Why is C++’s NULL typically an integer literal rather than a pointer like in C?

In C, a void* can be implicitly converted to any T*. As such, making NULL a void* is entirely appropriate.

But that’s profoundly dangerous. So C++ did away with such conversions, requiring you to do most pointer casts manually. But that would create source-incompatibility with C; a valid C program that used NULL the way C wanted would fail to compile in C++. It would also require a bunch of redundancy: T *pt = (T*)(NULL);, which would be irritating and pointless.

So C++ redefined the NULL macro to be the integer literal 0. In C, the literal 0 is also implicitly convertible to any pointer type and generates a null pointer value, behavior which C++ kept.

Now of course, using the literal 0 (or more accurately, an integer constant expression whose value is 0) for a null pointer constant was… not the best idea. Particularly in a language that allows overloading. So C++11 punted on using NULL entirely over a keyword that specifically means “null pointer constant” and nothing else.

Leave a Comment