Using NULL in C++? [duplicate]

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Closed 9 years ago.

Possible Duplicate:
Do you use NULL or 0 (zero) for pointers in C++?

Is it a good idea to use NULL in C++ or just the value 0?

Is there a special circumstance using NULL in C code calling from C++? Like SDL?

Answer 1

The downside of NULL in C++ is that it is a define for 0. This is a value that can be silently converted to pointer, a bool value, a float/double, or an int.

That is not very type safe and has lead to actual bugs in an application I worked on.

Consider this:

void Foo(int i);
void Foo(Bar* b);
void Foo(bool b);


main()
{
     Foo(0);         
     Foo(NULL); // same as Foo(0)
} 

C++11 defines a nullptr that is convertible to a null pointer but not to other scalars. This is supported in all modern C++ compilers, including VC++ as of 2008. In older versions of GCC there is a similar feature, but then it was called __null.

Answer 2

In C++ NULL expands to 0 or 0L. See this comment from Stroustrup:

Should I use NULL or 0? In C++, the definition of NULL is 0, so there is only an aesthetic difference. I prefer to avoid macros, so I use 0. Another problem with NULL is that people sometimes mistakenly believe that it is different from 0 and/or not an integer. In pre-standard code, NULL was/is sometimes defined to something unsuitable and therefore had/has to be avoided. That's less common these days.

If you have to name the null pointer, call it nullptr; that's what it's going to be called in C++0x. Then, "nullptr" will be a keyword.

Answer 3

Assuming that you don't have a library or system header that defines NULL as for example (void*)0 or (char*)0 it's fine. I always tend to use 0 myself as it is by definition the null pointer. In c++0x you'll have nullptr available so the question won't matter as much anymore.

Answer 4

I never use NULL in my C or C++ code. 0 works just fine, as does if (ptrname). Any competent C or C++ programmer should know what those do.

Answer 5

From crtdbg.h (and many other headers):

#ifndef NULL
#ifdef __cplusplus
#define NULL    0
#else
#define NULL    ((void *)0)
#endif
#endif

Therefore NULL is 0, at least on the Windows platform. So no, not that I know of.