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C typedef interpretation of const pointers

21 Feb 2012
c pointer typedef programming

(From http://stackoverflow.com/questions/2253738/c-typedef-interpretation-of-const-pointers)

Firstly, sample codes:

Case 1:

typedef char* CHARS;
typedef CHARS const CPTR;   // constant pointer to chars

Textually replacing CHARS becomes:

typedef char* const CPTR;   // still a constant pointer to chars

Case 2:

typedef char* CHARS;
typedef const CHARS CPTR;   // constant pointer to chars

Textually replacing CHARS becomes:

typedef const char* CPTR;   // pointer to constant chars

In case 2, after textually replacing CHARS, the meaning of the typedef changed. Why is this so? How does C++ interpret this definition?

Answers:

There’s no point in analyzing typedef behavior on the basis of textual replacement. Typedef-names are not macros, they are not replaced textually.

As you noted yourself

typedef CHARS const CPTR;

is the same thing as

typedef const CHARS CPTR;

This is so for the very same reason why

typedef const int CI;

has the same meaning as

typedef int const CI;

Typedef-name don’t define new types (only aliases to existing ones), but they are “atomic” in a sense that any qualifiers (like const) apply at the very top level, i.e. they apply to the entire type hidden behind the typedef-name. Once you defined a typedef-name, you can’t “inject” a qualifier into it so that it would modify any deeper levels of the type.