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Nullptr vs NULLNULL is 0 (zero) i.e. integer constant zero with C-style typecast to void* , while nullptr is prvalue of type nullptr_t , which is an integer literal that evaluates to zero.
nullptr is a keyword that represents zero as an address (its type is considered a pointer-type), while NULL is the value zero as an int . If you're writing something where you're referring to the zero address, rather than the value zero, you should use nullptr .
In C++11 and beyond, a pointer that is ==NULL will also ==nullptr and vice versa. Uses of NULL other than comparing with a pointer (like using it to represent the nul byte at the end of a string) won't work with nullptr .
The nullptr keyword can be used to test if a pointer or handle reference is null before the reference is used. Function calls among languages that use null pointer values for error checking should be interpreted correctly. You cannot initialize a handle to zero; only nullptr can be used.
nullptr has type std::nullptr_t. It's implicitly convertible to any pointer type. Thus, it'll match std::nullptr_t or pointer types in overload resolution, but not other types such as int.
0 (aka. C's NULL bridged over into C++) could cause ambiguity in overloaded function resolution, among other things:
f(int);
f(foo *);
(Thanks to Caleth pointing this out in the comments.)
You can find a good explanation of why it was replaced by reading A name for the null pointer: nullptr, to quote the paper:
This problem falls into the following categories:
Improve support for library building, by providing a way for users to write less ambiguous code, so that over time library writers will not need to worry about overloading on integral and pointer types.
Improve support for generic programming, by making it easier to express both integer 0 and nullptr unambiguously.
Make C++ easier to teach and learn.
Here is Bjarne Stroustrup's wordings,
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 called in C++11. Then, "nullptr" will be a keyword.
One reason: the literal 0 has a bad tendency to acquire the type int, e.g. in perfect argument forwarding or more in general as argument with templated type.
Another reason: readability and clarity of code.
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