nullptr 究竟是什么? [英] What exactly is nullptr?

问题描述

我们现在拥有具有许多新功能的 C++11.一个有趣且令人困惑的(至少对我而言)是新的 nullptr.

We now have C++11 with many new features. An interesting and confusing one (at least for me) is the new nullptr.

好吧，不再需要讨厌的宏 NULL.

Well, no need anymore for the nasty macro NULL.

int* x = nullptr;
myclass* obj = nullptr;

仍然，我不明白 nullptr 是如何工作的.例如，维基百科文章说:

Still, I am not getting how nullptr works. For example, Wikipedia article says:

C++11 通过引入一个新的关键字来纠正这个问题，作为一个特殊的空指针常量:nullptr.它是 type nullptr_t，可以隐式转换，并且可以与任何指针类型或指向成员的指针类型相比较.除了 bool 之外，它不能隐式转换或与整型比较.

C++11 corrects this by introducing a new keyword to serve as a distinguished null pointer constant: nullptr. It is of type nullptr_t, which is implicitly convertible and comparable to any pointer type or pointer-to-member type. It is not implicitly convertible or comparable to integral types, except for bool.

关键字和类型的实例如何?

How is it a keyword and an instance of a type?

另外，你有没有另一个例子(除了维基百科的那个)，其中 nullptr 优于旧的 0?

Also, do you have another example (beside the Wikipedia one) where nullptr is superior to good old 0?

推荐答案

关键字和类型的实例如何?

How is it a keyword and an instance of a type?

这并不奇怪.true 和 false 都是关键字，作为文字，它们有一个类型( bool ).nullptr 是 std::nullptr_t 类型的指针文字，它是一个纯右值(你不能使用 &).

This isn't surprising. Both true and false are keywords and as literals they have a type ( bool ). nullptr is a pointer literal of type std::nullptr_t, and it's a prvalue (you cannot take the address of it using &).

• 4.10 关于指针转换说std::nullptr_t 类型的纯右值是空指针常量，可以转换整型空指针常量到 std::nullptr_t.不允许相反方向.这允许为指针和整数重载函数，并传递 nullptr 以选择指针版本.传递 NULL 或 0 会混淆选择 int 版本.

• 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.

nullptr_t 到整数类型的转换需要一个 reinterpret_cast，并且与 (void*)0<的转换具有相同的语义/code> 到整数类型(定义映射实现).reinterpret_cast 不能将 nullptr_t 转换为任何指针类型.如果可能，依靠隐式转换或使用 static_cast.

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.

标准要求 sizeof(nullptr_t) 为 sizeof(void*).

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