Qfyilyi

I have the following code:

MyType x = NULL;

NetBeans gave me a suggestion to change it to this:

MyType x = __null;

I looked it up and found that __null is called a "compiler keyword", which I assumed to mean it's used internally for the compiler. I don't understand why NetBeans suggested to change it to a compiler keyword.

What's the difference between NULL and __null in c++?

__null is a g++ internal thing that serves roughly the same purpose as the standard nullptr added in C++11 (acting consistently as a pointer, never an integer).

NULL is defined as 0, which can be implicitly used as integer, boolean, floating point value or pointer, which is a problem when it comes to overload resolution, when you want to call the function that takes a pointer specifically.

In any event, you shouldn't use __null because it's a g++ implementation detail, so using it guarantees non-portable code. If you can rely on C++11 (surely you can by now?), use nullptr. If not, NULL is your only portable option.

NULL is the old C symbol for a null pointer. C++ traditionally have used 0 for null pointers, and since the C++11 standard nullptr.

Considering that x doesn't seem to be a pointer then you can't initialize x to be a null pointer, and the __null symbol is perhaps some compiler-internal symbol for a null value (which is a concept that doesn't really exist in standard C++).

If you want x to initialized to some default state, then you have to rely on the MyClass default constructor to initialize the objects and its member variables to some suitable default values.

NULL has been overtaken from C into C++ and - prior to C++11 - adopted its C meaning:

until C++11: The macro NULL is an implementation-defined null pointer
constant, which may be an integral constant expression rvalue of
integer type that evaluates to zero.

C++11 then introduced a dedicated null pointer literal nullptr of type std::nullptr_t. But - probably for backward compatibility - makro NULL was not removed; its definition was just a bit relaxed in that compilers may now define it either as integral or as pointer type:

C++11 onwards: an integer literal with value zero, or a prvalue of
type std::nullptr_t

If you use NULL, then you get implementation-defined behaviour in overload resolution. Consider, for example, the following code with a compiler that uses the integral-version of NULL-makro. Then a call using NULL as parameter passed to a function may lead to ambiguities:

struct SomeOverload {



    SomeOverload(int x) {

        cout << "taking int param: " << x << endl;

    }

    SomeOverload(void* x) {

        cout << "taking void* param: " << x << endl;

    }

};



int main() {



    int someVal = 10;



    SomeOverload a(0);

    SomeOverload b(&someVal);



    // SomeOverload c(NULL);  // Call to constructor is ambiuous

    SomeOverload d(nullptr);

}

So it is recommended to use nullptr where ever you want to express pointer type.

And don't use __null, as this is a compiler-specific, non-portable constant; nullptr, in contrast, is perfectly portable.