我怎样才能拥有一个随时间持续存在但不在同一个 class 的所有成员之间共享的局部变量？

Question

I am programming c++ in Unreal Engine. I have some functions that use variables whose value I need to last in time and that are only used within that function. The problem is that if I declare them as static local variables, when I have several members of the same class they all share the same value, and I would like each class to have its own instance of a local variable that lasts over time. How can I achieve this?

Answer 1

If you really want that level of encapsulation, you could make a separate class for each of your member variables and put the function(s) that operate on that variable in that class. You can then inherit from any number of those separate classes to build a class that has several variables.

Example:

struct func_x {
    func_x(int X) : x{X} {}

    int function_using_x() {
        return x;
    }

private:
    int x; // only one member variable
};

struct func_y {
    func_y(int Y) : y{Y} {}

    int function_using_y() {
        return y;
    }
    
private:
    int y; // only one member variable
};


struct foo : func_x, func_y { // inherit from both
    foo(int x, int y) : func_x(x), func_y(y) {}
};

Usage:

#include <iostream>

int main() {
    foo f{1, 2};
    std::cout << f.function_using_x() << f.function_using_y() << '\n'; // 12
}

Answer 2

you need 2 variables for that. one for counter (declare local but nut static) and change in one instance and another static for following the first variable;

but notice every time you need that static you should update it