重新声明错误 [英] Redeclaration Error

问题描述

我了解声明和定义之间的区别 当我遇到疑问时，我正在练习一些问题，以下代码要求我列出代码段中的错误.

I have understood the difference between declaration and definition And I was practicing some question when I hit the doubt, the below code asked me to list out the error in the snippet.

f(int a,int b)
{
     int a;
     a=20;
     return a; 
}

为什么这会给出a的重新声明错误? 它不应该给出a的多个定义，因为在:

Why does this gives re-declaration error of a? Shouldn't it give multiple definition of a because in:

• f(int a,int b) —在这里a定义正确吗?
• 在函数正文中，是否再次定义了int a?

• f(int a,int b) — here a is defined right?
• and in the function body, int a is defined again?

那为什么不定义多个错误呢?

So why not multiple definition error?

推荐答案

定义始终是声明.区别在于，定义还可以为您声明的值提供任何价值.

A definition is always a declaration. The difference is that a definition also gives whatever you declare some value.

在您的示例中， 只是一个重新声明错误:

In your example, by the way, it is only a redeclaration error:

f(int a, /* Defines a */
int b)
{
     int a; /* Declares a - error! */
     a=20; /* initializes a */
     return a; 
}

您可能打算这样做:

f(int a, /* Defines a */
int b)
{
     int a = 20; /* Declares and defines a - error! */
     return a; 
}

但是在这种情况下，大多数编译器也会抛出重新声明"错误.例如，GCC会引发以下错误:

But in this case, most compilers will throw a "redeclaration" error too. For example, GCC throws the following error:

错误:"a"被重新声明为另一种符号

Error: 'a' redeclared as a different kind of symbol

这是因为a最初是作为参数定义的，与函数范围内的变量定义不同.正如编译器看到的那样，您正在重新声明与新声明具有不同品种"的东西，因此，无论您的非法声明是否为定义，它都不会在意，因为它在功能参数和功能局部变量方面对定义"的看法有所不同.

That is because a is originally defined as a parameter, which is different from a variable definition inside the function's scope. As the compiler sees that you're re-declaring something that is of a different "breed" than your new declaration, it can't care less if your illegal declaration is a definition or not, because it regards "definition" differently in terms of function parameters and function local variables.

但是，如果您这样做:

int c = 20;
int c = 20;

例如，

GCC 会引发 redefinition 错误，因为这两个c -s都是函数的局部变量.

GCC, for example, throws a redefinition error, because both c-s are the function's local variables.

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