5.2 Scope and Storage

Scope describes where a name can be used. A variable declared inside a function is local to that function.

int add(int a, int b) {
    int total = a + b;
    return total;
}

int main() {
    // total is not visible here
    return 0;
}

Parameters are also local variables. In C++, ordinary parameters are passed by value, which means the function receives copies of the arguments.

void change(int x) {
    x = 100;
}

int main() {
    int number = 5;
    change(number);
    std::cout << number << std::endl;  // still 5
    return 0;
}

This is why a simple swap function does not change the caller's variables when the parameters are passed by value.

void swap_values(int a, int b) {
    int temp = a;
    a = b;
    b = temp;
}

To change a caller's variable directly, C++ commonly uses references. You will see them more later, but the shape is worth recognizing now.

void swap_values(int& a, int& b) {
    int temp = a;
    a = b;
    b = temp;
}

A static local variable is initialized once and keeps its value between function calls.

void count_calls() {
    static int count = 0;
    count++;
    std::cout << count << std::endl;
}

Use local variables by default. They make programs easier to reason about because each function owns its temporary work.