Question

For reading complex pointer declarations there is the right-left rule.

But this rule does not mention how to read const modifiers.

For example in a simple pointer declaration, const can be applied in several ways:

char *buffer; // non-const pointer to non-const memory
const char *buffer; // non-const pointer to const memory
char const *buffer; // equivalent to previous declartion
char * const buffer = {0}; // const pointer to non-const memory
char * buffer const = {0}; // error
const char * const buffer = {0}; // const pointer to const memory

Now what about the use of const with a pointer of pointer declaration?

char **x; // no const;
const char **x;
char * const *x;
char * * const x;
const char * const * x;
const char * * const x;
const char * const * const x;

And what is an easy rule to read those declarations?
Which declarations make sense?

Is the Clockwise/Spiral Rule applicable?

Two real world examples

The method ASTUnit::LoadFromCommandLine uses const char ** to supply command line arguments (in the llvm clang source).

The argument vector parameter of getopt() is declared like this:

int getopt(int argc, char * const argv[], const char *optstring);

Where char * const argv[] is equivalent to char * const * argv in that context.

Since both functions use the same concept (a vector of pointers to strings to supply the arguments) and the declarations differ – the obvious questions are: Why do they differ? Makes one more sense than the other?

The intend should be: The const modifier should specify that the function does not manipulate strings of this vector and does not change the structure of the vector.

Answer 1

The const modifier is trivial: it modifies what precedes it, unless
nothing precedes it. So:

char const* buffer;  // const modifies char
char* const buffer;  // const modifies *

, etc. Generally, It’s best to avoid the forms where nothing precedes
the const, but in practice, you’re going to see them, so you have to
remember that when no type precedes the const, you have to logically
move it behind the first type. So:

const char** buffer;

is in fact:

char const** buffer;

, i.e. pointer to pointer to const char.

Finally, in a function declaration, a [] after reads as a * before.
(Again, it’s probably better to avoid this misleading notation, but
you’re going to see it, so you have to deal with it.) So:

char * const argv[],  //  As function argument

is:

char *const * argv,

a pointer to a const pointer to a char.