Ce este o clasa abstracta?
Raspuns
O clasa abstracta reprezinta un concept general care necesita variante concrete pentru a putea fi utilizat.
De exemplu, animal reprezinta un concept general iar variantele lui concrete por fi caine, pisica s.a.m.d.
In C++, o clasa este abstracta daca
- are cel putin o functie virtuala pura;
Exemplu [1]- Cod: Selectaţi tot
class Animal
{
public:
// contains at least one pure virtual function
// (a function which has pure virtual specifier '=0').
virtual void say_something() = 0;
virtual void do_somethnig() = 0;
// 'Animal' is an abstract class.
};
- mosteneste cel putin o functie la care ultima suprascriere este virtuala pura.
Exemplu [2]- Cod: Selectaţi tot
class Mammal : public Animal
{
public:
// (1) does not override 'say_something' pure virtual function;
// (2) overrides 'do_somethnig' having pure virtual specifier ' = 0'.
virtual void do_somethnig() = 0;
// given (1) or (2), 'Mammal' class is still abstract
};
Exemplu [3]- Cod: Selectaţi tot
class Dog : public Mammal
{
public:
// Finally, 'Dog' is not an abstract class
virtual void say_something()
{
std::cout << "Bark, bark!";
}
virtual void do_somethnig()
{
std::cout << "Bite you!";
}
};
O clasa abstracta nu poate fi instantiata
Exemplu [4]
- Cod: Selectaţi tot
int main()
{
Animal a; // Error: cannot instantiate abstract class
Mammal* pM = new Mammal; // Error: cannot instantiate abstract class
Dog rex; // OK, 'Dog' is not an abstract class
// ...
}
Ca o consecinta, clasele abstracte nu pot fi folosite ca tip pentru parametri, return sau cast.
Exemplu [5]
- Cod: Selectaţi tot
void f1(Animal a); // Error: cannot instantiate abstract class
Animal f2(); // Error: cannot instantiate abstract class
Dog f3(); // OK, Dog is not an abstract class
int main()
{
Animal* p = &(Animal)f3(); // Error: cannot instantiate abstract class
// ...
}
Totusi, pot fi folositi pointeri sau referinte la clase abstracte.
Exemplu [6]
- Cod: Selectaţi tot
void f4(Animal& rA); // OK
void f5(Animal* pA); // OK
Animal& f6(); // OK
Animal* f7(); // OK
int main()
{
Animal* p = (Animal*)&f3(); // OK
Animal& r = (Animal&)f3(); // OK
// ...
}
Resurse
- ISO/IEC 14882:2003 - 10.4 Abstract classes
- [MSDN]Abstract Classes (C++)
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