Designates a null pointer constant of type std::nullptr_t, which is convertible to any raw pointer type. Although you can use the keyword nullptr without including any headers, if your code uses the type std::nullptr_t, then you must define it by including the header <cstddef>.
The nullptr keyword is also defined in C++/CLI for managed code applications and is not interchangeable with the ISO Standard C++ keyword. If your code might be compiled by using the /clr compiler option, which targets managed code, then use __nullptr in any line of code where you must guarantee that the compiler uses the native C++ interpretation. For more information, see nullptr (C++ Component Extensions).
Avoid using NULL or zero (0) as a null pointer constant; nullptr is less vulnerable to misuse and works better in most situations. For example, given func(std::pair<const char *, double>), then calling func(std::make_pair(NULL, 3.14)) causes a compiler error. The macro NULL expands to 0, so that the call std::make_pair(0, 3.14) returns std::pair<int, double>, which is not convertible to func()'s std::pair<const char *, double> parameter type. Calling func(std::make_pair(nullptr, 3.14)) successfully compiles because std::make_pair(nullptr, 3.14) returns std::pair<std::nullptr_t, double>, which is convertible to std::pair<const char *, double>.