在枚举中重新声明变量

Question

In C, If we re-declare variable inside enum , then compiler gives an error that "'i' redeclared as different kind of symbol".It Ok.

#include <stdio.h>

int i = 10;

struct S 
{ 
    enum 
    {
        i = 20
    }e; 
};

int main()
{
    printf("%d\n", i);
}

But, In C++, If we redeclare variable inside enum, then it's working fine.

#include <iostream>
using namespace std;

int i = 10;

struct S 
{ 
    enum 
    {
        i = 20
    }e; 
};

int main()
{
    cout<<i<<endl;
}

I don't understand, Why doesn't C++ compiler gives an error for redeclaration variable?

Answer 1

It doesn't give a re-declaration error because the enumerator is introduced into class scope. Recall that a struct and class are mostly interchangeable in C++. The scope of S contains the enumerator i .

In C however, struct S doesn't define a scope. There are only 4 types of scope in C: function, file, block, and function prototype. As such, i is introduced into file scope where the variable i is already defined.