Function pointer in a std::thread Args… list

Question

I am trying to give a function pointer to my std::thread arguments list but I get a bunch of compile errors that I don't understand (@ MSVC\14.28.29333\include\thread(43,14): error C2672: 'invoke': fonction correspondante surchargée introuvable [overloaded function not found] ).

I could write a mcve that gives that same error.

#include <thread>
#include <vector>

template<typename T>
void worker(std::vector<T>& data_set, void(*do_something)(T&)) {
    for (T& t : data_set)
        (*do_something)(t);
}

template<typename T>
std::vector<T> get_data(void(*do_something)(T&), size_t sz) {

    //only 1 thread as example
    std::vector<T> data_set(sz);
    std::thread t1(worker<T>, data_set, do_something); //compile error
    t1.join();


    worker<T>(data_set, do_something); //this on the other hand does compile

    return data_set;
}

void do_something_int(int& i) {
    i = 1;
}

void do_something_float(float& f) {
    f = 2.1f;
}

void do_something_char(char& c) {
    c = 'a';
}

int main(int argc, char** argv) {

    auto data_set_int = get_data(&do_something_int, 100);
    auto data_set_float = get_data(&do_something_float, 100);
    auto data_set_char = get_data(&do_something_char, 100);

    return 0;
}

Funny thing is if I call the worker in a non-threaded way everything is fine. I don't know what the compiler is expecting.

Answer 1

The problem is that your function accepts the argument by non-const lvalue reference. std::thread will pass an rvalue into the funtion, and the non-const lvalue reference cannot be bound to rvalues.

In order to pass an lvalue, you must use a reference wrapper:

std::thread t1(worker<T>, std::ref(data_set), do_something);

Always be careful to ensure lifetime of the referred object when referring to automatic objects in separate threads.