Multi-dimensional vector initialization

问题:

I have following std::vector declaration:

std::vector<std::vector<std::vector<int> > > m_input;

I am initializing it as follows:

    m_input.resize (100);
    m_output.resize(100);
    for (int i = 0; i < 100; ++i) {
        m_input [i].resize(100);
        m_output[i].resize(100);
        for (int j = 0; j < 100; ++j){
            m_input [i][j].resize(100);
            m_output[i][j].resize(100);
        }
    }

How can I achieve this via the member initializer list?

回答1:

std::vector<T> has a constructor that takes two arguments, a number of elements and an initial value. In your case, you want to initialize m_input with 100 copies of a std::vector<std::vector<int> > , so it'd be : m_input(100, X). Now, that X in turn is a vector of 100 std::vector<int>, which in turn contains a hundred ints:

: m_input(100, std::vector<std::vector<int> >(100, std::vector<int>(100, 0)))

回答2:

my_class::my_class()
 : m_input(100, std::vector< std::vector<int> >(100, std::vector<int>(100) ))
{
}

That said, implementing a multi-dimensional field should be done by projecting into a one-dimensional one, as Viktor said in his comment to the question.

回答3:

If you can assert that your vector dimensions are going to be of a fixed length, then why not use std::array?

For example:

std:array<std::array<std::array<int, 100>, 100>, 100>

That way you can take advantage of all the memory being contiguously allocated (as hinted at by Viktor_Sehr in the comments), without the added implementation woes of accessing a 1-dimensional array in a 3-dimensional way.