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2010-05-25T14:21:17Z

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2010-05-25T10:30:08Z

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==Dynamic cast==

<source lang="cpp">#include <iostream>
using std::cout; // this file uses std::cout

class Animal
{
public:
Animal():itsAge(1) {
cout << "Animal constructor...\n";
}
virtual ~Animal() {
cout << "Animal destructor...\n";
}
virtual void Speak() const {
cout << "Animal speak!\n";
}
protected:
int itsAge;
};

class Cat: public Animal
{
public:
Cat() {
cout << "Cat constructor...\n";
}
~Cat() {
cout << "Cat destructor...\n";
}
void Speak()const {
cout << "Meow\n";
}
void Miao() const {
cout << "Miao\n";
}
};

class Dog: public Animal
{
public:
Dog() {
cout << "Dog Constructor...\n";
}
~Dog() {
cout << "Dog destructor...\n";
}
void Speak()const {
cout << "Woof!\n";
}
};

int main()
{
const int NumberAnimals = 3;
Animal* animalArray[NumberAnimals];

int i;

animalArray[0] = new Dog;
animalArray[1] = new Cat;
animalArray[2] = new Cat;

for (i=0; i<NumberAnimals; i++)
{
animalArray[i]->Speak();

Cat *pRealCat = dynamic_cast<Cat *> (animalArray[i]);

if (pRealCat)
pRealCat->Miao();
else
cout << "not a cat!\n";

delete animalArray[i];
cout << "\n";
}

return 0;
}</source>
<pre class=codeResult>Animal constructor...
Dog Constructor...
Animal constructor...
Cat constructor...
Animal constructor...
Cat constructor...
Woof!
not a cat!
Dog destructor...
Animal destructor...
Meow
Miao
Cat destructor...
Animal destructor...
Meow
Miao
Cat destructor...
Animal destructor...</pre>

==dynamic_cast: Cast from Base * to Base *==

<source lang="cpp">#include <iostream>
using namespace std;
class Base {
public:
virtual void f() {
cout << "Inside Base\n";
}
};
class Derived : public Base {
public:
void f() {
cout << "Inside Derived\n";
}
};
int main()
{
Base *bp, b_ob;
Derived *dp, d_ob;
bp = dynamic_cast<Base *> (&b_ob);
if(bp) {
cout << "Cast from Base * to Base * OK.\n";
bp->f();
} else
cout << "Error\n";
cout << endl;
return 0;
}</source>
<pre class=codeResult>Cast from Base * to Base * OK.
Inside Base</pre>

==dynamic_cast: Cast from Base * to Derived * (wrong)==

<source lang="cpp">#include <iostream>
using namespace std;
class Base {
public:
virtual void f() {
cout << "Inside Base\n";
}
};
class Derived : public Base {
public:
void f() {
cout << "Inside Derived\n";
}
};
int main()
{
Base *bp, b_ob;
Derived *dp, d_ob;
dp = dynamic_cast<Derived *> (&b_ob);
if(dp)
cout << "Error\n";
else
cout << "Cast from Base * to Derived * not OK.\n";
cout << endl;
return 0;
}</source>
<pre class=codeResult>Cast from Base * to Derived * not OK.</pre>

==dynamic_cast: Cast from Derived * to Base *==

<source lang="cpp">#include <iostream>
using namespace std;
class Base {
public:
virtual void f() {
cout << "Inside Base\n";
}
};
class Derived : public Base {
public:
void f() {
cout << "Inside Derived\n";
}
};
int main()
{
Base *bp, b_ob;
Derived *dp, d_ob;
bp = dynamic_cast<Base *> (&d_ob);
if(bp) {
cout << "Cast from Derived * to Base * OK.\n";
bp->f();
} else
cout << "Error\n";

cout << endl;
return 0;
}</source>
<pre class=codeResult>Cast from Derived * to Base * OK.
Inside Derived</pre>

==dynamic_cast: Cast from Derived * to Derived *==

<source lang="cpp">#include <iostream>
using namespace std;
class Base {
public:
virtual void f() {
cout << "Inside Base\n";
}
};
class Derived : public Base {
public:
void f() {
cout << "Inside Derived\n";
}
};
int main()
{
Base *bp, b_ob;
Derived *dp, d_ob;
dp = dynamic_cast<Derived *> (&d_ob);
if(dp) {
cout << "Cast from Derived * to Derived * OK.\n";
dp->f();
} else
cout << "Error\n";
cout << endl;
return 0;
}</source>
<pre class=codeResult>Cast from Derived * to Derived * OK.
Inside Derived</pre>

==Use dynamic_cast to replace typeid==

<source lang="cpp">#include <iostream>
#include <typeinfo>
using namespace std;
class Base {
public:
virtual void f() {}
};
class Derived : public Base {
public:
void derivedOnly() {
cout << "Is a Derived Object.\n";
}
};
int main()
{
Base *bp, b_ob;
Derived *dp, d_ob;
// use typeid
bp = &b_ob;
if(typeid(*bp) == typeid(Derived)) {
dp = (Derived *) bp;
dp->derivedOnly();
}
else
cout << "Cast from Base to Derived failed.\n";
return 0;
}</source>
<pre class=codeResult>Cast from Base to Derived failed.</pre>

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