enforce order of function calls?

Question

Say I have a abstract base class and I want to have a pure virtual method which must be implemented by the derived class but I want to make sure that the derived method calls functions in a particular order what could I do to enforce it ?

IE

base class
virtual void doABC()=0;
virtual void A()=0;
virtual void B()=0;
virtual void C()=0;


// must call ABC in the correct order 
derived class public base
void doABC();

This is just so I have a better understanding on how to design my classes to enforce someone to use my class correctly.

Answer 1

You're looking for the template method pattern :

http://en.wikipedia.org/wiki/Template_method_pattern

Something along these lines:

class BaseAlgorithm
{
protected:
    virtual void firstStep() = 0;
    virtual void secondStep() = 0;
    virtual void finalStep() = 0;
public:
    void runAlgorithm()
    {
        firstStep();
        secondStep();
        finalStep();
    }
};

class ConcreteAlgorithm : BaseAlgorithm
{
    virtual void firstStep() {};
    virtual void secondStep() {};
    virtual void finalStep() {};
};

You basically force extending classes to implement all intermediate steps, but you keep them protected or private - document this - and only call runAlgorithm() , which ties the smaller pieces together.

Answer 2

There are actually two particular ways, depending on whether you go with inheritance or parameterization.

If you with inheritance, it is the Template Method pattern:

class Base {
public:
    void doit() {
        this->do1();
        this->do2();
    }
private:
    virtual void do1() = 0;
    virtual void do2() = 0;
};

And if you go with parameterization, it is the Strategy pattern:

class Strategy {
public:
    virtual void do1() = 0;
    virtual void do2() = 0;
};

void doit(Strategy& s) {
    s.do1();
    s.do2();
}

From the website:

Strategy is like Template Method except in its granularity. [Coplien, C++ Report, Mar 96, p88]

Template Method uses inheritance to vary part of an algorithm. Strategy uses delegation to vary the entire algorithm. [GoF, p330]

Strategy modifies the logic of individual objects. Template Method modifies the logic of an entire class. [Grand, p383]

I recommend you familiarize yourself with them.

Answer 3

The simplest answer could be if You remove virtual from doABC(), so that it can not be derived by child class. Call the virtual methods inside doABC() in correct order.