The easy way to call pre and post function execution code pieces

Well at times you want to execute some while you enter some function, something like enable or disable certain objects or set some flags before execution and reset them after you have finished this function.

The most straightforward way of doing it is putting code for it in the beginning and end of the function. But if this code of your grows in size and there are several return statements added to it, then you maintaining this code will become difficult.

So the easy way out ? Well its simple. Create a new class with just constructor and destructor. In the constructor call the code which you want to execute initially, and the destructor to have the post processing code. Make sure that copy constructors is made private just to avoid complications.

Example:

class magicalObject
{
    public:
        magicalObject()
        {
            // do pre processing job here
        };
        ~magicalObject()
        {
            // do post processing job here
        };
    private:
        magicalObjet(magicalObject & obj);      
}

void somefunction()
{
    magicalObject myMagicalObject;     // pre processing job done now

    // do your job

    return;
    // no worries the post processing job will be done my
    // magical object
}

The importance of Interface classes

I am sure you might have heard about interfaces. But then why do you need them. You anyways are going to implement those classes. Why take the unnecessary overhead of writing interfaces for them as well.

Actually its not necessary to write a interface class for all classes that you write. But there are certain cases when its much much better if you have them in place.

First what are interface classes. Well this is a base class which has pure declaration of all the public methods need in the target class. All these methods are make pure virtual functions. No implementation is added to this class. For example if you write a class Time, you would want to declare an interface named, ITime. So the class declarations would be as follows.

class ITime 
{
      public : int getHour() = 0;
                   int getMinute()= 0;
                   int getSecond()= 0;
                   void setHour(int hour)= 0;
                   void setMinute(int min)= 0;
                   void setSecond(int sec)= 0;
};
class Time: public ITime
{
      public : int getHour() = 0;
                   int getMinute()= 0;
                   int getSecond()= 0;
                   void setHour(int hour)= 0;
                   void setMinute(int min)= 0;
                   void setSecond(int sec)= 0;
      private: int m_hour, m_min, m_sec;
};

The interface class has no implementation but the concrete class does. Concrete class contains all additional private member variables needed as well as constructors and destructors, as needed.

When do you need it? For classes which are instantiated at a few locations but used at a lot of other places, it makes sense to create interfaces for them.

But why do all this circus. Well the reason is pretty straight forward, to allow users of this class not care about whatever is there in the actual implementation. All internally used methods and variables are not known to the user of this class. Also since the concrete class headers are not included at all places where this class is being used, changes in internal implementation need not lead to compilation of each of the other classes. Only if something is changed in Interface class, recompilation would take place in all places else its a faster process.

This is more or less what I understand as a need for Interfaces. Please do let me know if you think otherwise.

using array of pointer to member function

Seems like a crazy at the first instance, but this is an amazing use of the facility provided by C++ which is called, pointer to member of a class. This can be a function or a variable. But where to use it. Well one of cases is when one needs to call different function of same signature based on some parameter. Surely this can be done via other trivial means as well certain situation may call for special measures. This can used in that case.

How to create and use it is taken straight out of C++ FAQLite . Link to specific page is here.

Say we have a class A as following

class A
{
public : 
    void a(int x);
    void b(int x);
    void c(int x);
    void d(int x);
}

//create array of pointer to member function
int ((A::*ptr)[4])(int x) = {&A::a, &A::b, &A::c, &A::d};

//using it in function
void testFunc(int val1, int val2)
{
    ObjectOfA.*(ptr[val1])(val2);
}

So our testfunction will call a,b,c or d from ObjectOfA depending on the val1 with parameter val2. Though obviously there are better ways to do it, but then this is also one complicated but shorter(codewise) one.

Why pointers are so very important in C++

Pointer are the life line of any C/C++ programmer. The reason is not just the flexibility, but also the additional speed that a pointer adds to your program. The reason is simple, the constructor of your class is not called. Well its actually safer if you send any class object, since the actual object does not get modified. But then its a safe design and will lead to some penalty in terms of speed. Sometimes, all you care for is speed, then its important to understand what happens while a function is being called.

Well in C++ this is what happens when you call a function

1.  All registers are saved on the stack.

2. Stack is saved, usually by incrementing the stack pointer.

3. Parameters to the function are saved to the new stack.

4. Function execution starts

Well the reverse of thing happens when the function returns.

1. Parameters in the stack are destroyed.

2. Saved stack is retrieved, usually my manipulating the stack pointers.

3. All saved registers are restored.

As you can make from the above actions, there is an additional overhead of calling the constructor and destructor of the parameters. This is definitely not going to happen for free. There is a some penalty which you would take.

Its then upto you to decide what you want. Some safe programming or avoiding the penalty.