Question

I’m pretty new to C++ language. I’m using it with Qt to create a cross-platform user interface.
The user interface will show different graphs and plots by using the same data. So I would like to have my data stored in a class (Data_Class) accessible from all the other classes.
In the data class, I am using private static variables and public static methods (get/set).

What is not clear to me is how to access the data class from the other classes. In other words, how to get data values in Class_B (from Data_Class) after I write them in Class_A (to Data_Class).

Answer 1

Assuming that this is syntax an oriented question:

class Data_Class {
public:
    static int a;
    static int GetA() const { return a; }
}

In ClassA:

Data_Class::a;
Data_Class::GetA();

Don’t forget to add Data_Class static members definitions, for example into dataclass.cpp:

static int Data_Class::a = 0;

Otherwise you’ll get an undefined reference linker error.

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You also may consider using singleton pattern (according to comments, singleton is bad idea and you should avoid it if possible) as Aubin suggested which will take care of adding new static member definitions:

class Data_Class {
private:
    Data_Class();  // Prevents multiple instances

    int a;
    int b;

public:
    // Will initialize instance if needed
    static Data_Class &GetInstance(){
       static Data_Class instance;
       return instance;
    }


    int GetA() const {return a;}
};

Usage:

Data_Class::GetInstance().GetA()

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Or go with little more complex but more flexible Dependency injection which is according to wiki:

Dependency injection is a software design pattern that allows a choice
of component to be made at run-time rather than compile time. This can
be used, for example, as a simple way to load plugins dynamically or
to choose mock objects in test environments vs. real objects in
production environments. This software design pattern injects the
dependent element (object or value etc) to the destination
automatically by knowing the requirement of the destination.