You may have used code such as [1]:

int* a = f(); const int* b = a; g(b); // Success

Here we obtain a pointer b to the same object that a points to, but we say that it is now immutable (through b, at least).

However, C++ does not allow a programmer to add constness more than one layer of indirection away [2]:

int** a = f(); const int** b = a; g(b); // error: invalid conversion from 'int**' to 'const int**'

Of course, you can still make the pointer itself const [3]:

int** a = f(); int** const b = a; g(b); // Success

So it turns out that you can add constness to the pointer [3], or to the immediate pointee [1], but you can't add constness to the pointee of the pointee [2].

The question is: why not? How does adding constness violate const-correctness? Answers on a postcard, please.

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Update: As Kniht correctly pointed out in the comments (and, in fact, as the standard itself explains in a note), this is invalid for very good reason:

#include <cassert> int main() { char* p = 0; //char const** a = &p; // not allowed, but let's pretend it is char const** a = (char const**)&p; // instead force the cast to compile char const* orig = "original"; *a = orig; // type of *a is char const*, which is the type of orig, this is allowed assert(p == orig); // oops! char* points to a char const* }