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c++pointersconstantsmutators

C++ Type const* p and Type const* const p

I have this Type const* p method. Is it sure that p's pointee, *p will never be modified ?

Same thing with Type const* const q.

Solution

  • Type const* p is a pointer to const object with type Type. To be read left to right, with pointer pointing to type defined by everything before the star. The same way, Type const* const q is a const pointer to a const object with type Type.

    Also, *p cannot be modified through p. p is defined so as to const-point to *p and promise not to modify it. However, *p, p's pointee, can be modified by any other pointer pointing at it.

    For example, we can have

    Type t;
Type const* pc = &t;
Type *pnc = &t;

    pc promises not to alter t, pnc does not. Let's say class Type bears a const inspect() const method and a non-const mutate() method. Then we could have

    pc->inspect();
pnc->inspect();
pnc->mutate();

    whereas this one would rise compiler's error:

    pc->mutate();

    Type const* const q is a pointing to a const object and *q cannot be modified through q, just like with p. What is more, pointer q cannot be modified: it cannot be assigned a pointee a second time.

    Also, although this may sounds very strange, you are allowed to change in code the object of type Type pointed by pointer Type const* p -- but not through p.