問題描述

為什么在創建 std::thread 時不能通過引用傳遞對象?

Why can't you pass an object by reference when creating a std::thread ?

例如下面的代碼段給出了一個編譯錯誤:

For example the following snippit gives a compile error:

#include <iostream>
#include <thread>

using namespace std;

static void SimpleThread(int& a)  // compile error
//static void SimpleThread(int a)     // OK
{
    cout << __PRETTY_FUNCTION__ << ":" << a << endl;
}

int main()
{
    int a = 6;

    auto thread1 = std::thread(SimpleThread, a);

    thread1.join();
    return 0;
}

錯誤:

In file included from /usr/include/c++/4.8/thread:39:0,
                 from ./std_thread_refs.cpp:5:
/usr/include/c++/4.8/functional: In instantiation of ‘struct std::_Bind_simple<void (*(int))(int&)>’:
/usr/include/c++/4.8/thread:137:47:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(int&); _Args = {int&}]’
./std_thread_refs.cpp:19:47:   required from here
/usr/include/c++/4.8/functional:1697:61: error: no type named ‘type’ in ‘class std::result_of<void (*(int))(int&)>’
       typedef typename result_of<_Callable(_Args...)>::type result_type;
                                                             ^
/usr/include/c++/4.8/functional:1727:9: error: no type named ‘type’ in ‘class std::result_of<void (*(int))(int&)>’
         _M_invoke(_Index_tuple<_Indices...>)
         ^

我已改為傳遞指針，但有更好的解決方法嗎?

I've changed to passing a pointer, but is there a better work around?

推薦答案

使用 reference_wrapper 使用 std::ref:

Explicitly initialize the thread with a reference_wrapper by using std::ref:

auto thread1 = std::thread(SimpleThread, std::ref(a));

(或 std::cref 而不是 std::ref，視情況而定).根據 cppreference on std:thread 的注釋:

(or std::cref instead of std::ref, as appropriate). Per notes from cppreference on std:thread:

線程函數的參數按值移動或復制.如果需要將引用參數傳遞給線程函數，則必須對其進行包裝(例如使用 std::ref 或 std::cref).

The arguments to the thread function are moved or copied by value. If a reference argument needs to be passed to the thread function, it has to be wrapped (e.g. with std::ref or std::cref).

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