passing template (without a specification) to an std::thread() gives error : <unresolved overloaded function type> matching Error

user_9694134 0
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So, my task is to pass both vectors char & int in the template with two threads.

Why it cannot thread to reading the two containers ? Some push will be helpful.

#include <iostream>
#include <thread>
#include <vector>

template<typename T>

void check(std::vector<T> values){

     for(T& it : values){

        std::cout<< it <<std::endl;
     }

}

int main()
{

  std::vector<int> int_values = {1,2,3,4,5};
  std::vector<char> char_values = {'a','b','c','d','e'};

    std::thread th1(check, int_values);
    std::thread th2(check, char_values);

    th1.join();
    th2.join();

    return 0;
}

The Error is:

error: no matching function for call to std::thread::thread(<unresolved overloaded function type>,std::vector<int>&)

templates multithreading c++
2 Answers

check is not the name of a function, it is the name of a template. std::thread needs a function object so just using check won't work.

You can use check<int> to specify the int specialization of the check template be used, or you could use a lambda and let the compiler figure it out like

std::thread th1([](const auto & var) { check(var); }, std::ref(int_values));
// or even shorter
std::thread th1([&]{ check(int_values); });
user_43424980

template deduction doesn't work with std::thread, so use

  std::thread th1(check<int>, int_values);
  std::thread th2(check<char>, char_values);

Another thing, why do you use pass by value here I think you should change that to be

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

template<typename T>

void check(const std::vector<T> & values){

    for(const T& it : values){

        std::cout<< it <<std::endl;
    }

}

int main()
{

    std::vector<int> int_values = {1,2,3,4,5};
    std::vector<char> char_values = {'a','b','c','d','e'};

    std::thread th1(check<int>, std::ref( int_values ));
    std::thread th2(check<char>, std::ref( char_values ));

    th1.join();
    th2.join();

    return 0;
}

and std::ref() is to enable you to pass by reference because std::thread copies all args in an internal storage

user_96382720
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