Why objects are not used to acces the static class members?

Answer 1

Static members are members of the class, not an instance of the class.

All instances of a class have access to all their static members. Derived classes have access to both protected and public statics, while public statics are fully accessible.

Think of static members as being global variables or functions, but scoped to the class rather than being globally accessible (and therefore publicly accessible).

Consider the following example. A private static member variable, count, is declared. Note that it must be initialised immediately after the class declaration, but outside of a code block. Were it declared public it could be initialised in a code block, but the class needs to maintain control over it, thus it is completely hidden from all outside influence.

The two constructors increment count while the destructor decrements count. Thus count maintains a running total of all the instances of the class (much like a reference counter). Note that all instances of the class have direct access to the static member. Note also that if other parametrised constructors are declared, each must increment countindependently.

The public static accessor method, GetCount(), returns the current value of count (returning by value, not by reference). There is no equivalent set mutator, thus countis a read-only variable -- it cannot be altered from outside of the class.

#include

class MyClass{

public:

MyClass(){ ++count; }

MyClass(const MyClass & copy){ ++count; }

~MyClass(){ --count; }

public:

static unsigned int GetCount(){ return( count ); }

private:

static unsigned int count;

};

// Initialise the static member:

unsigned int MyClass::count = 0;

void PrintCount()

{

printf( "There %s currently %u instance%s of MyClass.\n",

MyClass::GetCount() 1 ? "" : "s" );

}

int main()

{

MyClass * myClass[10];

// At this point there are no instances.

PrintCount();

printf( "\nCreating instances of MyClass...\n" );

for( int i=0; i<10; ++i )

{

myClass[i] = new MyClass();

PrintCount();

}

printf( "\nDestroying instances of MyClass...\n" );

for( int i=0; i<10; ++i )

{

delete( myClass[i] );

PrintCount();

}

return( 0 );

}

Answer 2

When a method is static it can be called without instantiating the class of which it belongs. This means that it is possible to call the method before the class it belongs to has been created.