const 指针的 C++ typedef 解释 [英] C++ typedef interpretation of const pointers

问题描述

首先，示例代码:

案例一:

typedef char* CHARS;
typedef CHARS const CPTR;   // constant pointer to chars

文字替换 CHARS 变为:

Textually replacing CHARS becomes:

typedef char* const CPTR;   // still a constant pointer to chars

案例 2:

typedef char* CHARS;
typedef const CHARS CPTR;   // constant pointer to chars

文字替换 CHARS 变为:

Textually replacing CHARS becomes:

typedef const char* CPTR;   // pointer to constant chars

在案例 2 中，在文本替换 CHARS 后，typedef 的含义发生了变化.为什么会这样?C++ 如何解释这个定义?

In case 2, after textually replacing CHARS, the meaning of the typedef changed. Why is this so? How does C++ interpret this definition?

推荐答案

在文本替换的基础上分析 typedef 行为是没有意义的.Typedef 名称不是宏，它们不会被文本替换.

There's no point in analyzing typedef behavior on the basis of textual replacement. Typedef-names are not macros, they are not replaced textually.

正如你自己所说的

typedef CHARS const CPTR;

和

typedef const CHARS CPTR;

出于同样的原因

typedef const int CI;

与

typedef int const CI;

Typedef-name 不定义新类型(仅是现有类型的别名)，但从某种意义上说，它们是原子的"，任何限定符(如 const)都适用于最顶层，即它们适用于隐藏在typedef-name 后面的整个 类型.一旦定义了 typedef-name，就不能在其中注入"限定符，以便修改类型的任何更深层次.

Typedef-name don't define new types (only aliases to existing ones), but they are "atomic" in a sense that any qualifiers (like const) apply at the very top level, i.e. they apply to the entire type hidden behind the typedef-name. Once you defined a typedef-name, you can't "inject" a qualifier into it so that it would modify any deeper levels of the type.

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