Question

I hope that I am providing enough information and that I’ve titled this correctly.

In general, I want to have a class that stores data in my application, and I need several other classes to access the same data. essentially sharing the data between multiple classes.

Short/Concise Code follows:

example.cpp (main application)

// example.cpp : Defines the entry point for the console application.
//
#include "AnotherClass.h"
#include "ObjectClass.h"
#include "stdafx.h"
#include <iostream>  
#include <iomanip>

using namespace std;  
//Prototype
static void do_example ( );

int main()  

{  
    do_example ( );
}

static void do_example ( ) {


    MyObject.a = 5;
    cout <<"MyObject.a value set in main application is "<<MyObject.a<<"\n";

    AnotherClass    m_AnotherClass;

    m_AnotherClass.PrintValue();

}       

ObjectClass.h

class ObjectClass {
public:
    ObjectClass(); // Constructor
    int a; // Public variable
} static MyObject;

ObjecClass.cpp

#include "ObjectClass.h"

ObjectClass::ObjectClass() {

    a = 0;

}

AnotherClass.h

class AnotherClass {
public:
    AnotherClass(); // Constructor

    void PrintValue(); // Public function
    int value; // Public variable

};

AnotherClass.cpp

#include "AnotherClass.h"
#include "ObjectClass.h"
#include "stdafx.h"
#include <iostream>  
#include <iomanip>

using namespace std; 

AnotherClass::AnotherClass() {

    value = MyObject.a;

}

void AnotherClass::PrintValue() {

    cout <<"value in AnotherClass is "<<value<<"\n";
    cout <<"it should be the same."<<"\n";
}

But the value is the default value of 0, as if it is a new instance of MyObject. But it should be pulling the value of 5 from the static MyObject.

What am I missing?

Answer 1

A static class instance is a static variable itself. What you expect to happen also makes sense, however, your code does not show how the static instance is handled. In fact, if both MyClassInstances refer to the same object, then you don’t even need static declaration.

Also, static variables are defined in cpp files. If you define it in a header, the cpp file (compilation unit) that includes it will define a separate static variable. So, define the static object in the main cpp file and use extern MyStaticClass in the header file. The linker will then link the uses to the same variable.