c++ - 为什么 "non-standard syntax; use ' &' to create a pointer to member"在 CTOR 中使用线程？

Question

代码如下:

#include <iostream>
#include <thread>

struct PickRandomFile {
    PickRandomFile() {
        std::thread t1(taskScanPaths);
    }

    inline void taskScanPaths() {
        // my task
    }
};

int main() {
    PickRandomFile pickRandomFile;

    return 0;
}

msvc 说 PickRandomFile::taskScanPaths':非标准语法；使用“&”创建指向成员 ThreadTrigger 的指针

怎么了？我通常在 gcc 中执行此操作。

Answer 1

自由函数“退化为指针”(类似于数组)所以

std::thread t1(taskScanPaths);

如果 taskScanPaths 是一个免费函数就好了，它会和

有同样的效果

std::thread t1(&taskScanPaths);

但是，对于类成员函数，您需要地址来获取指向成员函数的指针(并且您需要指定类)，如

std::thread t1(&PickRandomFile::taskScanPaths,this);

另请注意，您需要将对象/指针传递给实例，以便线程可以实际调用该方法。

来自cppreference的一些相关引述:

Pointer to member functions

A pointer to non-static member function f which is a member of class C can be initialized with the expression &C::f exactly. Expressions such as &(C::f) or &f inside C's member function do not form pointers to member functions.

它没有明确提到 f，但是因为 &f 没有形成指向成员函数的指针，所以可以安全地假设 f也不形成指向成员函数的指针。

另一方面:

Pointers to functions

A pointer to function can be initialized with an address of a non-member function or a static member function. Because of the function-to-pointer implicit conversion, the address-of operator is optional: