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.

Instead of the static variable and check in getRandom, why not simply have the srand call in the main function before you do anything? Because the way it is now it's not thread-safe. — Some programmer dude
– Some programmer dude, Commented Aug 18, 2016 at 16:03
OT, if c++11 is an option, you could use std::to_string(). — Bob__
– Bob__, Commented Aug 18, 2016 at 18:06

Community · Accepted Answer · 2017-05-23 12:33:02Z

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) );

edited May 23, 2017 at 12:33

CommunityBot

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answered Aug 18, 2016 at 17:21

UmNyobe

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