什么是nullptr？ [英] What exactly is nullptr?

问题描述

我们现在有C ++ 11有许多新的功能。一个有趣的和混乱的（至少对我来说）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.

讨厌的宏 NULL 。

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

我还是没有得到 nullptr 作品。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种情况，以作为一个区分的空指针常量：nullptr。它是类型nullptr_t ，它是隐式可转换的，并可与任何指针类型或指针成员类型相比。它不是隐式可转换的或可比的整数类型，除了bool。

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.

它是一个关键字和一个类型的实例？

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

此外，还有另一个例子（维基百科旁边），其中 nullptr c $ c> 0 ？

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

推荐答案

和类型的实例？

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

这并不奇怪。 true 和 false 是关键字和文字，它们有一个类型（ bool ）。 nullptr 是 std :: nullptr_t 类型的指针文字，它是一个prvalue你不能使用& ）的地址。关于指针转换的

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 c $ c> std :: nullptr_t 是一个空指针常量，一个整数空指针常量可以转换为 std :: nullptr_t 。不允许相反方向。这允许为指针和整数重载一个函数，并通过 nullptr 来选择指针版本。传递 NULL 或 0 会混淆地选择 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 到整型类型需要 reinterpret_cast ，并且具有与向整数类型（定义的映射实现）的（void *）0 的转换相同的语义。 reinterpret_cast 无法将 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） ）。

The Standard requires that sizeof(nullptr_t) be sizeof(void*).

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