Why doesn't the C++11 compiler complain when I initialize a local variable with the same name multiple times within the same scope?

Question

So if I initialize a variable of the same type and name within the main function the compiler will complain of the re declaration, for example:

for (int i = 0; i < 5; i++)
{
    int a{5};
    int a{5}; // error int a previously declared here
}

But there are no errors if I compile this instead:

for (int i = 0; i < 5; i++)
{
    int a{5};
}

To my understanding, this looks as if I have initialized "int a" multiple times within the same scope which obviously would cause an error. If somebody could explain what's actually happening it would be greatly appreciated! Thanks!

Answer 1

You are confusing scope and lifetime, it seems.

 for (int i = 0; i < 5; i++) { int a{5}; } 

To my understanding, this looks as if I have initialized "int a" multiple times within the same scope which obviously would cause an error.

Why would that obviously cause an error? a is always the same variable , in every iteration of the loop. But it always points to a different object , whose lifetime begins and ends with the current iteration.

If such a thing would not work, then you could not even call the same function two times:

void f()
{
    int x = 0;
}

f();
f(); // why should this be an error?

Note that this question goes far beyond g++, and even far beyond C++. Every C-derived programming language works exactly like that. At least I've never seen one which doesn't.

Answer 2

To my understanding, this looks as if I have initialized "int a" multiple times within the same scope

You are right about the "multiple times" part, but you are wrong about "same scope": the scope of the second int a is nested inside the outer scope, in which the first int a is declared. Hence, the second declaration is legal: it hides the outer declaration, which the language allows.

int a = 5;
cout << a << endl;
{
    int a = 6;
    cout << a << endl;
}
cout << a << endl;

Demo above produces this output:

5
6
5

Answer 3

I think your getting confused with declaring variable and initializing it

for (int i = 0; i < 5; i++)
{
    int a{5}; // In this part you are declaring and initializing the variable
    int a{5}; // error int a previously declared here
    // same with this case as well
}

If you want to do that you can use this

 for (int i = 0; i < 5; i++)
    {
         int a{5}; 
         a=5; // error int a previously declared here

     }

at this point int a{5} This will be the declaration of a variable and initialization of variable in the for loop scope

and a=5; and here reinitialize that particular variable in the same scope