为何以上代码在vs2020中可以编译通过，可运行，在gcc7.3.0下编译报错？

Question

为何以上代码在vs2020中可以编译通过，可运行，在gcc7.3.0下编译报错？

template<class T, class...ReqContextT>
class AsyncRequestSchedulerT2 final {
public:
    void start(std::unique_ptr<MyClass1> c, ReqContextT&& ...reqContext)
    {
        using AsyncRequestScheduler = AsyncRequestSchedulerT2<T, ReqContextT...>;
        if (c != nullptr) {
            std::vector<std::thread> threads(c->count());
            for (std::size_t pos = 1; pos < c->count(); pos++) {
                threads[pos - 1] = std::thread(&AsyncRequestScheduler::processRequest, *this, std::forward<ReqContextT>(reqContext)...);
            }
            for (std::size_t pos = 0; pos < threads.size(); pos++) {
                threads[pos].join();
            }

        }
        processRequest(std::forward<ReqContextT>(reqContext)...);
    }

    void processRequest(ReqContextT&& ...reqContext)
    {
        std::cout << "processRequest" << std::endl;
    }
};

gcc编译错误

                 from AsyncRequestSchedulerTest2.cpp:12:
/usr/include/c++/7.3.0/thread: In instantiation of ‘struct std::thread::_Invoker<std::tuple<void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext> >’:
/usr/include/c++/7.3.0/thread:127:22:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&); _Args = {MyAsyncRequestSchedulerT<int, AsyncContext&>&, AsyncContext&}]’
test/dt_tests/AsyncRequestSchedulerTest2.cpp:102:41:   required from ‘void MyAsyncRequestSchedulerT<T, ReqContextT>::start(std::unique_ptr<SchedulePolicy>, ReqContextT&& ...) [with T = int; ReqContextT = {AsyncContext&}]’
test/dt_tests/AsyncRequestSchedulerTest2.cpp:120:61:   required from here
/usr/include/c++/7.3.0/thread:240:2: error: no matching function for call to ‘std::thread::_Invoker<std::tuple<void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext> >::_M_invoke(std::thread::_Invoker<std::tuple<void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext> >::_Indices)’
  operator()()
  ^~~~~~~~
/usr/include/c++/7.3.0/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 (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext>]
    _M_invoke(_Index_tuple<_Ind...>)
    ^~~~~~~~~
/usr/include/c++/7.3.0/thread:231:4: note:   template argument deduction/substitution failed:
/usr/include/c++/7.3.0/thread: In substitution of ‘template<long unsigned int ..._Ind> decltype (std::__invoke(_S_declval<_Ind>()...)) std::thread::_Invoker<std::tuple<void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext> >::_M_invoke<_Ind ...>(std::_Index_tuple<_Ind1 ...>) [with long unsigned int ..._Ind = {0, 1, 2}]’:
/usr/include/c++/7.3.0/thread:240:2:   required from ‘struct std::thread::_Invoker<std::tuple<void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext> >’
/usr/include/c++/7.3.0/thread:127:22:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&); _Args = {MyAsyncRequestSchedulerT<int, AsyncContext&>&, AsyncContext&}]’
test/dt_tests/AsyncRequestSchedulerTest2.cpp:102:41:   required from ‘void MyAsyncRequestSchedulerT<T, ReqContextT>::start(std::unique_ptr<SchedulePolicy>, ReqContextT&& ...) [with T = int; ReqContextT = {AsyncContext&}]’
test/dt_tests/AsyncRequestSchedulerTest2.cpp:120:61:   required from here
/usr/include/c++/7.3.0/thread:233:29: error: no matching function for call to ‘__invoke(std::__tuple_element_t<0, std::tuple<void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext> >, std::__tuple_element_t<1, std::tuple<void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext> >, std::__tuple_element_t<2, std::tuple<void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext> >)’
    -> decltype(std::__invoke(_S_declval<_Ind>()...))
                ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/7.3.0/tuple:41:0,
                 from /usr/include/c++/7.3.0/bits/unique_ptr.h:37,
                 from /usr/include/c++/7.3.0/memory:80,
                 from /googletest/googletest/include/gtest/gtest.h:54,
                 from test/dt_tests/AsyncRequestSchedulerTest2.cpp:5:
/usr/include/c++/7.3.0/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)
     ^~~~~~~~
/usr/include/c++/7.3.0/bits/invoke.h:89:5: note:   template argument deduction/substitution failed:
/usr/include/c++/7.3.0/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 (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&); _Args = {MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext}]’:
/usr/include/c++/7.3.0/thread:233:29:   required by substitution of ‘template<long unsigned int ..._Ind> decltype (std::__invoke(_S_declval<_Ind>()...)) std::thread::_Invoker<std::tuple<void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext> >::_M_invoke<_Ind ...>(std::_Index_tuple<_Ind1 ...>) [with long unsigned int ..._Ind = {0, 1, 2}]’
/usr/include/c++/7.3.0/thread:240:2:   required from ‘struct std::thread::_Invoker<std::tuple<void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext> >’
/usr/include/c++/7.3.0/thread:127:22:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&); _Args = {MyAsyncRequestSchedulerT<int, AsyncContext&>&, AsyncContext&}]’
test/dt_tests/AsyncRequestSchedulerTest2.cpp:102:41:   required from ‘void MyAsyncRequestSchedulerT<T, ReqContextT>::start(std::unique_ptr<SchedulePolicy>, ReqContextT&& ...) [with T = int; ReqContextT = {AsyncContext&}]’
test/dt_tests/AsyncRequestSchedulerTest2.cpp:120:61:   required from here
/usr/include/c++/7.3.0/bits/invoke.h:89:5: error: no type named ‘type’ in ‘struct std::__invoke_result<void (MyAsyncRequestSchedulerT<int, AsyncContext&>::*)(AsyncContext&), MyAsyncRequestSchedulerT<int, AsyncContext&>, AsyncContext>’
make[2]: *** [test/dt_tests/CMakeFiles/AsyncRequestSchedulerTest2.dir/build.make:63: test/dt_tests/CMakeFiles/AsyncRequestSchedulerTest2.dir/AsyncRequestSchedulerTest2.cpp.o] Error 1
make[2]: Leaving directory 'build'
make[1]: *** [CMakeFiles/Makefile2:4765: test/dt_tests/CMakeFiles/AsyncRequestSchedulerTest2.dir/all] Error 2
make[1]: Leaving directory 'build'
make: *** [Makefile:84: all] Error 2

Answer 1

该回答引用自GPT-3.5, 由博主 GISer Liu 编写：

问题分析

用户遇到的编译错误主要是因为在GCC 7.3.0下编译时，std::thread无法正确处理成员函数指针和对象的组合。具体来说，std::thread在处理成员函数指针时需要一个指向对象的指针或引用，而在当前代码中，std::thread的构造函数无法正确推导出类型。

解决方案

为了解决这个问题，我们需要确保std::thread能够正确地处理成员函数指针和对象的组合。可以通过以下几种方法来解决：

1. 使用std::bind或lambda表达式：将成员函数和对象绑定在一起，确保std::thread能够正确处理。
2. **使用std::ref**：将对象引用传递给std::thread，确保引用传递正确。

具体修改方法

方法一：使用std::bind

#include <iostream>
#include <thread>
#include <vector>
#include <memory>
#include <functional> // 添加此头文件以使用std::bind

template<class T, class... ReqContextT>
class AsyncRequestSchedulerT2 final {
public:
    void start(std::unique_ptr<MyClass1> c, ReqContextT&& ...reqContext)
    {
        using AsyncRequestScheduler = AsyncRequestSchedulerT2<T, ReqContextT...>;
        if (c != nullptr) {
            std::vector<std::thread> threads(c->count());
            for (std::size_t pos = 1; pos < c->count(); pos++) {
                threads[pos - 1] = std::thread(std::bind(&AsyncRequestScheduler::processRequest, this, std::forward<ReqContextT>(reqContext)...));
            }
            for (std::size_t pos = 0; pos < threads.size(); pos++) {
                threads[pos].join();
            }
        }
        processRequest(std::forward<ReqContextT>(reqContext)...);
    }

    void processRequest(ReqContextT&& ...reqContext)
    {
        std::cout << "processRequest" << std::endl;
    }
};

方法二：使用lambda表达式

#include <iostream>
#include <thread>
#include <vector>
#include <memory>

template<class T, class... ReqContextT>
class AsyncRequestSchedulerT2 final {
public:
    void start(std::unique_ptr<MyClass1> c, ReqContextT&& ...reqContext)
    {
        using AsyncRequestScheduler = AsyncRequestSchedulerT2<T, ReqContextT...>;
        if (c != nullptr) {
            std::vector<std::thread> threads(c->count());
            for (std::size_t pos = 1; pos < c->count(); pos++) {
                threads[pos - 1] = std::thread([this, &reqContext...]() {
                    this->processRequest(std::forward<ReqContextT>(reqContext)...);
                });
            }
            for (std::size_t pos = 0; pos < threads.size(); pos++) {
                threads[pos].join();
            }
        }
        processRequest(std::forward<ReqContextT>(reqContext)...);
    }

    void processRequest(ReqContextT&& ...reqContext)
    {
        std::cout << "processRequest" << std::endl;
    }
};

解释

1. **使用std::bind**：

   • std::bind将成员函数指针和对象绑定在一起，形成一个可调用对象，这样std::thread可以直接使用这个可调用对象。
   • 需要包含<functional>头文件。

2. 使用lambda表达式：

   • lambda表达式捕获this指针和参数，形成一个可调用对象，这样std::thread可以直接使用这个可调用对象。
   • 代码更加简洁和直观。

总结

通过使用std::bind或lambda表达式，可以确保std::thread能够正确处理成员函数指针和对象的组合，从而解决在GCC 7.3.0下的编译错误问题。这两种方法都可以有效地解决这个问题，选择其中一种即可。

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