Search code examples
c++pointerssemantics

C++ pointer notation

So I am learning C++ right now, and I just started to get into Pointers, and I thought I would understand the semantics pretty well till iI saw this in one of the recommended solutions to an exercise int the Book I am learning with: 

const char* sa = * ( static_cast < const char* const* > (a));

I understand everything aside the const* in the Type Parameter. Why is it there, what does it do?

EDIT: corrected unclear formulation

Solution

  • * in a type means that the type is a pointer to the type on the left side of the asterisk.

    const in a type means that the type to the left of const is constant. For an object, const means that the value may not be modified. For a reference, const means that the object may not be modified through the reference.

    char is a type that represents an integer encoded narrow character object.

    const char is a const char.

    const char* is a pointer to a const char.

    const char* const is a const pointer to a const char.

    const char* const* is a pointer to a const char* const.

    Note that the pointer is indirected:

    * ( static_cast < const char* const* > (a));
^ indirection operator

    When a pointer is indirected, the result is a reference (lvalue) to the pointed object. If a const char* were indirected, the resulting lvalue would have the type const char. Clearly such lvalue couldn't be used to initialize the object const char* sa.

    When a const char* const* is indirected, the result will be a reference (lvalue) to an object of type const char* const. Such value can be used to initialize const char* sa.

    A simpler example without casts:

    const char c;               // c cannot be modified
const char* const a = &c;   // a points to charcter object c
                            // a cannot be modified
const char* sa = *a;        // sa points to a as well
sa = nullptr;               // sa can be modified; it no longer points to a