C++ pointer notation

Question

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

Doesn't your book explain it, in the chapters, prior to such code snippet? — Algirdas Preidžius, Aug 30 '18 at 14:57
`const char* const*` is a pointer to a constant pointer to a `char` that is constant. When I say "constant pointer" it means that the pointer itself is constant and can't be changed. — Some programmer dude, Aug 30 '18 at 14:59
@AlgirdasPreidžius No, it is in one of the recommended solutions to one of the exercises — patrick ahrens, Aug 30 '18 at 14:59
@Someprogrammerdude Oooh, not I get it, thank you very much! — patrick ahrens, Aug 30 '18 at 15:00
@patrickahrens If the book doesn't provide necessary information, to do its own exercises, you should switch it for [a good C++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — Algirdas Preidžius, Aug 30 '18 at 15:03
Related: a type translator https://cdecl.org/ (You'll need to give it something more like `const char* const* foo`) — UKMonkey, Aug 30 '18 at 15:12
You can write that type expression more consistently as either `char const* const*` or as `ptr_>`. For the the consistent prefix `const` notation in the last example, you need to define the type builder `ptr_` or something like it. E.g. `template< class Type > using ptr_ = Type*;`. — Cheers and hth. - Alf, Aug 30 '18 at 15:15

eerorika · Accepted Answer · 2018-08-30T15:35:44.063

* 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

C++ pointer notation

1 Answers1