线程构造函数初始化 C++

Question

I have been attempting to write a simple program to experiment with vectors of threads. I am trying to create a thread at the moment, but I am finding that I am running into an error that my constructor is not initializing properly, with the error that there is no matching constructor for std::thread matching the argument list. Here is what I have done:

#include <functional>
#include <iostream>
#include <numeric>
#include <thread>
#include <vector>

int sum = 0; 
void thread_sum (auto it, auto it2, auto init) {
  sum = std::accumulate(it, it2, init);
}

int main() {
  // * Non Multi-Threaded
  // We're going to sum up a bunch of numbers.
  std::vector<int> toBeSummed;
  for (int i = 0; i < 30000; ++i) {
    toBeSummed.push_back(1);
  }
  // Initialize a sum variable
  long sum = std::accumulate(toBeSummed.begin(), toBeSummed.end(), 0);

  std::cout << "The sum was " << sum << std::endl; 
  // * Multi Threaded
  // Create threads
  std::vector<std::thread> threads;
  std::thread t1(&thread_sum, toBeSummed.begin(), toBeSummed.end(), 0);
  std::thread t2(&thread_sum, toBeSummed.begin(), toBeSummed.end(), 0);
  threads.push_back(std::move(t1));
  threads.push_back(std::move(t2));
  return 0; 
}

The line that messes up is the following:

  auto t1 =
      std::thread {std::accumulate, std::ref(toBeSummed.begin()), 

It is an issue with the constructor. I have tried different combinations of std::ref, std::function, and other wrappers, and tried making my own function lambda object as a wrapper for accumulate.

Here is some additional information:

The error message is: atomics.cpp:28:7: error: no matching constructor for initialization of 'std::thread'

Moreover, when hovering over the constructor, it tells me that the first parameter is of <unknown_type>.

Other attempts I have tried:

• Using references instead of regular value parameters
• Using std::bind
• Using std::function
• Declaring the function in a variable and passing that as my first parameter to the constructor
• Compiling with different flags, like std=c++2a

EDIT :

1. I will leave the original issue as a means for others to learn from my mistakes. As the answer I accept will show, this is due to my excessive usage of auto. I had read a C++ book that basically said "always use auto, it's much more readable, Like Python and dynamic typing, but with the performance of C++." yet clearly this cannot always be done. The using keyword provides the readability while still the safety. Thank you for the answers!

Answer 1

The problems you're encountering are because std::accumulate is an overloaded function template, so the compiler doesn't know what specific function type to treat it as when passed as an argument to the thread constructor. Similar problems arise with your thread_sum function because of the auto parameters.

You can choose a specific overload/instantiation of std::accumulate as follows:

  std::thread t2(
      (int(*)(decltype(toBeSummed.begin()), decltype(toBeSummed.end()), int))std::accumulate,
       toBeSummed.begin(), toBeSummed.end(), 0);

Answer 2

The problem is your excessive use of auto . You can fix it by changing this one line:

void thread_sum (auto it, auto it2, auto init) {

To this:

using Iter = std::vector<int>::const_iterator;
void thread_sum (Iter it, Iter it2, int init) {