Question

This is a good old C array:

int a[10];

And this is a good old C array that is const:

const int b[10];

In C++, there seem to be two ways to define std::arrays that are const:

std::array<const int, 10> c;
const std::array<int, 10> d;

Are these two definitions equivalent? If so, what is the idiomatic one? If not, what are the differences?

Answer 1

Well, the original const int b[10]; is only useful when you can initialize the array, so both of the std::array examples don’t work in practice.

1:

std::array<const int, 10> c;

This is the closest to const int c[10];. The problem is there will be no default constructor for it, and because the elements are not mutable, it’s worthless to use this. You must provide some initialization for it in the constructor. As-is, it will give a compiler error because the default constructor did not initialize the elements.

This code means that c is mutable, but the elements themselves are not. In practice, however, there are no mutations on c that don’t affect the elements.

2:

const std::array<int, 10> d;

This means d is not mutable, but the elements are of mutable type int. Because const will propagate to the members, it means the elements are still not mutable by the caller. Similar to the above example, you will need to initialize d because it’s const.

In practice, they will both behave similarly with respect to mutability, because mutable operations on array always touch the elements.