Portably safe to pass NULL/zero to dynamic_cast?

c++ null dynamic-cast
sdg picture sdg · Mar 1, 2011 · Viewed 18.3k times · Source

Out of habit for checking null pointers, I have sometimes written:

MyClass * c = someBasePtr ? dynamic_cast<MyClass*>(someBasePtr) : 0;
if (c) {...

In effect, checking for a null pointer before passing to dynamic cast, and also checking the return.

I then read in the MSDN documentation

A null pointer value is converted to the null pointer value of the destination type by dynamic_cast.

It appears then that I could remove the ?: construct safely. Is this C++ portable?

Such that the new code would be

MyClass * c = dynamic_cast<MyClass*>(someBasePtr);
if (c) {...

Of course presuming that someBasePtr is either null or valid, i.e. not wild pointing to garbage...

Answer

user405725 picture user405725 · Mar 1, 2011

§5.2.7/4:

If the value of v is a null pointer value in the pointer case, the result is the null pointer value of type R.

So you don't have to check for a null pointer yourself. Same goes for operator delete, deleting a null pointer has no effect.

Related questions

Checking for NULL pointer in C/C++

In a recent code review, a contributor is trying to enforce that all NULL checks on pointers be performed in the following manner: int * some_ptr; // ... if (some_ptr == NULL) { // Handle null-pointer error } else { // Proceed } instead of int * some_ptr; // ... …

Read More
NULL vs nullptr (Why was it replaced?)

I know that in C++ 0x or NULL was replaced by nullptr in pointer-based applications. I'm just curious of the exact reason why they made this replacement? In what scenario is using nullptr over NULL beneficial when dealing with pointers?

Read More
error: ‘NULL’ was not declared in this scope

I get this message when compiling C++ on gcc 4.3 error: ‘NULL’ was not declared in this scope It appears and disappears and I don't know why. Why? Thanks.

Read More