C++ Tutorial/Class/static instance variable

Refer to static variable through its class name

<source lang="cpp">#include <iostream> using namespace std;

class MyClass { public:

 static int num; 
 void display() { cout << num << endl; } 

};

int MyClass::num; // define num

int main() {

 MyClass a, b; 

 // Set num through its class name. 
 MyClass::num = 100; 

 a.display();
 b.display();

 // Set num through an object. 
 a.num = 200; 

 a.display();
 b.display();

 return 0; 

}</source>

100
100
200
200

static data members

<source lang="cpp">#include <iostream>

class MyClass
{
public:
    MyClass(int age = 1):intValue(age){
       InstanceCount++; 
    }
    virtual ~MyClass() { 
       InstanceCount--; 
    }
    virtual int getValue() { 
       return intValue; 
    }
    virtual void setValue(int age) { 
       intValue = age; 
    }
    static int InstanceCount;

private:
    int intValue;

};

int MyClass::InstanceCount = 0;

int main()
{
    const int count = 5;
    MyClass *MyClassHouse[count];
    int i;
    for (i = 0; i<count; i++)
        MyClassHouse[i] = new MyClass(i);

    for (i = 0; i<count; i++)
    {
        std::cout << "There are ";
        std::cout << MyClass::InstanceCount;
        std::cout << " left!\n";
        std::cout << "Deleting the one which is ";
        std::cout << MyClassHouse[i]->getValue();
        delete MyClassHouse[i];
        MyClassHouse[i] = 0;
    }
    return 0;
}</source>

There are 5 left!
Deleting the one which is 0There are 4 left!
Deleting the one which is 1There are 3 left!
Deleting the one which is 2There are 2 left!
Deleting the one which is 3There are 1 left!
Deleting the one which is 4

Use a static instance variable

<source lang="cpp">#include <iostream> using namespace std;

class MyClass {

 static int num; 

public:

 void setI(int i) { num = i; }; 
 void display() { cout << num << " "; } 

};

int MyClass::num; // define num

int main() {

 MyClass a, b; 

 a.display(); // prints 0 
 b.display(); // prints 0 

 a.setI(10); // set static num to 10 

 a.display(); // prints 10 
 b.display(); // also prints 10 

 return 0; 

}</source>

0 0 10 10