定义和变量声明与某个值 [英] Definition and declaration of variable with a value

问题描述

只是 K＆放大器开始; - [R 并在第2章，有一行：

Just started with K & R and on the 2nd chapter, there is the line:

声明列出变量是
  使用和他们有什么类型的国家和
  也许什么初始值

所以：

INT X = 42 是定义的

和 INT X 是声明的也是一个的定义的，因为每一个定义的是的声明的

and int x is a declaration but also a definition since every definition is a declaration.

但是，当我们分配如 K＆安培的初值; - [R 说，不，使的声明的一个的定义的？

But when we assign an intial value like K & R say, doesn't that make the declaration a definition?

推荐答案

您混淆了两件事情：

1. 的声明状态（声明的）什么对象的*类型，名称和范围是


2. 的定义的定义的什么对象的内容

1. A declaration states (declares) what an object's* type, name and scope is
2. A definition defines what a object's content is

*对象为：变量，函数等等，而不是一个面向对象的对象

* object as in: variable, function etc., not an OOP object.

一个定义，因此很多时候也是一个声明，因为你不能定义什么是一个对象，当你没有说明对象的类型是什么。最容易记住仅仅是：每一个定义是一个声明，但并不是每一个声明是一个定义

A definition is hence very often also a declaration, since you cannot define what is in an object when you do not state what the type of the object is. Easiest to remember is just: "Every definition is a declaration, but not every declaration is a definition"

对于变量

有只有1个没有定义的变量声明方式：

There is only 1 way to declare without defining the variable:

extern typeX variable_name

这告诉编译器，有一个叫VARIABLE_NAME类型而TYPEx变量，而不是在哪里得到它。
每一个其他的方式来声明一个变量也是一个定义，因为它告诉编译器预留空间，这或许能给它一个初始值。

This tells the compiler that there is a variable called variable_name with type typeX, but not where to get it. Every other way to declare a variable is also a definition, since it tells the compiler to reserve space for it and perhaps give it an initial value.

不同的是在结构和功能更加清晰：

The difference is much clearer in structs and functions:

对于结构

一个声明：

struct some_struct{
    int a;
    int b;
}

此声明some_struct编译器A和B既是int型的结构变量。

This declares some_struct to the compiler with a and b as struct variables both with type int.

只有当你定义它们的空间被保留，并且可以使用它们：

Only when you define them space is reserved and you can use them:

void foo(){
    struct some_struct s;
    s.a = 1; // For this to work s needs to be defined
}

对于函数

不同的是更清楚

声明：

// This tells the compiler that there is a function called "foo" that returns void and takes void arguments
void foo();

一个定义可能是像上面那样（在结构中的一部分）

A definition could be like the one above (in the struct part)

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