Null pointer

wikipedia Null pointer

cppreference Pointer declaration # Null pointers

Pointers of every type have a special value known as null pointer value of that type. A pointer whose value is null does not point to an object or a function (dereferencing a null pointer is undefined behavior), and compares equal to all pointers of the same type whose value is also null.

To initialize a pointer to null or to assign the null value to an existing pointer, the null pointer literal nullptr, the null pointer constant NULL, or the implicit conversion from the integer value 0 may be used.

Zero- and value-initialization also initialize pointers to their null values.

Null pointers can be used to indicate the absence of an object (e.g. function::target()), or as other error condition indicators (e.g. dynamic_cast). In general, a function that receives a pointer argument almost always needs to check if the value is null and handle that case differently (for example, the delete expression does nothing when a null pointer is passed).

CppCoreGuidelines ES.47: Use nullptr rather than 0 or NULL

Reason

Readability. Minimize surprises: nullptr cannot be confused with an int. nullptr also has a well-specified (very restrictive) type, and thus works in more scenarios where type deduction might do the wrong thing on NULL or 0.

Example

Consider:

void f(int);
void f(char*);
f(0);         // call f(int)
f(nullptr);   // call f(char*)

Enforcement

Flag uses of 0 and NULL for pointers. The transformation might be helped by simple program transformation.