When should a variable be declared in the header file?

Question

I have this class with a lot of functions (around 30) and each function declares a new integer value that is used to store the value from the conversion of a string from the stack to an integer. This means I keep declaring new integers so many times in the same class.

Here is an example of a function that declares these integers:

void Interpreter::add()
{
    val1 = std::stoi(stack.back());
    stack.pop_back();
    val2 = std::stoi(stack.back());
    stack.pop_back();

    stack.push_back(std::to_string(val1 + val2));
}

class Interpreter 
{
private:
    int val1, val2;

}

My question is: is it better to declare these variables only once in the header file and then re-use them in each function?

I'd like to know if there is some kind of convention on this matter.

Answer 1

Don't use member variables just to save on typing – you're going to encounter bugs that are incredibly hard to find.

Add a little abstraction,

int Interpreter::pop() 
{ 
    // Add error handling here.
    int i = std::stoi(stack.back());
    stack.pop_back();
    return i;
}

void Interpreter::push(int i)
{
    stack.push_back(std::to_string(i));
}

and then you can write

void Interpreter::add()
{
    push(pop() + pop());
}