checking a function was called or not

Let's say I have following code:

// I'll be providing these 3
int a = 0;
void setA(){ a = 1000;}
void processA() { std::cout << a; }

// User will call them
int main(){
    // setA();    // <-- Notify about this
    processA();
    return 0;
}

Here I must make sure that someone has called setA() before calling processA(). I've thought about 2 ways to do this:

• call setA() just inside processA() function or just dump the contents of setA() inside processA()
• maintain a boolean flag and check at later state to make sure setA() has been called

Now 1st method makes sense but the problem is setA() function would be called a lot earlier than processA() and I just cannot do this. 2nd way is fine but is runtime dependent.

Is there a way to make this work in compile time, so that there is a message/warning or a compiler warning would be the best way to alert user that he/she hasn't called setA() before processing it.