Accessing protected static members of Superclass by Sub-classes in C++

Question

I have a question. If I have a static member in the superclass, how do I allow all sub-classes of this superclass access and use the static member.

Eg

/*Superclass*/
class Commands {
   protected:
            static Container database;
};

/*Sub class*/
class Add: public Commands {
   public:
            void add_floating_entry(std::string task_description);  
};

/*This gives me an error. add_floating_task is a method of the Container Class*/
void Add::add_floating_entry(string task_description)
{
   database.add_floating_task(task_description);
}

May I know what is wrong here? Thanks in advance!

EDIT:

The Container class is as follows

class Container {
private:
   vector<Task_Info*> calendar[13][32];
   vector<Task_Info*> task_list;
public:
   void add_floating_task(std::string task_description);
};

The error given is: "Use of undeclared identifier "database"

Answer 1

Define that static member out of the class declaration:

class Commands {
protected:
   static Container database; // <-- It's just a declration
};

Container Commands::database; // <-- You should make a definition
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The declaration of a static data member in its class definition is not a definition ... The definition for a static data member shall appear in a namespace scope enclosing the member's class definition.

Your way to make it protected is OK to make it accessible for derived classes.

Answer 2

Your code looks ok except missing definition of static Commands member database . You need to define database outside commands class

Container Commands::database;

§ 9.4.2 Static members

Static members obey the usual class member access rules (Clause 11) . When used in the declaration of a class member, the static specifier shall only be used in the member declarations that appear within the member-specification of the class definition.

As database is a protected member of base class Commands , derived class Add should be able to access it by :: operator or . operator from object.

Answer 3

Since static member is shared between all objects. Commands::database should be OK.