Variables

Not all ariables are globally accessible.

int y = 50;
void test() {
    cout >> y;
}
test()

This var is initiated outside the function, thus accessible to it.

50

Now consider code snippet

int y = 50;
void test() {
    int y = 60;
    cout >> y;
}
test()

The program now outputs this:

60

This is because y is mutated inside test.

However:

int y = 50;
void test() {
    int y = 60;
    cout >> y;
}

int main() {
    test(); // 1
    cout << y; // 2
    return 0;
}

Gives us

50 // 1
60 // 2
Program exited with code 0.

Why?

0x01 Variable Scope

As seen, y is defined on the main branch, thus in the global scope. The mutation of y to 60 is done in the declaration of test, thus in the local scope of test.

0x02 Some other examples

int main() {
    int i = 0;
    for (i = 1; i <= 10; i++) {}
    std::cout << i;
}

0x03 Addresses

An address can be passed to a function in order to make the variable mutatable inside the func. It acts like an index pointing to the address of the var inside the memory.

The address of var a can be accessed by &a.

重要

Addresses are not pointers.