我正在为 std::thread
实现一个包装器这将允许我在线程完成执行后检索任意返回值。当我使用 C++11
时,我用的是旧版 ARM
不支持完全支持原子 int 的体系结构,这意味着我不能使用 std::future
, std::promise
, std::packaged_task
,以及许多 STL 线程功能(我至少得到了 std::thread
)。我正在测试 gcc 4.8.4
.
在执行我的实现时,我遇到了 this bug ,这让我无法使用 lambda 捕获可变参数模板参数。不幸的是,我目前无法将我的编译器升级到 4.9。
我正在尝试使用 std::bind
实现解决方法,但我遇到了很多问题。我不确定这些是编译器错误还是我的实现错误。这是来源:
#include <iostream>
#include <memory>
#include <thread>
#include <unistd.h>
#include <pthread.h>
class ConcurrentTaskBase
{
public:
ConcurrentTaskBase(int priority, const std::function<void()>& runTask)
: m_thread(),
m_active(true)
{
auto wrap = [this](int priority, const std::function<void()>& runTask)
{
//Unrelated pthread stuff that I commented out
// sched_param param{priority};
//
// int err = pthread_setschedparam(pthread_self(), SCHED_RR, ¶m);
// if (err)
// cout << "failed to set new priority: " << err << endl;
runTask();
};
m_thread = std::thread(wrap, priority, runTask);
}
virtual ~ConcurrentTaskBase(void)
{
waitForCompletion();
}
void waitForCompletion(void)
{
if (m_active)
{
m_thread.join();
m_active = false;
}
}
private:
std::thread m_thread;
bool m_active;
};
template<class R, class... ArgTypes>
class ConcurrentTask;
template<class R, class... ArgTypes>
class ConcurrentTask<R(ArgTypes...)> : public ConcurrentTaskBase
{
public:
ConcurrentTask(int priority, const std::function<R(ArgTypes...)>& task, ArgTypes&&... args)
: ConcurrentTaskBase(priority, bindTask(task, std::forward<ArgTypes>(args)...))
{}
std::shared_ptr<R> getReturn(void) noexcept
{
waitForCompletion();
return m_storage;
};
private:
static std::function<void(void)> bindTask(const std::function<R(ArgTypes...)>& task, ArgTypes&&... args)
{
auto action = [task](ArgTypes&&... args) -> void
{
//Eventually m_storage = std::make_shared<R>(task(std::forward<ArgTypes>(args)...)); after bugs are fixed
task(std::forward<ArgTypes>(args)...);
return;
};
std::function<void(void)> bound = std::bind(action, std::forward<ArgTypes>(args)...);
return bound;
};
std::shared_ptr<R> m_storage;
};
int testFunction(int val)
{
std::cout << "Was given " << val << std::endl;
return val + 10;
}
int main()
{
ConcurrentTask<int(int)> task(20, testFunction, 5);
// shared_ptr<int> received = task.getReturn();
// testFunction(*received);
return 0;
}
这是我的编译器输出:
16:31:00 **** Incremental Build of configuration Debug for project TestLinuxMint ****
make all
Building file: ../src/TestLinuxMint.cpp
Invoking: GCC C++ Compiler
g++ -std=c++0x -O0 -g3 -Wall -pthread -c -fmessage-length=0 -MMD -MP -MF"src/TestLinuxMint.d" -MT"src/TestLinuxMint.o" -o "src/TestLinuxMint.o" "../src/TestLinuxMint.cpp"
../src/TestLinuxMint.cpp: In instantiation of ‘static std::function<void()> ConcurrentTask<R(ArgTypes ...)>::bindTask(const std::function<_Res(_ArgTypes ...)>&, ArgTypes&& ...) [with R = int; ArgTypes = {int}]’:
../src/TestLinuxMint.cpp:58:84: required from ‘ConcurrentTask<R(ArgTypes ...)>::ConcurrentTask(int, const std::function<_Res(_ArgTypes ...)>&, ArgTypes&& ...) [with R = int; ArgTypes = {int}]’
../src/TestLinuxMint.cpp:91:53: required from here
../src/TestLinuxMint.cpp:76:90: error: conversion from ‘std::_Bind_helper<false, ConcurrentTask<R(ArgTypes ...)>::bindTask(const std::function<_Res(_ArgTypes ...)>&, ArgTypes&& ...) [with R = int; ArgTypes = {int}]::__lambda1&, int>::type {aka std::_Bind<ConcurrentTask<R(ArgTypes ...)>::bindTask(const std::function<_Res(_ArgTypes ...)>&, ArgTypes&& ...) [with R = int; ArgTypes = {int}]::__lambda1(int)>}’ to non-scalar type ‘std::function<void()>’ requested
std::function<void(void)> bound = std::bind(action, std::forward<ArgTypes>(args)...);
^
make: *** [src/TestLinuxMint.o] Error 1
16:31:01 Build Finished (took 319ms)
问题似乎在 line 76
上,其中从 std::bind(*) to std::function<void(void)>
转换失败.这段代码肯定还在开发中,但我需要解决这个问题才能继续前进。我在这里查看了其他多篇关于 SO 的帖子,但它们似乎都能够毫无问题地在可变参数模板参数上使用 std::bind。
解决方案
这是我想出的最终解决方案(关于这个问题),感谢 kzraq 和 this post .
来源:
#include <iostream>
#include <memory>
#include <utility>
#include <vector>
#include <thread>
#include <type_traits>
#include <typeinfo>
#include <tuple>
#include <memory>
//------------------------------------------------------------------------------------------------------------
template <std::size_t... Ints>
struct idx_sequence
{
using type = idx_sequence;
using value_type = std::size_t;
static constexpr std::size_t size() noexcept { return sizeof...(Ints); }
};
//------------------------------------------------------------------------------------------------------------
template <class Sequence1, class Sequence2>
struct _merge_and_renumber;
template <std::size_t... I1, std::size_t... I2>
struct _merge_and_renumber<idx_sequence<I1...>, idx_sequence<I2...> >
: idx_sequence<I1..., (sizeof...(I1)+I2)...>
{
};
//------------------------------------------------------------------------------------------------------------
template <std::size_t N>
struct make_idx_sequence : _merge_and_renumber<make_idx_sequence<N/2>, make_idx_sequence<N - N/2> >
{
};
template<> struct make_idx_sequence<0> : idx_sequence<> { };
template<> struct make_idx_sequence<1> : idx_sequence<0> { };
//------------------------------------------------------------------------------------------------------------
template<typename Func, typename Tuple, std::size_t... Ints>
auto applyImpl(Func&& f, Tuple&& params, idx_sequence<Ints...>)
-> decltype(f(std::get<Ints>(std::forward<Tuple>(params))...))
{
return f(std::get<Ints>(std::forward<Tuple>(params))...);
};
template<typename Func, typename Tuple>
auto apply(Func&& f, Tuple&& params)
-> decltype(applyImpl(std::forward<Func>(f),
std::forward<Tuple>(params),
make_idx_sequence<std::tuple_size<typename std::decay<Tuple>::type>::value>{}))
{
return applyImpl(std::forward<Func>(f),
std::forward<Tuple>(params),
make_idx_sequence<std::tuple_size<typename std::decay<Tuple>::type>::value>{});
};
class ConcurrentTaskBase
{
public:
ConcurrentTaskBase(int priority, const std::function<void()>& task)
: m_thread(),
m_active(true)
{
auto wrap = [this](int priority, const std::function<void()>& task)
{
//Unrelated pthread stuff that I commented out
sched_param param{priority};
int err = pthread_setschedparam(pthread_self(), SCHED_RR, ¶m);
if (err)
std::cout << "failed to set new priority: " << err << std::endl;
task();
};
m_thread = std::thread(wrap, priority, task);
}
virtual ~ConcurrentTaskBase(void)
{
waitForCompletion();
}
void waitForCompletion(void)
{
if (m_active)
{
m_thread.join();
m_active = false;
}
}
private:
std::thread m_thread;
bool m_active;
};
template<class R, class... ArgTypes>
class ConcurrentTask;
template<class R, class... ArgTypes>
class ConcurrentTask<R(ArgTypes...)> : public ConcurrentTaskBase
{
public:
ConcurrentTask(int priority, const std::function<R(ArgTypes...)>& task, ArgTypes&&... args)
: ConcurrentTaskBase(priority, bindTask(task, std::forward<ArgTypes>(args)...))
{}
std::shared_ptr<R> getReturn(void) noexcept
{
waitForCompletion();
return m_storage;
}
private:
std::function<void(void)> bindTask(const std::function<R(ArgTypes...)>& task, ArgTypes&&... args)
{
auto params = std::make_tuple(args...);
return [this, task, params](){m_storage = std::make_shared<R>(apply(task, params));};
};
std::shared_ptr<R> m_storage;
};
template<class... ArgTypes>
class ConcurrentTask<void(ArgTypes...)> : public ConcurrentTaskBase
{
public:
ConcurrentTask(int priority, const std::function<void(ArgTypes...)>& task, ArgTypes&&... args)
: ConcurrentTaskBase(priority, bindTask(task, std::forward<ArgTypes>(args)...))
{}
private:
std::function<void(void)> bindTask(const std::function<void(ArgTypes...)>& task, ArgTypes&&... args)
{
auto params = std::make_tuple(args...);
return [this, task, params](){apply(task, params);};
};
};
// Example stuff
struct MyStruct
{
int x;
int y;
};
int testFunction(MyStruct val)
{
std::cout << "X is " << val.x << " Y is " << val.y << std::endl;
return val.x + 10;
}
void printMe(int x)
{
std::cout << "Printing " << x << std::endl;
}
int main()
{
ConcurrentTask<int(MyStruct)> task(20, testFunction, {5, -21});
std::shared_ptr<int> received = task.getReturn();
std::cout << "Return value is " << *received << std::endl;
ConcurrentTask<void(int)> voidTask(25, printMe, -123);
return 0;
}
最佳答案
这或多或少是 Yakk 写的。也许我不太理解你的想法,但对我来说,你似乎过度设计了它并且你过早地使用了 std::function
。此外,ArgTypes&&
不会是转发/通用引用的列表,因为它们不是在 bindTask
中推导的。
gcc 4.8.2 上的以下 compiles successfully:
为 C++11 获取您自己的 integer_sequence
。由 Xeo 提供。
编写 apply
将元组参数应用于函数(也许这可以改进):
template<typename Func, typename Tuple, unsigned int... is>
auto apply_impl(Func&& f, Tuple&& params, seq<is...>)
// -> decltype(f(std::get<is>(std::forward<Tuple>(params))...)) // C++11 only
{
using std::get; // enable ADL-lookup for get in C++14
return f(get<is>(std::forward<Tuple>(params))...);
}
template<typename Func, typename Tuple>
auto apply(Func&& f, Tuple&& params)
// -> decltype(apply_impl(std::forward<Func>(f), std::forward<Tuple>(params),
// GenSeq<std::tuple_size<typename std::decay<Tuple>::type>::value>{}))
// C++11 only
{
return apply_impl(std::forward<Func>(f), std::forward<Tuple>(params),
GenSeq<std::tuple_size<typename std::decay<Tuple>::type>::value>{});
}
简化您的 bindTask
(尽管此时我将其作为模板保留):
auto params = make_tuple(args...);
std::function<void(void)> bound = [task,params]{ apply(task, params); };
return bound;
在 C++14 中执行 [task=std::move(task),params=std::move(params)]
以避免不必要的复制。
关于c++ - 无法将 std::bind 与可变模板参数一起使用,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/40096098/