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nullptr: A Type-safe and Clear-Cut Null Pointer-2 : Page 2




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Presenting the Problem
In C++, the macro NULL is a synonym for the literal 0. For this reason, most C++ programs use 0 directly to initialize, assign, and compare null pointers. Whether you're using NULL or 0, your C++ apps are subjected to bugs and maintenance problems that are caused by the ambiguity of 0. Consider these two overloaded versions of a function called func:

void func(int); void func (char *); int main() { func(0); //which func is called? }

The function call func(0) is always resolved as func(int), never as func(char *)—even if you meant to call the latter with a null pointer argument! To call the latter, you must use an explicit cast expression:

func( (char *)0); //ugly but unavoidable

Most C++ programmers aren't aware of this trap; they mistakenly assume that passing NULL as the argument of func will invoke the char* overload:

func(NULL); //still calls func(int)

Remember: In C++, NULL is a synonym for the literal 0. It's never implemented as void* because NULL must also be compatible with pointers to members:

struct A {/**/}; int (A::*pmf)(char *)=NULL;

However, there is a subtler problem involving NULL and 0. When you modify a large corpus of code automatically (by using search and replace or a script), it's very difficult to distinguish between code statements in which 0 stands for a numeric value and statements that use 0 as a null pointer. Using NULL is only a partial solution because NULL is replaced with the literal 0 during the preprocessing stage. Furthermore, you have to #include a standard header file in order to use NULL. What you want is a strongly-typed, built-in keyword whose meaning is unambiguous and explicit. This is exactly what nullptr is.

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