Assigning a pointer variable to a const int in C++?

Question

I'm wondering if anyone can explain the following to me: If I write

int i = 0;
float* pf = i;

I get a compile error (gcc 4.2.1):

error: invalid conversion from ‘int’ to ‘float*’

Makes sense - they are obviously two completely different types. But if instead I write

const int i = 0;
float* pf = i;

It compiles without error. Why should the 'const' make a difference on the right hand side of the assignment? Isn't part of the idea of the 'const' keyword to be able to enforce type constraints for constant values?

Any explanation I have been able to come up with feels kind of bogus. And none of my explanations also explain the fact that

const int i = 1;
float* pf = i;

fails to compile. Can anyone offer an explanation?

Georg Fritzsche · Accepted Answer · 2010-05-13 00:00:55Z

10

Your second example simply happens to be covered by the conversion rules as specified in §4.10/1 (C++03):

A null pointer constant is an integral constant expression (5.19) rvalue of integer type that evaluates to zero. A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of pointer to object or pointer to function type.

answered May 13, 2010 at 0:00

Georg Fritzsche

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