C++11 Threads: Error passing a vector to a thread function

Question

I am working on a multithreaded median function as part of a larger project. I have little C++ experience. The median function below should take a vector of 3 dimensional int vectors, and return a 3 dimensional vector of ints where each entry is the median value of all the entries in that index in the input vectors. So if the input is <<3,2,1>,<1,2,3>,<2,2,2>>, the return in <2,2,2>. This code will be used in the implementation of a median blur for use on real-time video, hence the desire to multithread it.

#include <thread>
#include <iostream>
#include <mutex>
#include <vector>
#include <algorithm>
#include "median.h"

// mutex to protect bgrPixel (possibly not needed)
std::mutex mtx;


std::vector<int> median(const std::vector<std::vector<int> >& input)
{
    std::vector<int> bgrPixel;              // Vector to store median BGR value
    std::thread first(thread_function, bgrPixel, input, 0); // thread for each colour channel
    std::thread second(thread_function, bgrPixel, input, 1);
    std::thread third(thread_function, bgrPixel, input, 2);
    first.join();
    second.join();
    third.join(); 
    return bgrPixel;
}

void thread_function(std::vector<int>& bgrPixel, const std::vector<std::vector<int> >&                 input1, int channel)
{

    std::vector<int> input = input1[channel];  // copy the colour channel
    std::sort(input.begin(), input.end());
    int size = input.size();
    if (size %2 == 0)   // get the median
    {
        mtx.lock();
        bgrPixel[channel] = (input[size/2] + input[size/2 + 1])/2;
        mtx.unlock();
    } else
    {
        mtx.lock();
        bgrPixel[channel] = input[(size-1)/2];
        mtx.unlock();
    }
}

The problem I am having is, at compile time, g++ (and clang also) gives a fairly unintelligible error:

 g++ -std=c++11 -pthread -o median median.cpp

 In file included from /usr/include/c++/4.8.2/thread:39:0,
                  from median.cpp:1:
 /usr/include/c++/4.8.2/functional: In instantiation of ‘struct std::_Bind_simple<void           (*(std::vector<int>, std::vector<std::vector<int> >, int))(std::vector<int>&, const      std::vector<std::vector<int> >&, int)>’:
 /usr/include/c++/4.8.2/thread:137:47:   required from ‘std::thread::thread(_Callable&&,           _Args&& ...) [with _Callable = void (&)(std::vector<int>&, const      std::vector<std::vector<int> >&, int); _Args = {std::vector<int, std::allocator<int> >&,      const std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int,      std::allocator<int> > > >&, int}]’
 median.cpp:15:58:   required from here
 /usr/include/c++/4.8.2/functional:1697:61: error: no type named ‘type’ in ‘class                std::result_of<void (*(std::vector<int>, std::vector<std::vector<int> >, int))     (std::vector<int>&, const std::vector<std::vector<int> >&, int)>’
        typedef typename result_of<_Callable(_Args...)>::type result_type;
                                                         ^
 /usr/include/c++/4.8.2/functional:1727:9: error: no type named ‘type’ in ‘class      std::result_of<void (*(std::vector<int>, std::vector<std::vector<int> >, int))     (std::vector<int>&, const std::vector<std::vector<int> >&, int)>’
          _M_invoke(_Index_tuple<_Indices...>)
          ^

I have found a similar error message c++11 Thread class how to use a class member function, but it does not deal specifically with my problem. Any help would be much appreciated, I fully expect this is because I don't know what I am doing :P

EDIT: Prototypes for thread_function and median are included from the header file median.h.

Answer 1

Replace

std::thread first(thread_function, bgrPixel, input, 0);

by

std::thread first(thread_function, std::ref(bgrPixel), std::ref(input), 0);

Live example: http://coliru.stacked-crooked.com/a/630775aafc3d4642