quote:Original post by Fruny

quote:Original post by Peon
Couldn''t you just code a NEW constructor for the boy object? I would just copy the duplicated code into the Boy constructor, and add the height there.

Such a terribly bad idea would cost your C++ license.

I''m confused; please elaborate.

Couldn''t you just code a NEW constructor for the boy object? I would just copy the duplicated code into the Boy constructor, and add the height there.

I''m confused; please elaborate.

If you don''t explicitely call a base class constructor, the compiler will automatically insert a call to the base class default constructor. If this constructor doesn''t exist (like in the original poster''s example - he has defined a constructor with arguments, inhibiting the compiler-generated default constructor), your program will not compile.

Now, assuming you do have a default constructor, the base class members are likely to have been declared private. If so, the constructor in the derived class cannot access them - your program will not compile.

Assuming further that those members are accessible, you are doing twice as much work as is needed, first initialising the class in the base class default constructor and then *again* in the Boy constructor - which you said duplicates the base class constructor code. People often claim C++ classes are slow... but that fault lies on those who misuse the language, not the language itself.

Finally ... you have duplicated the base class code in the child class. Fine. But what if you then modify the base class ? You will have to go and modify *all* the class that derive from it to update their constructors. And that''s what object-oriented programming is intended to avoid. Imagine working in a large project, you would have to be informed of each and every change in the base class : "Oh, I changed the implementation class Foo again, if you are inheriting from it, please replace the corresponding code fragments in your class constructors with the following...".

That''s just insanity. Inheritance does mean you reuse your parent''s code directly - not that you manually duplicate it. That way, so long as the public interface of the class remains the same, you''re fine. Furthermore, the code for the base class might just not be available to you.


Anyway, here''s "the solution".

class Human{  int age;  std::string name;public:  Human( int a, const std::string& n );};Human::Human( int a, const std::string& n ): age(a), name(n)  // Initializer list - use them, love them.{}class Boy{  int height;public:  Boy( int a, const std::string& n, int h );};Boy::Boy( int a, const std::string& n, int h ): Human(a,b), height(h){}

Thomas Sauder was basically right on every point.

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Awesome, thanks for the detailed clarification. I had been doing it my way before (copying the code) and was running into exactly the problems you described: having to change a bunch of derived classes if the base changed. My HD decided to destroy itself ridding myself of days of work, but perhaps it was a message telling me my design was poor I''ll have to keep this in mind when I rewrite it.