Why Const is "Wicked Weird"

This might be surprising:

typedef char * c_string;
typedef const char * const_c_string;
const c_string x;
const_c_string y;
y = x; // this is illegal!

In order to understand this you need to know a few things about C++.

First: a type specifier has parts:

• Some kind of typename, like int.
• Possible a "cv" (const-volatile) qualifier like "const".
• Possibly the word typedef.
• Other stuff...

The order doesn't matter!!!! In other words:

typedef const int number_t;
typedef int const number_t;

Those are both the same. But it gets a little werider:

int typedef const number_t;

basically int, typedef and const are all parsed together - they can trade places.

Pointers however, most certainly can't. So these things are the same:

const char * t;
char const * t;

But this one is very different:

char * const t;

If you read the C++ spec, the syntax for the pointer part of a type-spec is actually * [cv-qualifier] -- that is, if the word const follows the *, it acts on the pointer.

The way I think of this is: the word const can move around, but it can never cross a *.

So now we can understand why the above didn't work...

typedef char * c_string;
typedef char const * const_c_string;

c_string const x; // this is char * const
const_c_string y; // this is char const *

When we view the "expansion" of the typedef with const moved around, we can see how these are not the same.

(You might wonder, if we had written const c_string whether it would be different from const c_string. Well, remember, the const can move around a named type without having any effect. Essentially the pointer has been "baked into" the type using typedef before we applied const. So const has to apply to the pointer, not to the data itself.)