fastest way converting multidimensional std::vector into one array

Question

I want to copy as little as possible. At the moment I'm using num_t* array = new num_t[..] and then copying each value of the multidimensional vector into array in a for-loop.

I'd like to find a better way to do this.

Answer 1

As you stated in the comments your inner vectors of your vector<vector<T>> structure are of the same size. So what you are actually trying to do is to store a m x n matrix.

Usually such matrices are not stored in multi-dimensional structures but in linear memory. The position (row, column) of a given element is then derived based on an indexing scheme of which row-major and column-major order are used most often.

Since you already state that you will copy this data on to a GPU, this copying is then simply done by copying the linear vector as a whole. You will then use the same indexing scheme on the GPU and on the host.

If you are using CUDA, have a look at Thrust. It provides thrust::host_vector<T> and thrust::device_vector<T> and simplifies copying even further:

thrust::host_vector<int> hostVec(100); // 10 x 10 matrix
thrust::device_vector<int> deviceVec = hostVec; // copies hostVec to GPU

Answer 2

For arithmetic types you can use function memcpy. For example

#include <iostream>
#include <vector>
#include <cstring>

int main()
{
    std::vector<std::vector<int>> v =
    {
        { 1 },
        { 1, 2 },
        { 1, 2, 3 },
        { 1, 2, 3, 4 }
    };

    for ( const auto &row : v )
    {
        for ( int x : row ) std::cout << x << ' ';
        std::cout << std::endl;
    }
    std::cout << std::endl;

    size_t n = 0;
    for ( const auto &row : v ) n += row.size();

    int *a = new int[n];
    int *p = a;

    for ( const auto &row : v )
    {
        std::memcpy( p, row.data(), row.size() * sizeof( int ) );
        p += row.size();
    }        

    for ( p = a; p != a + n; ++p ) std::cout << *p << ' ';
    std::cout << std::endl;

    delete []a;
}      

The program output is

1 
1 2 
1 2 3 
1 2 3 4 

1 1 2 1 2 3 1 2 3 4