Tag - Cpp - Chouquette's blog

Sunday, October 5 2008

Cuda... episode 2

By Hugo on Sunday, October 5 2008, 15:16 - GPU

Le premier programme en cuda, c'est fait, maintenant il serait bon de ne pas avoir a se prendre la tete avec les allocations memoires, ou tout du moins les simplifier un peu...

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Saturday, September 13 2008

Calling contructor without parameter

By Hugo on Saturday, September 13 2008, 15:29 - C++

Cpp

Maybe something like that already happens to you : You wan't to build an instance of your class without calling the constructor :

class   A
{
    private:
        A(){};
        ~A(){};
};
 
int main()
{
    A   a();
}

But this will compile on g++ without any problem... why is that ? Because you're not building an instance of your class, your declaring a function called "a", which will return an instance of A. If you pass parameters, g++ will understand this a a constructor call, but if you wan't to call a 0 parameter constructor, you will have to call your CTOR without parenthesis...

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Wednesday, September 3 2008

The almost useless c++ trick of the day

By Hugo on Wednesday, September 3 2008, 23:37 - C++

Cpp
Tricks

When I learn c++, I remember my teacher telling : "A pure virtual method is a method without implementation", well that's not entirelly true... Actually, you can have a method beeing both pure virtual, and implementated. How is that possible ? well i must say that I don't know, since every assertions I ever made concerning how a compiler handles pure virtual methods was using the fact that a pure virtual was a null pointer in the class vtable... since you can have a body in it, it's obviously not null anonyme...

Anyway, implementing a pure virtual method is quite easy :

class  A
{
public :
	virtual   void pureVirtual() = 0
        {
            std::cout << "Me love otters" << std::endl;
        }
};

And this will compile without any problem !

The comportment of an implemented pure virtual method is quite simple, like any other pure virtual method, it prevents your class from beeing implemented. So here you can't have

A* a = new A();

it would print the usual error message : cannot instanciate abstract class. But what you can do, is call the implementation from a inherithed class of A, so here, you could do :

class B : A
{
public :
	virtual   void pureVirtual()
        {
            std::cout << "Not pure virtual anymore" << std::endl;
            A::pureVirtual();
        }
};
 
int main()
{
    B*b = new B();
    b->pureVirtual();    
}

which would print :

Not pure virtual anymore
Me love otters

And the worst part, is that even if a implemented pure virtual method seems a little bit of a non sense, not only it can be done, but it could be usefull, if you want to provide an interface with a default behavior, or probably many things else :)

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