Difference between const char*, char const*, const char const* & string storage

Question

First of all, what's the difference between:

(1) const char*
(2) char const*
(3) const char const*

I'm fairly certain I understand this fully, but I'd like someone to give me a sentence for each, specifically, so it sticks in my head. It's one of those things that I'm fine with until someone puts me on the spot and then it gets fuzzy!

Also, how are string-literalls stored by the compiler? This isn't homework, I'm just brushing up on C for interviews in case anyone cares.

Answer 1

(1) const char* 
(2) char const* 

This is a pointer (that you can change) to a char (or multiple chars) that you cannot change. In other words, all string literals.

(3) const char const*

This is doubled up. I think you were trying to go for the third position:

(4) const char * const

which is a pointer that you cannot change, to a char (or multiple chars) that you cannot change. You can use this for global pointers to literals that should not be accidentally changed.

The string literals are going to be (most likely) lumped right after your code, usually in a segment or section called "rodata".

Answer 2

1 and 2 are equivalent, and specify the type of a pointer to a const char. The pointer itself is not const. 3 is invalid, because it repeats "const". It's like saying const const int. The order is not relevant, so it's also like saying int const int.

In C99 it is valid to repeat const like that. But in C++, you cannot repeat it.

Also, how are string-literalls stored by the compiler?

They are stored in an unspecified way. But compilers are allowed to store them in a read-only portion of the program. So you cannot write to string literals. You are guaranteed that they stay allocated through the whole program lifetime (in other words, they have static storage duration).

This isn't homework, I'm just brushing up on C for interviews in case anyone cares.

You should be aware of the subtle differences between C and C++. In C99, like explained above, const const int is allowed. In C89 and C++ it is forbidden. In C++ you can however introduce a redundant const, if applied to a typedef that is itself const:

typedef int const cint;
cint const a = 0; // this const is redundant!

Same goes for template parameters.