const TypedeffedIntPointer not equal to const int *

Question

I have the following C++ code:

typedef int* IntPtr;
const int* cip = new int;
const IntPtr ctip4 = cip;

I compile this with Visual Studio 2008 and get the following error:

error C2440: 'initializing' : cannot convert from 'const int *' to 'const IntPtr'

Clearly my understanding of typedefs is not what is should be.

The reason I'm asking, I'm storing a pointer type in a STL map. I have a function that returns a const pointer which I would like to use to search in the map (using map::find(const key_type&). Since

const MyType* 

and

const map<MyType*, somedata>::key_type

is incompatible, I'm having problems.

Regards Dirk

Answer 1

When you write const IntPtr ctip4 , you are declaring a const-pointer-to-int , whereas const int * cip declares a pointer-to-const-int . These are not the same, hence the conversion is impossible.

You need to change the declaration/initialization of cip to

int * const cip = new int;

To resolve this issue in your example, you need to either change the key type of the map to const MyType * (whether or not it makes sense depends on your application, but I think that changing a MyType object via a pointer used as key in the map is unlikely), or fall back to const_casting the parameter to find:

#include <map>

int main()
{
    const int * cpi = some_func();

    std::map<const int *, int> const_int_ptr_map;
    const_int_ptr_map.find(cpi); //ok

    std::map<int *, int> int_ptr_map;
    int_ptr_map.find(const_cast<int *>(cpi)); //ok
}

Answer 2

const IntPtr is the same as int* const , not const int* .

That is, it is a const pointer to an int , not a pointer to a const int .

A solution would be to provide two typedefs:

typedef int* IntPtr;
typedef const int* ConstIntPtr;

and use ConstIntPtr when you need a pointer to a const int .