[ACCEPTED]-What does a const pointer-to-pointer mean in C and in C++?-constants

Accepted answer

Score: 67

Your colleague is wrong. That is a (non-const) pointer 2 to a (non-const) pointer to a const MyStructure. In 1 both C and C++.

Score: 65

In such cases the tool cdecl (or c++decl) can 1 be helpfull:

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

Score: 23

You were right in your interpretation. Here's 5 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 4 the alternatives of a pointer-to-pointer 3 with one const qualifier. The right-to-left 2 rule can be used to decipher the declarations 1 (at least in C++; I'm no C expert).

Score: 7

Your colleague is wrong, and it's the same 3 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 2 C++ 2008 gives the following errors for 1 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

Score: 5

You are right.

Another answer already pointed to the "Clockwise Spiral Rule". I 1 liked that one very much - a little elaborate, though.

Score: 3

As a corollary to the other comments, don't 4 put 'const' first. It really belongs after 3 the type. That would have clarified the 2 meaning immediately, just read it RTL as 1 usual:

MyStructure const** ppMyStruct;

Score: 0

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
}

Source: stackoverflow.com