为什么在c++ 11中使用nullptr ?是什么?为什么NULL是不充分的?

c++专家Alex Allain在这里说得很好(我用粗体加了重点):

...imagine you have the following two function declarations: void func(int n); void func(char *s); func( NULL ); // guess which function gets called? Although it looks like the second function will be called--you are, after all, passing in what seems to be a pointer--it's really the first function that will be called! The trouble is that because NULL is 0, and 0 is an integer, the first version of func will be called instead. This is the kind of thing that, yes, doesn't happen all the time, but when it does happen, is extremely frustrating and confusing. If you didn't know the details of what is going on, it might well look like a compiler bug. A language feature that looks like a compiler bug is, well, not something you want. Enter nullptr. In C++11, nullptr is a new keyword that can (and should!) be used to represent NULL pointers; in other words, wherever you were writing NULL before, you should use nullptr instead. It's no more clear to you, the programmer, (everyone knows what NULL means), but it's more explicit to the compiler, which will no longer see 0s everywhere being used to have special meaning when used as a pointer.

Allain在文章结尾写道:

不管这些——c++ 11的经验法则是，只要在过去使用NULL，就开始使用nullptr。

(我的话):

最后，不要忘记nullptr是一个对象——一个类。它可以在任何之前使用NULL的地方使用，但如果你因为某种原因需要它的类型，它的类型可以用decltype(nullptr)提取，或者直接描述为std::nullptr_t，这只是一个decltype(nullptr)的类型定义，如下所示:

定义在头<cstddef>:

See:

https://en.cppreference.com/w/cpp/types/nullptr_t 和https://en.cppreference.com/w/cpp/header/cstddef

namespace std
{
typedef decltype(nullptr) nullptr_t; // (since C++11)
// OR (same thing, but using the C++ keyword `using` instead of the C and C++ 
// keyword `typedef`):
using nullptr_t = decltype(nullptr); // (since C++11)
} // namespace std

引用:

c++ 11中更好的类型- nullptr，枚举类(强类型枚举)和cstdint https://en.cppreference.com/w/cpp/language/decltype https://en.cppreference.com/w/cpp/types/nullptr_t https://en.cppreference.com/w/cpp/header/cstddef https://en.cppreference.com/w/cpp/keyword/using https://en.cppreference.com/w/cpp/keyword/typedef

0 used to be the only integer value that could be used as a cast-free initializer for pointers: you can not initialize pointers with other integer values without a cast. You can consider 0 as a consexpr singleton syntactically similar to an integer literal. It can initiate any pointer or integer. But surprisingly, you'll find that it has no distinct type: it is an int. So how come 0 can initialize pointers and 1 cannot? A practical answer was we need a means of defining pointer null value and direct implicit conversion of int to a pointer is error-prone. Thus 0 became a real freak weirdo beast out of the prehistoric era. nullptr was proposed to be a real singleton constexpr representation of null value to initialize pointers. It can not be used to directly initialize integers and eliminates ambiguities involved with defining NULL in terms of 0. nullptr could be defined as a library using std syntax but semantically looked to be a missing core component. NULL is now deprecated in favor of nullptr, unless some library decides to define it as nullptr.

它如何既是关键字又是类型的实例?

这并不奇怪。true和false都是关键字，作为字面量，它们有一个类型(bool)。Nullptr是一个std::nullptr_t类型的指针字面值，它是一个prvalue(不能使用&获取它的地址)。

4.10 about pointer conversion says that a prvalue of type std::nullptr_t is a null pointer constant, and that an integral null pointer constant can be converted to std::nullptr_t. The opposite direction is not allowed. This allows overloading a function for both pointers and integers, and passing nullptr to select the pointer version. Passing NULL or 0 would confusingly select the int version. A cast of nullptr_t to an integral type needs a reinterpret_cast, and has the same semantics as a cast of (void*)0 to an integral type (mapping implementation defined). A reinterpret_cast cannot convert nullptr_t to any pointer type. Rely on the implicit conversion if possible or use static_cast. The Standard requires that sizeof(nullptr_t) be sizeof(void*).

从nullptr:一个类型安全和明确的空指针:

新的c++ 09 nullptr关键字指定了一个右值常量，用作通用空指针字面量，取代了有bug且弱类型的字面量0和臭名昭著的null宏。因此，Nullptr结束了30多年来的尴尬、歧义和错误。下面几节介绍nullptr功能，并展示它如何补救NULL和0的问题。

其他参考资料:

WikiBooks，带有示例代码。 Stack Overflow:在c++中，指针使用NULL还是0(零)? 模板 谷歌组:comp.lang.c++。有节制的编译器讨论

NULL need not to be 0. As long you use always NULL and never 0, NULL can be any value. Asuming you programme a von Neuman Microcontroller with flat memory, that has its interrupt vektors at 0. If NULL is 0 and something writes at a NULL Pointer the Microcontroller crashes. If NULL is lets say 1024 and at 1024 there is a reserved variable, the write won't crash it, and you can detect NULL Pointer assignments from inside the programme. This is Pointless on PCs, but for space probes, military or medical equipment it is important not to crash.

为什么在c++ 11中使用nullptr ?是什么?为什么NULL是不充分的?

c++专家Alex Allain在这里说得很好(我用粗体加了重点):

...imagine you have the following two function declarations: void func(int n); void func(char *s); func( NULL ); // guess which function gets called? Although it looks like the second function will be called--you are, after all, passing in what seems to be a pointer--it's really the first function that will be called! The trouble is that because NULL is 0, and 0 is an integer, the first version of func will be called instead. This is the kind of thing that, yes, doesn't happen all the time, but when it does happen, is extremely frustrating and confusing. If you didn't know the details of what is going on, it might well look like a compiler bug. A language feature that looks like a compiler bug is, well, not something you want. Enter nullptr. In C++11, nullptr is a new keyword that can (and should!) be used to represent NULL pointers; in other words, wherever you were writing NULL before, you should use nullptr instead. It's no more clear to you, the programmer, (everyone knows what NULL means), but it's more explicit to the compiler, which will no longer see 0s everywhere being used to have special meaning when used as a pointer.

Allain在文章结尾写道:

不管这些——c++ 11的经验法则是，只要在过去使用NULL，就开始使用nullptr。

(我的话):

最后，不要忘记nullptr是一个对象——一个类。它可以在任何之前使用NULL的地方使用，但如果你因为某种原因需要它的类型，它的类型可以用decltype(nullptr)提取，或者直接描述为std::nullptr_t，这只是一个decltype(nullptr)的类型定义，如下所示:

定义在头<cstddef>:

See:

https://en.cppreference.com/w/cpp/types/nullptr_t 和https://en.cppreference.com/w/cpp/header/cstddef

namespace std
{
typedef decltype(nullptr) nullptr_t; // (since C++11)
// OR (same thing, but using the C++ keyword `using` instead of the C and C++ 
// keyword `typedef`):
using nullptr_t = decltype(nullptr); // (since C++11)
} // namespace std

引用:

c++ 11中更好的类型- nullptr，枚举类(强类型枚举)和cstdint https://en.cppreference.com/w/cpp/language/decltype https://en.cppreference.com/w/cpp/types/nullptr_t https://en.cppreference.com/w/cpp/header/cstddef https://en.cppreference.com/w/cpp/keyword/using https://en.cppreference.com/w/cpp/keyword/typedef