Conversion from int** to const int**

Question

Why do I get in this code:

void foo ( const int ** );

int main() {
    int ** v = new int * [10];
    foo(v);

    return 0;
}

this error:

Answer 1

You're being misled here by C++'s confusing parsing rules for pointers. It might be clearer to look at this way:

typedef const int * ptr_to_const_int;
void foo( ptr_to_const_int *);
int main() {
    int ** v = new int * [10];
    foo(v);

    return 0;
}

What foo()'s parameter list promises is that you'll be passing it a pointer to a (pointer-to-constant-thing). But new int*[10] means "pointer to (pointer-to-not-constant-thing)".

So if foo were defined like this:

foo( const int **p )
{
  (*p); //<-- this is actually of type const int *
}

whereas I think you're expecting that

foo( const int **p )
{
  (*p); //<-- you're expecting this to be of type int *
  p = 0; //<-- and this to throw a compiler error because p is const
}

but it's not p that you're declaring to be constant, it's the thing it points to.

Anyway just use a typedef in this case and everything will be clear and readable.