Using thread pool library

Question

I'm trying to use Tyler Hardin's thread pool class. The library can be found here: https://github.com/Tyler-Hardin/thread_pool

My code is:

#include "thread_pool.hpp"

#include <windows.h>
#include <iostream>
#include <list>
#include <string>
#include <sstream>

using namespace std;

const int num_threads = 8;

int getRandom(int min, int max)
{
   return min + rand() % (max - min);
}

std::string to_string(int val)
{
    std::ostringstream ss;
    ss << val;
    std::string str = ss.str();
    return str;
}

string getResult(string param)
{
    int time = getRandom(0, 500);
    Sleep(time);
    return ("Time spend here: " + to_string(time));
}

int main()
{
    srand(time(NULL));

    thread_pool pool(num_threads);
    list<future<string>> results;

    for(int i=100; i<=100000; i++)
    {
        std::future<string> buff = pool.async( function<string(string)>(getResult), "MyString" );
        results.push_back( buff );
    }

    for(auto i=results.begin(); i != results.end(); i++)
    {
        i->get();
        cout << endl;
    }
    return 0;
}

But something seems to be wrong as I am facing with the following errors:

error: no matching function for call to 'thread_pool::async(std::function<std::basic_string<char>(std::basic_string<char>)>, const char [9])
error: use of deleted function 'std::future<_Res>::future(const std::future<_Res>&) [with _Res = std::basic_string<char>]'|

What am I doing wrong in this call:

std::future<string> buff = pool.async( function<string(string)>(getResult), "MyString" );

The program should print the sleep time withing every thread right after every thread finish their job.

Answer 1

Error 1 : function matching

Pretty sure the Windows compiler you are using doesn't know to match the string literal of type const char [9] to a std::string when matching async. This is two levels of implicit conversion, which is not allowed :

const char [9] 
--> const char* 
--> std::basic_string<char>(const char* s, const Allocator& alloc = Allocator() );

I am not sure whether the compiler should consider it to be a single or two separate implicit conversion.
Anyway, you could fix it by explicitly converting the parameter to a std::string

std::future<string> buff = pool.async( function<string(string)>(getResult), std::string("MyString") );

Error 2 : use of deleted ...

Use the move constructor. The copy constructor is marked as deleted

results.push_back( std::move(buff) );