如何正确扩展 std::thread 以包装生成的线程

Question

我正在尝试创建一个简单的包装器，供我的应用程序在 std::thread 中创建线程时使用。使用此应用程序线程包装器的唯一目的是确保在每个生成的线程上一致地调用某些代码。我认为这很简单，但是 std::thread 构造函数和参数传递相当复杂，我在这里遇到非常神秘的构建错误。

这是我正在尝试做的一个简单示例：

#include <thread>

class AppThread : public std::thread
{
    template< class Function, class... Args > 
    static void wrap( Function&& f, Args&&... args )
    {
        //Some code
        f( std::forward<Args>( args )... );
    }

public:
    template< class Function, class... Args > 
    explicit AppThread( Function&& f, Args&&... args ) : std::thread( AppThread::wrap<Function,Args...>,
                                                                      std::forward<Function>( f ), std::forward<Args>( args )... )
    {}
};

void runA() {}
void runB( int x ) {}

main()
{
    AppThread thread1 = AppThread( runA );
    //AppThread thread2 = AppThread( runB, 5 );
}

我希望能够在任何使用 std:thread 的地方加入 AppThread，因此扩展和覆盖构造函数似乎是最好的方法。但是将这些参数传递给我的包装方法会导致这一系列错误（gcc 7.2）

   In file included from thread_wrap.cpp:1:0:
gcc-7.2.0/include/c++/7.2.0/thread: In instantiation of ‘struct std::thread::_Invoker<std::tuple<void (*)(void (&)()), void (*)()> >’:
gcc-7.2.0/include/c++/7.2.0/thread:127:22:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(void (&)()); _Args = {void (&)()}]’
thread_wrap.cpp:15:130:   required from ‘AppThread::AppThread(Function&&, Args&& ...) [with Function = void (&)(); Args = {}]’
thread_wrap.cpp:24:41:   required from here
gcc-7.2.0/include/c++/7.2.0/thread:240:2: error: no matching function for call to ‘std::thread::_Invoker<std::tuple<void (*)(void (&)()), void (*)()> >::_M_invoke(std::thread::_Invoker<std::tuple<void (*)(void (&)()), void (*)()> >::_Indices)’
  operator()()
  ^~~~~~~~
gcc-7.2.0/include/c++/7.2.0/thread:231:4: note: candidate: template<long unsigned int ..._Ind> decltype (std::__invoke((_S_declval<_Ind>)()...)) std::thread::_Invoker<_Tuple>::_M_invoke(std::_Index_tuple<_Ind ...>) [with long unsigned int ..._Ind = {_Ind ...}; _Tuple = std::tuple<void (*)(void (&)()), void (*)()>]
    _M_invoke(_Index_tuple<_Ind...>)
    ^~~~~~~~~
gcc-7.2.0/include/c++/7.2.0/thread:231:4: note:   template argument deduction/substitution failed:
gcc-7.2.0/include/c++/7.2.0/thread: In substitution of ‘template<long unsigned int ..._Ind> decltype (std::__invoke(_S_declval<_Ind>()...)) std::thread::_Invoker<std::tuple<void (*)(void (&)()), void (*)()> >::_M_invoke<_Ind ...>(std::_Index_tuple<_Ind1 ...>) [with long unsigned int ..._Ind = {0, 1}]’:
gcc-7.2.0/include/c++/7.2.0/thread:240:2:   required from ‘struct std::thread::_Invoker<std::tuple<void (*)(void (&)()), void (*)()> >’
gcc-7.2.0/include/c++/7.2.0/thread:127:22:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(void (&)()); _Args = {void (&)()}]’
thread_wrap.cpp:15:130:   required from ‘AppThread::AppThread(Function&&, Args&& ...) [with Function = void (&)(); Args = {}]’
thread_wrap.cpp:24:41:   required from here
gcc-7.2.0/include/c++/7.2.0/thread:233:29: error: no matching function for call to ‘__invoke(std::__tuple_element_t<0, std::tuple<void (*)(void (&)()), void (*)()> >, std::__tuple_element_t<1, std::tuple<void (*)(void (&)()), void (*)()> >)’
    -> decltype(std::__invoke(_S_declval<_Ind>()...))
                ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
In file included from gcc-7.2.0/include/c++/7.2.0/tuple:41:0,
                 from gcc-7.2.0/include/c++/7.2.0/bits/unique_ptr.h:37,
                 from gcc-7.2.0/include/c++/7.2.0/memory:80,
                 from gcc-7.2.0/include/c++/7.2.0/thread:39,
                 from thread_wrap.cpp:1:
gcc-7.2.0/include/c++/7.2.0/bits/invoke.h:89:5: note: candidate: template<class _Callable, class ... _Args> constexpr typename std::__invoke_result<_Functor, _ArgTypes>::type std::__invoke(_Callable&&, _Args&& ...)
     __invoke(_Callable&& __fn, _Args&&... __args)
     ^~~~~~~~
gcc-7.2.0/include/c++/7.2.0/bits/invoke.h:89:5: note:   template argument deduction/substitution failed:
gcc-7.2.0/include/c++/7.2.0/bits/invoke.h: In substitution of ‘template<class _Callable, class ... _Args> constexpr typename std::__invoke_result<_Functor, _ArgTypes>::type std::__invoke(_Callable&&, _Args&& ...) [with _Callable = void (*)(void (&)()); _Args = {void (*)()}]’:
gcc-7.2.0/include/c++/7.2.0/thread:233:29:   required by substitution of ‘template<long unsigned int ..._Ind> decltype (std::__invoke(_S_declval<_Ind>()...)) std::thread::_Invoker<std::tuple<void (*)(void (&)()), void (*)()> >::_M_invoke<_Ind ...>(std::_Index_tuple<_Ind1 ...>) [with long unsigned int ..._Ind = {0, 1}]’
gcc-7.2.0/include/c++/7.2.0/thread:240:2:   required from ‘struct std::thread::_Invoker<std::tuple<void (*)(void (&)()), void (*)()> >’
gcc-7.2.0/include/c++/7.2.0/thread:127:22:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(void (&)()); _Args = {void (&)()}]’
thread_wrap.cpp:15:130:   required from ‘AppThread::AppThread(Function&&, Args&& ...) [with Function = void (&)(); Args = {}]’
thread_wrap.cpp:24:41:   required from here
gcc-7.2.0/include/c++/7.2.0/bits/invoke.h:89:5: error: no type named ‘type’ in ‘struct std::__invoke_result<void (*)(void (&)()), void (*)()>’

绝对可以在这里使用一些见解！谢谢

Answer 1

这是我认为 Sam 的目标示例。如您所见，它只有几行代码，所以我就此打住。

#include <iostream>
#include <thread>
#include <functional>

void preamble (void) { std::cout << "preamble\n"; }

template <class F, class ... Args> std::thread ThreadWrapper (F f, Args&& ... args)
{
    return std::thread ([f, args...] () { preamble (); f (std::forward <Args...> (args...)); });
};

int main()
{
    std::thread t = ThreadWrapper ([] (std::string s) { std::cout << s << "\n"; }, "42");
    t.join ();
}

输出：

preamble
42

Live demo

Answer 2

我今天早上醒来的时候就明白了:) 由于 std::thread 衰减它传递给函数的所有参数，因此当它调用我的 wrap 方法时，参数是衰减的类型。但是，当我将模板参数传递给 wrap<> 时，它会得到未衰减的类型：

std::thread( AppThread::wrap<Function,Args...>,

解决方案很简单，我需要在实例化模板包装方法之前衰减类型：

std::thread( AppThread::wrap<std::decay_t<Function>,std::decay_t<Args>...>,

如何正确扩展 std::thread 以包装生成的线程

How to properly extend std::thread to wrap spawned thread

c++

c++11

stdthread