为什么我不能多声明一个类 [英] Why can't I multi-declare a class

问题描述

我可以这样做

  extern int i; 
 extern int i; 
  

但我不能对类进行同样的操作

  A类{
 .. 
} 
 A类{
 .. 
} 
 
 
 
 
 >解决方案
以下是声明：
  extern int i; 
 A类; 
  
接下来两个是定义：
  int i; 
 class A {...} 
  
规则是：
 
 
 
 
 
定义也是一个声明。 
 
 
您必须先查看项目的声明，才能使用它。 
 
 
重新声明正确（必须相同）。
 
 
重新定义是错误（单一定义规则）。
 
 
 
I can do this 
extern int i;
extern int i;
But I can't do the same with a class
class A {
..
}
class A {
..
}
While in both cases no memory is being allocated.
解决方案
The following are declarations:
extern int i;
class A;
And the next two are  definitions:
int i;
class A { ... }
The rules are: 


a definition is also a declaration.  
you have to have 'seen' a declaration of an item before you can use it. 
re-declaration is OK (must be identical).
re-definition is an error (the One Definition Rule).


                        
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