lousybyte lousybyte - 6 months ago 40
C++ Question

Pointer to reference to const null value

This is the code:

const int x = 0;
const int& r = x;

const int* p1 = x; // Works (p1 = 0)
const int* p2 = r; // error C2440: 'initializing': cannot convert from 'const int' to 'const int *'

Why is p1 set to the value of x, but p2 not?

Is it because p2 cannot guarantee that x is const just by looking at r?

Or is there anything else subtle about references that causes this error?


The third line should provoke an error. It doesn't in VS2015. It's a bug in that compiler. Other compilers do emit error messages about this line.

Until C++11, any integral constant expression that evaluates to 0 is treated as a null pointer constant, so the first line is equivalent to const int* p1 = NULL;, It doesn't make p1 point to x.

Since C++11 (and VS2015 is supposed to support C++11) this is no longer the case. Roughly speaking, only literal zero and nullptr are valid null pointer constants.

The fourth line was never valid in any version of C++, because the type of r is a reference, not an integer.