是否有任何主要的C ++实现实际上将NULL定义为nullptr? [英] Does any major C++ implementation actually define `NULL` as `nullptr`?

问题描述

自C ++ 11起，标准允许宏NULL可以是值为零的整数文字或类型为std::nullptr_t的prvalue.

Since C++11, the Standard allows the macro NULL to either be a integer literal with value zero, or a prvalue of type std::nullptr_t.

任何标准库供应商决定将其NULL的定义从整数更改为nullptr，很可能会导致依赖C ++ 11之前代码的客户端损坏.

Any Standard Library vendor deciding to change their definition of NULL from an integer to nullptr would very likely cause breakage for clients relying on pre-C++11 code.

是否有任何主要的实现方式(例如，GCC，Clang，MSVC)将NULL定义为nullptr??或者，尽管有可能，没有人这样做吗?

Does any major implementation (e.g. GCC, Clang, MSVC) actually define NULL as nullptr? Or, despite the possibility, does nobody do this?

推荐答案

libstdc++ 有关__null的信息可以在此问题中找到:

Information about __null can be found in this question:

__null的实现是作为G ++内部实现的.基本上，内部程序会执行您reinterpret_cast<void *>(0)天真的期望.

The implementation of __null is as a G++ internal. Basically, the internal does what you would naively expect reinterpret_cast<void *>(0) to do.

其类型为魔术"，具体取决于上下文.这就是G ++必须将其作为内部实现的原因.没有常规类型提供精确正确的语义.它的行为大致类似于void *，但不完全相同.

Its type is 'magic', depending on context. That's the reason G++ had to implement it as an internal. No regular type provides the precisely-correct semantics. It acts roughly like void *, but not exactly.

---

libc++ 做几乎相同的事情:

// <cstddef>
//
// Don't include our own <stddef.h>; we don't want to declare ::nullptr_t.
#include_next <stddef.h>
#include <__nullptr>

---

Microsoft的STL也依赖于包含stddef.h :

---

Microsoft's STL also relies on including stddef.h:

#pragma once
#ifndef _CSTDDEF_
#define _CSTDDEF_
#include <yvals_core.h>
#if _STL_COMPILER_PREPROCESSOR

#include <stddef.h>

我在开源STL存储库中找不到stddef.h，但是vcruntime.h中提供了NULL的定义:

I could not find stddef.h in the open-source STL repository, but a definition of NULL is provided in vcruntime.h:

#ifndef NULL
    #ifdef __cplusplus
        #define NULL 0
    #else
        #define NULL ((void *)0)
    #endif
#endif

---

对icc 19.0.1的简单测试还显示NULL没有定义为nullptr:

---

A simple test on icc 19.0.1 also shows that NULL is not defined as nullptr:

#include <type_traits>
#include <cstddef>

static_assert(!std::is_same<decltype(NULL), std::nullptr_t>::value, "");
static_assert(!std::is_same<decltype(NULL), int>::value, "");
static_assert(std::is_same<decltype(NULL), long>::value, "");

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