NULL vs nullptr (Why was it replaced?) [duplicate]

问题:

This question already has answers here:

What exactly is nullptr? (12 answers)

Closed 6 years ago.

I know that in C++ 0x or NULL was replaced by nullptr in pointer-based applications. I'm just curious of the exact reason why they made this replacement?

In what scenario is using nullptr over NULL beneficial when dealing with pointers?

回答1:

nullptr is always a pointer type. 0 (aka. C's NULL bridged over into C++) could cause ambiguity in overloaded function resolution, among other things:

f(int);
f(foo *);

回答2:

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.

回答3:

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.

回答4:

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.