What does `const T* const` mean?

Question

I was reading a C++ template example, and part of the template signature confused me.

Am I reading this correctly:

const T* const foo;

Is foo a const* to a const T?

Answer 1

Yes, it's a constant pointer to a constant T. I.e., you can neither modify the pointer, nor the thing it points to.

const T* would only forbid you to modify anything the pointer points to, but it allows you (within the bounds of the language) to inspect the value at *(foo+1) and *(foo-1). Use this form when you're passing pointers to immutable arrays (such as a C string you're only supposed to read).

T * const would mean you can modify the T value pointed to by foo, but you cannot modify the pointer itself; so you can't say foo++; (*foo)++ because the first statement would increment (modify) the pointer.

T * would give you full freedom: you get a pointer into an array, and you can inspect and modify any member of that array.

Answer 2

This is a const pointer-to-const T. So if T was an int, then array is a pointer to an int that is const in two ways:

pointer-to-const: the value that the pointer is pointing to cannot be changed (or pointing to const int) const pointer: the memory address stored in the pointer cannot change

This is also the same as T const * const array

See wiki on const correctness.