如何正确扩展 std::thread 来包装生成的线程

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

本文关键字：线程 包装 thread 何正确 扩展 std      更新时间：2023-10-16

我试图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。 我认为这将是微不足道的，但是 std：：thread 构造函数和参数传递相当复杂，我在这里遇到了非常神秘的构建错误。

这是我正在尝试执行的操作的简单示例：

#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 );
}

我希望能够在使用 std：thread 的任何地方放入 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一连串错误(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

这是我认为山姆正在开车的一个例子。 如您所见，它只有几行代码，所以我就不说了。

#include <iostream>
#include <thread>
#include <functional>
void preamble (void) { std::cout << "preamblen"; }
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

现场演示

今天早上醒来时我就想通了:) 由于 std：：thread 衰减了它传递给函数的所有参数，因此当它调用我的包装方法时，参数是衰减的类型。 但是，当我传递模板参数进行包装时<>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>...>,
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