如何初始化未知大小的多维向量，C ++

Question

in my project I would need create variable multidimensional matrix by std:vector with variable size. I found correctly code below, which can create this:

template<typename T, std::size_t N>
struct md_vector
{
    using type = std::vector<typename md_vector<T, N - 1>::type>;
};

 template<typename T>
 struct md_vector<T, 1>
 {
    using type = std::vector<T>;
 };

 Template<typename T, std::size_t N>
 using md_vector_t = typename md_vector<T, N>::type;


//using
//this create 3 dimensional vectors of int - std:vector<std:vector<std:vector<int>>>
 md_vector_t<int, 3> matrix;

But now i dont know ho use this matrix? I would like initialize whole matrix to 0....by example 4 dimensional matrix, each vector 7 integers.... 7 x 7 x 7 x 7 integers ? And after initialize how i can assign value? matrix[0][0][0][0] - this i cannot, because I dont know dimension size.

Thank you very much for some advice...

Answer 1

The easiest way to deal with this insane situation is to use resize() . Let the compiler figure out how to initialize the vector:

md_vector_t<int, 4> matrix;

matrix.resize(7);

This is going to resize the first dimension. Now, you need to resize each individual element in the first dimension, in order to construct the second dimension.

for (auto &a:matrix)
    a.resize(7);

Before soon, you figure out how to let range iteration work for you. Something like this:

md_vector_t<int, 4> matrix;

matrix.resize(7);

for (auto &a:matrix)
{
    a.resize(7);
    for (auto &b:a)
    {
        b.resize(7);
        for (auto &c:b)
        {
             c.resize(7);
        }
    }
}

Counting this off on my fingers, I'm pretty fairly certain this'll initialize all four dimensions here.

Before C++11, this would've been agony.

Now, I'm not sure exactly why you say "matrix[0][0][0][0] - this i cannot, by i dont know dimension size". Because that's exactly how you would go around peeking and poking at this sucker.

Each element in the matrix will be value-constructed to 0. If you want to initialize the values to something else, just use the same nested range iteration approach to initialize the values.

Now, as far as a template for initializing a matrix of arbitrary dimensions, it should be trivial to take this same approach and use the same approach as your existing template: a specialization for a 1-dimensional vector, and a the default template that invokes resize() , then performs range iteration to recursively invoke itself on the values in its container.