send function pointer as function template in a threadpool class

Question

I'm trying to use a ThreadPool implementation from this fellow: https://github.com/progschj/ThreadPool

I'm having trouble adding 'functions' to the enqueue method...here is the implementation for the enqueue method below:

// add new work item to the pool
template<class T, class F>
Result<T> ThreadPool::enqueue(F f)
{
    Result<T> res;
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        tasks.push_back(std::function<void()>(
        [f,res]()
        {
            CallAndSet<T,F>()(res, f);
        }));
    }
    condition.notify_one();
    return res;
}

Here's what I'm using:

#include "ThreadPool.h"
#include <stdio.h>
#include <iostream>

int main() {
    // create a thread pool of 4 worker threads
    ThreadPool pool(4);

    // queue a bunch of "work items"
    for(int i = 0; i < 8; ++i) {
        pool.enqueue([i] {
            std::cout << "hello " << i << std::endl;

            std::cout << "world " << i << std::endl;
        });
    }
}

It is part of the sample code that is trying to show how to use the library...

output from compilation is:

scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
scons: building associated VariantDir targets: build
g++ -o build/main.o -c -std=c++11 -pthread -Wall -g main.cpp
main.cpp: In function 'int main()':
main.cpp:15:7: error: no matching function for call to 'ThreadPool::enqueue(main()::<lambda()>)'
main.cpp:15:7: note: candidate is:
In file included from main.cpp:1:0:
ThreadPool.h:117:15: note: template<class T, class F> Result<T> ThreadPool::enqueue(F)
ThreadPool.h:117:15: note:   template argument deduction/substitution failed:
main.cpp:15:7: note:   couldn't deduce template parameter 'T'
scons: *** [build/main.o] Error 1
scons: building terminated because of errors.

I'm pretty clueless when it comes to templating stuff..I have no idea why the above won't work...anyone have any ideas?

cheers

jarrett

Answer 1

You have to specify T explicitly as it is not part of the argument list and thus can not be reduced.

 pool.enqueue<TheType>(functor);

I can not guess what T is supposed to be from the snippet alone.