Declaring two global variables of same name in C

问题

I have declared two global variables of same name in C. It should give error as we cannot declare same name variables in same storage class.

I have checked it in C++ — it gives a compile time error, but not in C. Why?

Following is the code:

int a;
int a = 25;
int main()
{

   return 0;
}

Check it out at : Code Written at Ideone

I think this probably is the reason

Declaration and Definition in C

But this is not the case in C++. I think in C++, whether the variable is declared at global scope or auto scope the declaration and definition is happening at the same time.

Could anyone throw some more light on it.

Now when I define the variable two times giving it value two times it gives me error (instead of one declaration and one definition).

Code at : Two definitions now

int a;
int a;
int a;
int a = 25;

int main()
{
return 0;
}

回答1:

In C, multiple global variables are "merged" into one. So you have indeed just one global variable, declared multiple times. This goes back to a time when extern wasn't needed (or possibly didn't exist - not quite sure) in C.

In other words, this is valid in C for historical reasons so that we can still compile code written before there was a ANSI standard for C.

Although, to allow the code to be used in C++, I would suggest avoiding it.

来源：https://stackoverflow.com/questions/17388431/declaring-two-global-variables-of-same-name-in-c