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 typede开发者_开发知识库f changed. Why is this so? How does C++ interpret this definition?
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.
Typedef is not a simple textual substitution.
typedef const CHARS CPTR;
Means "the CPTR type will be a const CHARS thing." But CHARS is a pointer-to-char type, so this says "the CPTR type will be a const pointer-to-char type." This does not match what you see when you do a simple substituion.
In other words,
typedef char * CHARS;
is not the same as
#define CHARS char *
The typedef syntax is like a variable declaration, except that instead of declaring the target name to be a variable, it declares it as a new type name which can be used to declare variables of the type that the variable would be without the typedef.
Here's a simple process for figuring out what a typedef is declaring:
Remove the
typedef
keyword. Now you will have a variable declaration.const CHARS CPTR;
Figure out what type that variable is (some compilers have a
typeof()
operator which does exactly this and is very useful). Call that type T. In this case, a constant pointer to (non-constant) char.Replace the
typedef
. You are now declaring a new type (CPTR
) which is exactly the same type as T, a constant pointer to (non-constant) char.
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