Why does redefining string str doesn't show error?

Question

int N;
cin >> N;

for (int i = 0; i < N; i++) 
{
//here string is redefined per loop cycle, still no error 


    string str;
    cin >> str;


//ignore code below 

    for (int j = 0; j < str.length(); j++) 
    {
        if (j % 2 == 0)
            cout << str[j];
    }
    cout << " ";
    for (int j = 0; j < str.length(); j++) 
    {
        if (j % 2 != 0)
        {
            cout << str[j];
        }
    }

    cout << endl;
}

return 0;

Answer 1

It sounds like you're asking why this (cut down version) is valid:

for (int i = 0; i < N; i++)  {
    string str;
}

when the string is being redefined every time through the loop.

However, that's perfectly acceptable. The lifetime of the string object extends from immediately after the semicolon following str to the closing brace } , which is part of the loop itself. Hence it effectively comes into, and goes out of, existence for every single iteration of that loop ^(a) .

It's no different really to the snippet:

{
    int xyzzy = 42; // xyzzy now exists.
}                   // and no longer exists from here on.

---

^(a) Technically, using the as-if rule, it could reuse the same object, provided it was cleared out to an empty string each time.