Passing vector to a thread function in c++ - c++

Question

I am trying to run a multi-threaded matrix sum function, so that each line will sum up in a different thread. I have tried implementing all workarounds for passing vector to a cpp thread function in a template class but I still get this common error.
For the code:
template <typename T> class Matrix
{
// variables for matrix size and variables in a one dimension vector
unsigned int _rows;
unsigned int _cols;
vector<vector<T> > _matrix;
// Matrix template class functions declarations (all common operators and constructors)
void sumLine(vector<T>& first, vector<T>& second, vector<T>& result);
Matrix<T> operator+(const Matrix<T> & other) const;
};
// Matrix template class functions implmentations
template <typename T> void Matrix<T>::sumLine(vector<T>& first, vector<T>& second, vector<T>& result)
{
for (unsigned int colIdx = 0; colIdx < _cols; colIdx++)
{
result[colIdx] = first[colIdx] + second[colIdx];
}
}
template <typename T> Matrix<T> Matrix<T>::operator+(const Matrix<T> & other) const
{
vector<thread> threads;
vector<vector<T> > results;
vector<T> newRow(_cols);
results.resize(_rows, newRow);
for (unsigned int rowIdx = 0; rowIdx < _rows; rowIdx++)
{
vector<T> first = _matrix[rowIdx];
vector<T> second = other._matrix[rowIdx];
vector<T> result = results[rowIdx];
threads.push_back(thread(Matrix<T>::sumLine, std::ref(first), std::ref(second), std::ref(result)));
}
for (unsigned int thrdIdx = 0; thrdIdx < _rows; thrdIdx++)
{
threads[thrdIdx].join();
}
// do something with vector<vector<T>> results
}
and still after compiling with gcc I get:
In file included from /usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/thread:39:0,
from Matrix.hpp:12,
from main.cpp:13:
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/functional: In instantiation of 'struct std::_Bind_simple<std::_Mem_fn<void (Matrix<Complex>::*)(std::vector<Complex, std::allocator<Complex> >&, std::vector<Complex, std::allocator<Complex> >&, std::vector<Complex, std::allocator<Complex> >&)>(std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >, std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >, std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >)>':
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/thread:137:47: required from 'std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (Matrix<Complex>::*)(std::vector<Complex, std::allocator<Complex> >&, std::vector<Complex, std::allocator<Complex> >&, std::vector<Complex, std::allocator<Complex> >&); _Args = {std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >, std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >, std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >}]'
Matrix.hpp:404:102: required from 'Matrix<T> Matrix<T>::operator+(const Matrix<T>&) const [with T = Complex]'
main.cpp:59:14: required from here
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/functional:1665:61: error: no type named 'type' in 'class std::result_of<std::_Mem_fn<void (Matrix<Complex>::*)(std::vector<Complex, std::allocator<Complex> >&, std::vector<Complex, std::allocator<Complex> >&, std::vector<Complex, std::allocator<Complex> >&)>(std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >, std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >, std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >)>'
typedef typename result_of<_Callable(_Args...)>::type result_type;
^
/usr/lib/gcc/x86_64-pc-cygwin/4.9.3/include/c++/functional:1695:9: error: no type named 'type' in 'class std::result_of<std::_Mem_fn<void (Matrix<Complex>::*)(std::vector<Complex, std::allocator<Complex> >&, std::vector<Complex, std::allocator<Complex> >&, std::vector<Complex, std::allocator<Complex> >&)>(std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >, std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >, std::reference_wrapper<std::vector<Complex, std::allocator<Complex> > >)>'
_M_invoke(_Index_tuple<_Indices...>)
When Complex is a complex number class I wrote for the matrix template class, and my main function is trying to use this calculation with it. What is wrong here? If it is in the complex class, is there a simpler way to pass the parameters to the thread function to avoid this?

Answer 1

you should post your Complex class, it may be the problem.
also in your code:
vector<T> first = _matrix[rowIdx];
vector<T> second = other._matrix[rowIdx];
vector<T> result = results[rowIdx];
you should write it like this: vector<T>& first = _matrix[rowIdx];
because you are copying those vectors... or even const vector<T>& first = _matrix[rowIdx]; if you don't modify them.
then you can remove the std::ref

Answer 2

the following code compiles (with clang 3.5) and avoids passing references to temporaries to any thread ...
#include <vector>
#include <thread>
#include <stdexcept>
using namespace std; // for expose only (don't do this in real code!!)
template <typename T> class Matrix
{
using row = vector<T>;
size_t _rows, _cols;
vector<row> _matrix;
Matrix(vector<row>&&); // defined elsewhere
row sumLine(row const& first, row const& second) const
{
row result; // don't tempt the compiler to default
result.reserve(_cols); // initialise result[i] for all columns
for(size_t c=0; c!=_cols; ++c)
result.emplace_back(first[c]+second[c]);
return result; // return is fast (no copy!)
}
Matrix<T> operator+ (const Matrix<T>& other) const;
};
template <typename T>
Matrix<T> Matrix<T>::operator+(const Matrix<T>&other) const
{
if(other._cols != _cols)
throw std::runtime_error("column number mismatch in Matrix+Matrix");
if(other._rows != _rows)
throw std::runtime_error("row number mismatch in Matrix+Matrix");
vector<thread> threads; threads.reserve(_rows);
vector<row> result(_rows);
for(size_t r=0; r!=_rows; ++r)
threads.emplace_back([&,r]() {
result[r] = Matrix<T>::sumLine(_matrix[r],other._matrix[r]);
});
for(size_t r=0; r!=_rows; ++r)
threads[r].join();
return move(result);
}
template class Matrix<double>;