Is using NULL references OK?

Question

I came across this code:

void f(const std::string &s);

And then a call:

f( *((std::string*)NULL) );

A

Answer 1

You will sometimes see constructions like this in fairly esoteric template library code, but only inside a sizeof() where it is harmless.

Supposing you wanted to know the size of the return type of a function-like type F if it was passed a reference to a type T as an argument (both of those being template parameters). You could write:

sizeof(F(T()))

But what if T happens to have no public default constructor? So you do this instead:

sizeof(F(*((T *)0)))

The expression passed to sizeof never executes - it just gets analyzed to the point where the compiler knows the size of the result.