c++ - Passing an array by reference to thread

Question

I have an array that needs to be passed to multiple threads to be worked on. The array size is know at compile time. Here is a working reduction to the essentials where the function is called directly, not via a thread:

#include <thread>

template <class TYPE>
void function(TYPE& data, std::size_t row, std::size_t column){
    data[row-1][column-1]=2;
}

int main(){
    const int column(5),row(2);
    std::array<std::array<int, column>, row> arr;
    function(arr, row, column);
    return data[row-1][column-1];
}

The code returns 2, as expected. If I call the function via

    std::thread worker(function<std::array<std::array<int,column>,row>>, arr, row, column);

I get the following compiler error:

g++ Testo.cpp -o Testo -std=c++11 -lpthread
In file included from /usr/include/c++/4.8/thread:39:0,
                 from Testo.cpp:2:
/usr/include/c++/4.8/functional: In instantiation of ‘struct std::_Bind_simple<void (*(std::array<std::array<int, 5ul>, 4ul>, int, int))(std::array<std::array<int, 5ul>, 4ul>&, long unsigned int, long unsigned int)>’:
/usr/include/c++/4.8/thread:137:47:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(std::array<std::array<int, 5ul>, 4ul>&, long unsigned int, long unsigned int); _Args = {std::array<std::array<int, 5ul>, 4ul>&, const int&, const int&}]’
Testo.cpp:13:82:   required from here
/usr/include/c++/4.8/functional:1697:61: error: no type named ‘type’ in ‘class std::result_of<void (*(std::array<std::array<int, 5ul>, 4ul>, int, int))(std::array<std::array<int, 5ul>, 4ul>&, long unsigned int, long unsigned int)>’
       typedef typename result_of<_Callable(_Args...)>::type result_type;
                                                             ^
/usr/include/c++/4.8/functional:1727:9: error: no type named ‘type’ in ‘class std::result_of<void (*(std::array<std::array<int, 5ul>, 4ul>, int, int))(std::array<std::array<int, 5ul>, 4ul>&, long unsigned int, long unsigned int)>’
         _M_invoke(_Index_tuple<_Indices...>)
         ^

I can change my template to

template <class TYPE>
void function(TYPE data, std::size_t row, std::size_t column){
    data[row-1][column-1]=2;
}

and the compiler no longer complains, but main then returns 0 as the array is no longer passed by reference. What is the appropriate first argument for the thread to call the correct template to pass the array by reference, as is working when calling the function directly?

Answer 1

Your function expects a reference to the array but std::thread stores decayed copies of the bound arguments. Array-to-pointer decay results in a prvalue of pointer type. To fix, pass arr through a std::reference_wrapper to preserve the type:

std::thread worker(..., std::ref(arr), row, column);

You might also want to use a lambda so you won't have to explicitly pass the template arguments:

std::thread worker([&arr]{ return function(arr, row, column); });