C++
dynamic_cast
Learn safe downcasting with dynamic_cast.
dynamic_cast
dynamic_cast performs a safe downcast in polymorphic classes. It converts a base pointer to a derived type; if the conversion is invalid it returns nullptr.
Example: Animal* a = new Dog();. If you know a is really a Dog, write Dog* d = dynamic_cast<Dog*>(a);. If d is not nullptr, call d->fetch(). For dynamic_cast to work, the class must have at least one virtual method; otherwise it gives a compile error.
RTTI and nullptr check
dynamic_cast uses runtime type information (RTTI). It checks the actual type of the pointer through the vtable, so it only works on polymorphic classes.
If the types are incompatible, it throws std::bad_cast for references and returns nullptr for pointers. Using a pointer and checking with if (d) is safer. dynamic_cast carries a small performance cost but prevents crashes from bad downcasts.
Exercise Downcast to Dog with dynamic_cast
Define an Animal class with virtual void speak() and virtual ~Animal() {}. A Dog class inherits, adds speak() override and an extra void fetch() with body std::cout << "Ball";. In main write Animal* a = new Dog();, Dog* d = dynamic_cast<Dog*>(a);, if (d) d->fetch(); and delete a;.
Show example solution
#include <iostream>
class Animal {
public:
virtual void speak() {}
virtual ~Animal() {}
};
class Dog : public Animal {
public:
void speak() override {}
void fetch() { std::cout << "Ball"; }
};
int main() {
Animal* a = new Dog();
Dog* d = dynamic_cast<Dog*>(a);
if (d) d->fetch();
delete a;
return 0;
}
Test yourself
-
When is dynamic_cast used?
Dog* d = dynamic_cast<Dog*>(a);
- For safe downcasting
- For static variables
- For array access
- For type promotion
Show answer
For safe downcasting — dynamic_cast safely converts a base pointer to a derived type.
-
Match each piece to its role
- dynamic_cast
- Safe downcast
- Dog*
- Target type
- virtual ~Animal
- Required polymorphic base
- if (d)
- Null pointer check
-
Complete the dynamic_cast call
Dog* d = dynamic_cast<____>(a);
Show answer
Dog* d = dynamic_cast<Dog*>(a);
Next lesson: Virtual destructor
