...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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 <boost/asio/yield.hpp>
and may conversely be undefined as follows:
#include <boost/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_context_)); 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_context::post()
.
If preferred, an application can use macro names that follow a more typical naming convention, rather than the pseudo-keywords. These are:
BOOST_ASIO_CORO_REENTER
instead of reenter
BOOST_ASIO_CORO_YIELD
instead of yield
BOOST_ASIO_CORO_FORK
instead of fork
Header: boost/asio/coroutine.hpp
Convenience header: boost/asio.hpp