std::thread::_Invoker error while programming withthreads

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EDIT: I'm trying to learn how to use threads in c++. I've a problem with my code, it gives me the following error: 

no matching function for call to 'std::thread::_Invoker<std::tuple<void (matrix_product<int, 0, 0>::*)(matrix_wrap<int>&, int, const matrix_wrap<int>&, const matrix_wrap<int>&), matrix_wrap<int>, int, matrix_wrap<int>, matrix_wrap<int> > >::_M_invoke(std::thread::_Invoker<std::tuple<void (matrix_product<int, 0, 0>::*)(matrix_wrap<int>&, int, const matrix_wrap<int>&, const matrix_wrap<int>&), matrix_wrap<int>, int, matrix_wrap<int>, matrix_wrap<int> > >::_Indices)'
  operator()()

This is the piece of code that gives me the error (it was working before I wrote this):

void do_multiply_fico(matrix_wrap<T> result, matrix_wrap<T> lhs, matrix_wrap<T> rhs) {
        // Create an array of threads
        std::thread threads[lhs.get_height()];
        for (int i = 0; i < lhs.get_height(); ++i) {
            // Initialize each thread with the function responsible of multiplying only a part of the matrices
            threads[i] = std::thread(multiply_threading, result, i, lhs, rhs);
        }
        for (int i = 0; i < lhs.get_height(); ++i) {
            // Wait until each thead has finished
            threads[i].join();
        }
    }

    void multiply_threading(matrix_wrap<T>& result, const int thread_number, const matrix_wrap<T>& lhs, const matrix_wrap<T>& rhs){
        const unsigned height = result.get_height();
        const unsigned width = result.get_width();
        const unsigned span = lhs.get_width();
        assert(span==rhs.get_height());
        for (unsigned i=0; i!=height; ++i) {
            for (unsigned j = 0; j != width; ++j) {
                for (unsigned k = 0; k != span; ++k) {
                    result(i, j) += lhs(i, k) * rhs(k, j);
                }
            }
        }

    }

thanks in advance for the help.

multithreading c++
1 Answers

It looks like you're trying to construct std::thread from a member function. That's not going to work: you need an instance to invoke it.

You also have a more serious problem, in that reference arguments will, by default, be passed to the thread constructor as value types. You'd need to wrap them in std::ref to get it to compile and exhibit the expected behavior.

The easier way around all that, though, would just be to pass a lambda to std::thread:

threads[i] = std::thread([this, &result, i, &lhs, &rhs](){
   multiply_threading(result, i, lhs, rhs);
});

That way the wrapping of arguments is done through lambda capture, rather than through the vagaries of std::thread. (Reference capture can under certain circumstances be unsafe, but since you're joining everything in the same function you don't need to worry about that.) Note that i is captured by value, since you're going to change its value in later iterations and need the threads you already created to capture the value it had when you created them.

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