3 Dimensional Array in C++

Question

I am implementing a 3 dimensional array. Much like cells and sheets of an Excel document. x and y being the page size and z being the number of sheets. For now the x and y indices can be the same for every page.

I thought to approach this with vectors (let's for now just say for strings only), but declaring it as follows:

std::vector<std::string> sheets;
//x and y being the x-y coordinates and z being the number of pages.
int size = x*y*z;
sheets.reserve(size);

Thus giving me a nice contiguous piece of memory (as is definition of a std::vector), which is fast and efficient.

My question is: Is there a way of doing this with std::array? Is there a way that to create the array size when the object is created, or must this be know at compile time?

class sheetsObj
{
    int x, y, z;
    int size;
    //Where size is x * y * z.
    std::array<std::string, size> sheets;
public:
   sheetsObj(int xInd, int yInd, int zInd) : x{ xInd }, y{ yInd }, z{ zInd } {};
   ....
}

Thanks in advance.

Answer 1

The best rule of thumb is to use std::vector when you can, std::array when you have to. In this case, std::vector is the best option for you. As you say, the size of std::array must be known at compile-time, so this is a no-brainer if you need dynamically-sized storage.

Answer 2

std::array has its size fixed by a template parameter, thus evaluated at compile time and probably not suitable for your needs (unless all the sheets are designed to have the same size forever?).

A std::vector will perfectly meet your requirements, since the resizing overhead is not used by your program, and you will get your memory block as needed.

std::dynarray would probably be the best class for you to use, since it represents a runtime initializable array which remains fixed-sized during his lifetime. However, initially scheduled to be released with the C++14 standard, a vote has been cast to exclude it from the upcoming standard, leaving it to be proposed later into a separate Technical Specification.