如何使用默认参数将函数传递给std :: thread?

时间:2014-03-25 22:52:20

标签: c++ multithreading c++11 default-parameters

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。

我在下面编译的内容,但是当我取消注释线程“two”的两行时,我收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

有任何建议或解决方案吗?提前谢谢!

1 个答案:

答案 0 :(得分:12)

thread two (print_int);

您不能这样做,C ++中的默认参数仅用于呼叫站点(例如,当您自己调用print_int()时)作为语法快捷方式,但在例如时未使用std::thread调用作为参数传递给它的相同函数。问题是std::thread被赋予一个单参数函数的指针,但是没有传递参数来使用它,因此它不知道如何调用它,并且没有在给定指向函数的指针的情况下查找函数的默认参数的方法。

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(); } );