parent()
        and is_child()
        can be used to differentiate between parent and child. You would use these
        functions to alter subsequent control flow.
      
        Note that fork doesn't do
        the actual forking by itself. It is the application's responsibility to create
        a clone of the coroutine and call it. The clone can be called immediately,
        as above, or scheduled for delayed execution using something like io_service::post().
      
If preferred, an application can use macro names that follow a more typical naming convention, rather than the pseudo-keywords. These are:
ASIO_CORO_REENTER instead
            of reenter
          ASIO_CORO_YIELD instead
            of yield
          ASIO_CORO_FORK instead
            of fork
          
        Header: asio/coroutine.hpp
      
        Convenience header: asio.hpp