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//
// coroutine.h
// ~~~~~~~~~~~
//
// Copyright (c) 2003-2010 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
#ifndef COROUTINE_HPP
#define COROUTINE_HPP
// \brief Coroutine object
//
// A coroutine object maintains the state of a re-enterable routine. It
// is assignable and copy-constructible, and can be used as a base class
// for a class that uses it, or as a data member. The copy overhead is
// a single int.
//
// A reentrant function contains a CORO_REENTER (coroutine) { ... }
// block. Whenever an asynchronous operation is initiated within the
// routine, the function is provided as the handler object. (The simplest
// way to do this is to have the reentrant function be the operator()
// member for the coroutine object itself.) For example:
//
// CORO_YIELD socket->async_read_some(buffer, *this);
//
// The CORO_YIELD keyword updates the current status of the coroutine to
// indicate the line number currently being executed. The
// async_read_some() call is initiated, with a copy of the updated
// coroutine as its handler object, and the current coroutine exits. When
// the async_read_some() call finishes, the copied coroutine will be
// called, and will resume processing exactly where the original one left
// off--right after asynchronous call. This allows asynchronous I/O
// routines to be written with a logical flow, step following step, rather
// than as a linked chain of separate handler functions.
//
// When necessary, a coroutine can fork itself using the CORO_FORK keyword.
// This updates the status of the coroutine and makes a copy. The copy can
// then be called directly or posted to the ASIO service queue so that both
// coroutines will continue forward, one "parent" and one "child". The
// is_parent() and is_child() methods indicate which is which.
//
// The CORO_REENTER, CORO_YIELD and CORO_FORK keywords are implemented
// via preprocessor macros. The CORO_REENTER block is actually a large,
// complex switch statement. Because of this, inline variable declaration
// is impossible within CORO_REENTER unless it is done in a subsidiary
// scope--and if it is, that scope cannot contain CORO_YIELD or CORO_FORK
// keywords.
//
// Because coroutines are frequently copied, it is best to minimize copy
// overhead by limiting the size of data members in derived classes.
//
// It should be noted that when a coroutine falls out of scope its memory
// is reclaimed, even though it may be scheduled to resume when an
// asynchronous operation completes. Any shared_ptr<> objects declared in
// the coroutine may be destroyed if their reference count drops to zero,
// in which case the coroutine will have serious problems once it resumes.
// One solution so this is to have the space that will be used by a
// coroutine pre-allocated and stored on a free list; a new coroutine can
// fetch the block of space off a free list, place a shared pointer to it
// on an "in use" list, and carry on. The reference in the "in use" list
// would prevent the data from being destroyed.
class coroutine
{
public:
coroutine() : value_(0) {}
virtual ~coroutine() {}
bool is_child() const { return value_ < 0; }
bool is_parent() const { return !is_child(); }
bool is_complete() const { return value_ == -1; }
int get_value() const { return value_; }
private:
friend class coroutine_ref;
int value_;
};
class coroutine_ref
{
public:
coroutine_ref(coroutine& c) : value_(c.value_), modified_(false) {}
coroutine_ref(coroutine* c) : value_(c->value_), modified_(false) {}
~coroutine_ref() { if (!modified_) value_ = -1; }
operator int() const { return value_; }
int& operator=(int v) { modified_ = true; return value_ = v; }
private:
void operator=(const coroutine_ref&);
int& value_;
bool modified_;
};
#define CORO_REENTER(c) \
switch (coroutine_ref _coro_value = c) \
case -1: if (_coro_value) \
{ \
goto terminate_coroutine; \
terminate_coroutine: \
_coro_value = -1; \
goto bail_out_of_coroutine; \
bail_out_of_coroutine: \
break; \
} \
else case 0:
#define CORO_YIELD \
for (_coro_value = __LINE__;;) \
if (_coro_value == 0) \
{ \
case __LINE__: ; \
break; \
} \
else \
switch (_coro_value ? 0 : 1) \
for (;;) \
case -1: if (_coro_value) \
goto terminate_coroutine; \
else for (;;) \
case 1: if (_coro_value) \
goto bail_out_of_coroutine; \
else case 0:
#define CORO_FORK \
for (_coro_value = -__LINE__;; _coro_value = __LINE__) \
if (_coro_value == __LINE__) \
{ \
case -__LINE__: ; \
break; \
} \
else
#endif // COROUTINE_HPP
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