在没有副本构造函数的对象的成员函数中启动std :: thread

Question

I want to start a std::thread at a member function of an object that does not have a copy constructor. The standard way of starting a thread at a member function (see for example Start thread with member function ) needs a copy constructor. I thought I could get around this by using std::ref (see for example std::thread pass by reference calls copy constructor ), but no luck.

Here is my code. Thread t1 works, but not t2 . If I try to uncomment the two lines, it gives:

error: attempt to use a deleted function 
__invoke(_VSTD::move(_VSTD::get<0>(__t)), _VSTD::move(_VSTD::get<_Indices>(__t))...); ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/thread:357:5: note: 
in instantiation of function template specialization
'std::__1::__thread_execute<void (Dummy::*)(), std::__1::reference_wrapper<Dummy> , 1>' 
requested here __thread_execute(*__p, _Index()); ^ 
etc.

How do I start the thread directly at print() , without needing uselessFunction ?

Also, I'd like to understand the error better. Which "deleted function" is the compiler complaining about?

#include <iostream>
#include <thread>
using std::cout;
using std::endl;

class Dummy {
public:
    std::atomic<bool> flag;  // atomic kills implicitly created copy constructor
    void print() { std::cout << "hello!" << std::endl; }
};

void uselessFunction(Dummy &d) {
    d.print();
}

int main() {
    Dummy d{};
    std::thread t1(uselessFunction, std::ref(d));  // this works
    // std::thread t2(&Dummy::print, std::ref(d)); // does not work
    t1.join();
    // t2.join();
}

Answer 1

std::thread use std::invoke to call the function. std::invoke is smart enough to be able to take a pointer to an object and call a member function with it. That means you can change t2 to

std::thread t2(&Dummy::print, &d);

and get the correct behavior.