C++: Null Pointer

整理自 C++ Primer, 5th Edition

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There are several ways to obtain a null pointer:

int *p1 = nullptr;	// OK. equivalent to int *p1 = 0;
int *p2 = 0;		// OK.

// must #include <cstdlib>
int *p3 = NULL;		// OK. equivalent to int *p3 = 0;

int zero = 0;
p3 = zero;			// ERROR. cannot assign an int to a pointer

• The most direct approach is to initialize the pointer using the literal nullptr, which was introduced by C++11.
• Alternatively, we can initialize a pointer to the literal 0.
• Older programs sometimes use a preprocessor variable named NULL, which the cstdlib header defines as 0.
• It is illegal to assign an int variable to a pointer, even if the variable’s value happens to be 0.