如何使用默认参数将函数传递给std :: thread？

Question

所以我有gcc版本4.8.1,g ++版本4.6.4,使用标志:-std = c ++ 0x和-pthread.

我将问题简化为显示的代码并仍然得到原始错误.

我在下面编译,但当我取消注释线程"两"的两行时,我得到代码下面显示的错误消息

#include <iostream>
#include <thread>
using namespace std;

void print_int(int x=7);
void print_A(){
    cout << "A\n";
}

int main(){
    thread one (print_int,17);
    //thread two (print_int);
    thread three (print_A);

    one.join();
    //two.join();
    three.join();

    return 0;
}

void print_int(int x){
    cout << x << '\n';
}

我试图解析错误消息,但我仍然不知道发生了什么......

    In file included from /usr/include/c++/4.6/thread:39:0,
             from def_params.cpp:2:

    /usr/include/c++/4.6/functional: In member function ‘void std::_Bind_result<_Result, _Functor(_Bound_args ...)>::__call(std::tuple<_Args ...>&&, std::_Index_tuple<_Indexes ...>, typename std::_Bind_result<_Result, _Functor(_Bound_args ...)>::__enable_if_void<_Res>::type) [with _Res = void, _Args = {}, int ..._Indexes = {}, _Result = void, _Functor = void (*)(int), _Bound_args = {}, typename std::_Bind_result<_Result, _Functor(_Bound_args ...)>::__enable_if_void<_Res>::type = int]’:

    /usr/include/c++/4.6/functional:1378:24:   instantiated from ‘std::_Bind_result<_Result, _Functor(_Bound_args ...)>::result_type std::_Bind_result<_Result, _Functor(_Bound_args ...)>::operator()(_Args&& ...) [with _Args = {}, _Result = void, _Functor = void (*)(int), _Bound_args = {}, std::_Bind_result<_Result, _Functor(_Bound_args ...)>::result_type = void]’

    /usr/include/c++/4.6/thread:117:13:   instantiated from ‘void std::thread::_Impl<_Callable>::_M_run() [with _Callable = std::_Bind_result<void, void (*())(int)>]’

    def_params.cpp:26:9:   instantiated from here

    /usr/include/c++/4.6/functional:1287:4: error: too few arguments to function

任何建议或解决方案？提前致谢!

Answer 1

thread two (print_int);

你不能这样做,C++中的默认参数仅用于调用站点(例如,当你print_int()自己调用时)作为语法快捷方式,但是在例如std::thread调用作为参数传递给它的相同函数时不使用.问题是std::thread获得一个指向单参数函数的指针,但是没有传递参数来使用它,所以它不知道如何调用它,并且在给定指针的情况下无法查找函数的默认参数到功能.

最简单的解决方案是使用lambda来调用函数,因为该调用可以使用默认参数:

thread two( []{ print_int(); } );