我正在尝试调整Martinho Fernandes's sample std::future implementation以便它可以在C ++ 03下使用Boost 1.40运行，作为一种便宜的，间隙测量，直到我可以访问Boost 1.41或C ++ 11本身。
我的适应性并不美观，而且肯定不是最佳的，但我更希望它至少能起作用。但是，在 gcc版本4.4.1 20090725（Red Hat 4.4.1-2）下，它没有。
这是futures.h：
#include <boost/thread.hpp>
#include <boost/shared_ptr.hpp>
#include <stdexcept>
template <typename T>
class future;
template <typename T>
class promise;
namespace detail {
template <typename T>
struct future_shared_state
{
public:
void wait(boost::mutex& m) const
{
boost::mutex::scoped_lock lock(m, boost::adopt_lock);
while (!(state || error))
available.wait(lock);
}
T& get()
{
if (state)
return *state;
if (error)
throw *error;
throw std::runtime_error("WTF");
}
template <typename U>
void set_value(const U& value)
{
state = value;
available.notify_all();
}
void set_exception(boost::shared_ptr<std::exception> e)
{
error = e;
available.notify_all();
}
private:
mutable boost::condition_variable available;
boost::optional<T> state;
boost::shared_ptr<std::exception> error;
friend class promise<T>;
friend class future<T>;
mutable boost::mutex m;
};
}
template <typename T>
struct future
{
public:
future() {}
~future() {}
T get()
{
boost::shared_ptr<detail::future_shared_state<T> > old_box;
swap(box, old_box);
boost::mutex::scoped_lock lock(old_box->m);
old_box->wait(old_box->m);
return old_box->get();
}
bool valid() const
{
return !!box;
}
void wait() const
{
boost::mutex::scoped_lock lock(box->m);
box->wait(box->m);
}
private:
boost::shared_ptr<detail::future_shared_state<T> > box;
friend class promise<T>;
future(boost::shared_ptr<detail::future_shared_state<T> > const& box) : box(box) {}
};
template <typename T>
struct promise
{
public:
promise() : box(new detail::future_shared_state<T>) {}
~promise() {}
void swap(promise& other)
{
box.swap(other.box);
}
future<T> get_future()
{
return future<T>(box);
}
void set_value(T const& value)
{
box->set_value(value);
}
void set_exception(boost::shared_ptr<std::exception> e)
{
box->set_exception(e);
}
private:
boost::shared_ptr<detail::future_shared_state<T> > box;
};
template<typename T>
void swap(promise<T>& lhs, promise<T>& rhs)
{
lhs.swap(rhs);
}
执行：
#include "futures.h"
#include <iostream>
#include <boost/thread.hpp>
void foo(promise<unsigned int> p)
{
sleep(1);
p.set_value(42);
}
int main()
{
promise<unsigned int> p;
future <unsigned int> f(p.get_future());
boost::thread t(&foo, p);
std::cout << f.get() << std::endl;
t.join();
}
// g++ -O0 -g test.cpp -lboost_thread -pthread -o test
结果始终如下：
42
test: /usr/local/include/boost/thread/pthread/mutex.hpp:45: boost::mutex::~mutex(): Assertion `!pthread_mutex_destroy(&m)' failed.
Aborted
来自gdb的Backtrace：
#0 0x00679422 in __kernel_vsyscall ()
#1 0x002ac781 in *__GI_raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#2 0x002ae04a in *__GI_abort () at abort.c:88
#3 0x002a58de in *__GI___assert_fail (assertion=0x805de56 "!pthread_mutex_destroy(&m)",
file=0x805de24 "/usr/local/include/boost/thread/pthread/mutex.hpp", line=45, function=0x805e071 "boost::mutex::~mutex()")
at assert.c:78
#4 0x0804bdd5 in boost::mutex::~mutex (this=0x806c9c0, __in_chrg=<value optimized out>)
at /usr/local/include/boost/thread/pthread/mutex.hpp:45
#5 0x0804d020 in detail::future_shared_state<unsigned int>::~future_shared_state (this=0x806c980,
__in_chrg=<value optimized out>) at futures.h:35
#6 0x0804d099 in boost::checked_delete<detail::future_shared_state<unsigned int> > (x=0x806c980)
at /usr/local/include/boost/checked_delete.hpp:34
#7 0x0804d69c in boost::detail::sp_counted_impl_p<detail::future_shared_state<unsigned int> >::dispose (this=0x806c9e0)
at /usr/local/include/boost/smart_ptr/detail/sp_counted_impl.hpp:78
#8 0x0804bb68 in boost::detail::sp_counted_base::release (this=0x806c9e0)
at /usr/local/include/boost/smart_ptr/detail/sp_counted_base_gcc_x86.hpp:145
#9 0x0804bbfe in boost::detail::shared_count::~shared_count (this=0xbffff634, __in_chrg=<value optimized out>)
at /usr/local/include/boost/smart_ptr/detail/shared_count.hpp:217
#10 0x0804c2d4 in boost::shared_ptr<detail::future_shared_state<unsigned int> >::~shared_ptr (this=0xbffff630,
__in_chrg=<value optimized out>) at /usr/local/include/boost/smart_ptr/shared_ptr.hpp:169
#11 0x0804c535 in promise<unsigned int>::~promise (this=0xbffff630, __in_chrg=<value optimized out>) at futures.h:125
#12 0x0804b937 in main () at test.cpp:19
除了糟糕的风格外，我的互斥体出了什么问题？
答案 0 :(得分：5)
future_shared_state::value和future_shared_state::error上的数据竞赛，set_value和set_exception访问它们而未获取wait用来保护它们的互斥锁。
您遇到的实际问题是由于您在boost::mutex::scoped_lock的来电者中使用future_shared_state::wait：您成功避免在adopt_lock内使用future_shared_state::wait两次锁定互斥锁，但是两个scoped_lock析构函数都会运行和解锁互斥锁两次。
通过将锁定全部锁定到future_shared_state（Demo at Coliru），可以轻松解决这两个问题：
namespace detail {
template <typename T>
struct future_shared_state
{
public:
void wait() const
{
boost::mutex::scoped_lock lock(m);
while (!(state || error))
available.wait(lock);
}
T& get()
{
if (state)
return *state;
if (error)
throw *error;
throw std::runtime_error("WTF");
}
template <typename U>
void set_value(const U& value)
{
{
boost::mutex::scoped_lock lock(m);
state = value;
}
available.notify_all();
}
void set_exception(boost::shared_ptr<std::exception> e)
{
{
boost::mutex::scoped_lock lock(m);
error = e;
}
available.notify_all();
}
private:
mutable boost::condition_variable available;
boost::optional<T> state;
boost::shared_ptr<std::exception> error;
mutable boost::mutex m;
};
}
template <typename T>
class promise;
template <typename T>
struct future
{
public:
future() {}
~future() {}
T get()
{
boost::shared_ptr<detail::future_shared_state<T> > old_box;
swap(box, old_box);
old_box->wait();
return old_box->get();
}
bool valid() const
{
return !!box;
}
void wait() const
{
box->wait();
}
private:
boost::shared_ptr<detail::future_shared_state<T> > box;
friend class promise<T>;
future(boost::shared_ptr<detail::future_shared_state<T> > const& box) : box(box) {}
};
template <typename T>
struct promise
{
public:
promise() : box(new detail::future_shared_state<T>) {}
~promise() {}
void swap(promise& other)
{
box.swap(other.box);
}
future<T> get_future()
{
return future<T>(box);
}
void set_value(T const& value)
{
box->set_value(value);
}
void set_exception(boost::shared_ptr<std::exception> e)
{
box->set_exception(e);
}
private:
boost::shared_ptr<detail::future_shared_state<T> > box;
};