C / C ++中的typedef是否真的通过组合复合类型（例如int *）来创建NEW类型？ [英] Does typedef in C/C++ really create a NEW type by combining the compound type (e.g. int*)?

问题描述

最近，我发现代码中的 typedef 的工作原理与我提出的完全不同。像这样的示例：

recently I found typedef in my code works really different than what I proposed. An example like this:

typedef int *ptype;
ptype v1 = 0, v2 = 0;

结果：v1和v2均被定义为指向 int 类型。但是，如果您在第二句中将 ptype 替换为 int * ，则为 int * v1 = 0，v2 = 0; 或 int * v1 = 0，v2 = 0; ，只有v1将是指针，而v2是正常的 int 。看来 typedef 并不能做简单的替换。更重要的是，使修饰符复杂化，例如：

The result: both v1 and v2 were defined as a pointer to int type. But if you simply replace ptype by int * in the second sentence as int *v1 = 0, v2 = 0; or int* v1 = 0, v2 =0;, only v1 will be the pointer and v2 is normal int. It seems typedef does not do a simple replacement. What's more, when it comes to complicate modifier like:

typedef int *ptype;
const ptype v3 = 0;

结果将是：v3是 const 指针，而不是指向 const int的指针，如果我们编写 const int * v3 = 0; 。在上面的代码中， const 是整个 ptype 而不是 int <的修饰符。 / code>位于 ptype 中。因此，看起来真的像 typedef 组合了复合类型 int * 并创建了一个新类型。

The result will be: v3 is a const pointer, not a pointer to the const int if we write const int *v3 = 0;. In the above code const stands as the modifier to the whole ptype, not the int inside ptype. Thus it really looks like typedef combines the compound type int* and creates a new type.

但是，权威的C / C ++参考网站 cplusplus 说 typedef不会创建不同的类型。它只会创建现有类型的同义词。所以我真的很困惑，希望有人可以帮助解释 typedef 的行为。谢谢！

However, the authoritative C/C++ reference website cplusplus says "typedef does not create different types. It only creates synonyms of existing types." so I was really confused and hope someone can help explain the behavior of typedef. Thanks!

推荐答案

同义词并不表示文本替换。 ptype 并没有被预处理器或任何东西扩展为 int * 。

"Synonym" does not mean "text replacement". ptype is not literally expanded to int * by the preprocessor or anything.

这意味着您可以执行以下操作：

What it means is that you can do things like:

typedef int *ptype;
ptype a;
int *b;

a = b;   // OK

该分配有效，因为 ptype 和 int * 是同一类型；

The assignment is valid because ptype and int * are the same type; there is no type conversion or cast required.

typedef 只是让您为现有类型命名。但是该名称将现有类型的各个方面组合成一个不可分割的实体，例如 ptype a，b; 等同于 ptype a; ptype b; （和 const ptype 的意思是 const指向int的指针，因为 ptype 表示指向int的指针。

typedef simply lets you give a new name to an existing type. But that name combines every aspect of the existing type into an indivisible entity, such that e.g. ptype a, b; is equivalent to ptype a; ptype b; (and const ptype means "const pointer-to-int" because ptype means "pointer-to-int").

换句话说，由 typedef <创建的新名称 / code>的行为就像内置关键字一样，但声明这些名称表示的实际类型相同。

In other words, the new names created by typedef behave like built-in keywords as far as declaring things goes, but the actual types represented by those names are the same.

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