C++ Primer, Fifth Edition

5.2. Statement Scope

We can define variables inside the control structure of the if, switch, while, and for statements. Variables defined in the control structure are visible only within that statement and are out of scope after the statement ends:

while (int i = get_num()) // i is created and initialized on each iteration    cout << i << endl;i = 0;  // error: i is not accessible outside the loop

If we need access to the control variable, then that variable must be defined outside the statement:

// find the first negative elementauto beg = v.begin();while (beg != v.end() && *beg >= 0)    ++beg;if (beg == v.end())    // we know that all elements in v are greater than or equal to zero

The value of an object defined in a control structure is used ...