What are the features of static data members in c?

Answer 1

A non-static member function has a hidden argument called the this-pointer which points to the data of the specific instance of the class. Static member functions can be called without reference to a specific instance of the class so it is completely what instance if any you mean. You do this by treating them as functions in a namespace with the same name as your class.

Here's an illustrative example:

class CPerson

{ private:

int m_Age; static int m_NumPeople = 0;

public: void SetAge( int Age ){ this->m_Age = Age }; // Correct, if you omit "this->" the compiler will still infer its existance.

CPerson( void ){ m_NumPeople++ };

~CPerson( void ){ m_NumPeople-- };

static int GetAge( void ){ return this->m_Age }; // Wrong, this-> is ambiguous for static functions.

static int GetPeople( void ){ return m_NumPeople}; // Correct, people count is static.

};

There is only one integer m_NumPeople, it behaves like a global variable but the compiler will only let member functions access it since it is private.

m_NumPeople is created with a default value of zero before you even instantiate your first CPerson. As you instantiate CPersons the constructor is called when each instance is created and the instance counter named m_NumPeople is incremented. As these instances are destroyed the destructor decrements m_NumPeople.

Since GetPeople is static you can call it without having access to any instance of the class like so: CPerson::GetPeople();

The compiler has no mechanism to infer which object you called the static function on or even guarantee that the object has ever been instantiated, therefor CPerson::GetAge() is ambiguous and the compiler spits out an error. CPerson::SetAge( 42) is now allowed either. SetAge needs a this-pointer to find the correct instance of m_Age to set.

When you have some instance of the object you use the . operator or -> operator like so: CPerson Nicholas( ); Nicholas.SetAge( 42 ); Now the compiler can uniquely identify the CPerson in question as Nicholas. It will pass the pointer &Nicholas to the SetAge function. Calling conventions vary, but if you're using microsofts visual studio compiler and compiling 32-bit code it will pass the this pointer in the ECX register and the function parameters will be pushed onto the stack in reverse order(but in this case there's only one).

Answer 2

In C++, a static data member of a class is a member that is shared, or common, between all instances of that class.

A static member function is a function that can only access static data members. It does not have a this parameter, and cannot therefore locate the instance data.

Static data and function members can be used to track class specific things, such as statistics for all instances of a class, or to provide class specific things, such as custom allocators.

Answer 3

C does not support static data members. You are probably talking about C++ static data members. These are data members that are local to the class in which they are declared, rather than local to objects of the class as per non-static data members.

Answer 4

static is not a class it is a storage class which is used to declare a local-scope variable.