Question

I cannot explain myself the following code:

   double d = 100;

    double const d1 = 30;

    double* const p = &d; // Line 1
    double* const p1 = &d1; // Line 2

In the above code, Line 1 is ok, but Line 2 produces the error:

"error C2440: 'initializing' : cannot convert from 'const double *__w64 ' to 'double *const '"

Can anyone elaborate on that, please?
(I am using VS C++ 2005, running on Win XP SP3)

Answer 1

The type double* const is a const pointer to a non-const double. If you want a pointer to a const double, you have to use double const* (or double const* const if you want a const pointer to a const double).

In C++, with a simple pointer to a double, you the const-ness of both the pointer itself (ie, can you make it point at another location) and the const-ness of the value (can you change the value through the pointer) can be configured independently. This gives you four very similar, but incompatibles types:

double const* const p1; // Const pointer to const double
                        //  . you can't have the pointer point to another address
                        //  . you can't mutate the value through the pointer

double const* p2;       // Non-const pointer to const double
                        //  . you can have the pointer point to another address
                        //  . you can't mutate the value through the pointer

double* const p3;       // Const pointer to double
                        //  . you can't have the pointer point to another address
                        //  . you can mutate the value through the pointer

double* p4;             // Non-const pointer to non-const double
                        //  . you can have the pointer point to another address
                        //  . you can mutate the value through the pointer