What does a const pointer-to-pointer mean in C and in C++?

In such cases the tool cdecl (or c++decl) can be helpfull:

 [flolo@titan ~]$ cdecl explain "const struct s** ppMyStruct" declare ppMyStruct as pointer to pointer to const struct s 

You were right in your interpretation. Here's another way to look at it:

const MyStructure * *ppMyStruct; // ptr --> ptr --> const MyStructure MyStructure *const *ppMyStruct; // ptr --> const ptr --> MyStructure MyStructure * *const ppMyStruct; // const ptr --> ptr --> MyStructure 

These are all the alternatives of a pointer-to-pointer with one const qualifier. The right-to-left rule can be used to decipher the declarations (at least in C++; I'm no C expert).

Your colleague is wrong, and it's the same for C and C++. Try the following:

typedef struct foo_t { int i; } foo_t; int main() { foo_t f = {123}; const foo_t *p = &f; const foo_t **pp = &p; printf("f.i = %d\n", (*pp)->i); (*pp)->i = 888; // error p->i = 999; // error } 

Visual C++ 2008 gives the following errors for the last two lines:

error C2166: l-value specifies const object error C2166: l-value specifies const object 

GCC 4 says:

error: assignment of read-only location '**pp' error: assignment of read-only location '*p' 

G++ 4 says:

error: assignment of data-member 'foo_t::i' in read-only structure error: assignment of data-member 'foo_t::i' in read-only structure 

You are right.

Another answer already pointed to the "Clockwise Spiral Rule". I liked that one very much - a little elaborate, though.

As a corollary to the other comments, don't put 'const' first. It really belongs after the type. That would have clarified the meaning immediately, just read it RTL as usual:

MyStructure const** ppMyStruct; 

void Foo( int * ptr, int const * ptrToConst, int * const constPtr, int const * const constPtrToConst ) { *ptr = 0; // OK: modifies the pointee ptr = 0; // OK: modifies the pointer *ptrToConst = 0; // Error! Cannot modify the pointee ptrToConst = 0; // OK: modifies the pointer *constPtr = 0; // OK: modifies the pointee constPtr = 0; // Error! Cannot modify the pointer *constPtrToConst = 0; // Error! Cannot modify the pointee constPtrToConst = 0; // Error! Cannot modify the pointer }