RTTI In C++: Dynamic Type Checking With Examples

RTTI is a C++ mechanism that allows you to determine an object’s type at runtime. It’s essential when working with polymorphism and inheritance hierarchies, but it comes with trade-offs you need to understand.

Key RTTI Operators

typeid() — Returns a std::type_info object representing the type. You can compare type information or get a human-readable name via .name().

dynamic_cast — Safely converts pointers or references down an inheritance hierarchy. For pointers, a failed cast returns nullptr; for references, it throws std::bad_cast.

Both require virtual functions in the base class. RTTI is disabled if your base class lacks a virtual destructor or virtual methods.

Safe Downcasting with dynamic_cast

The primary use case for RTTI is safe downcasting:

class Animal {
public:
    virtual ~Animal() {}
    virtual void speak() = 0;
};

class Dog : public Animal {
public:
    void speak() override { std::cout << "Woof!\n"; }
    void fetch() { std::cout << "Fetching...\n"; }
};

class Cat : public Animal {
public:
    void speak() override { std::cout << "Meow!\n"; }
};

void process(Animal* a) {
    if (Dog* d = dynamic_cast<Dog*>(a)) {
        d->fetch();  // Safe; we confirmed it's a Dog
    } else if (Cat* c = dynamic_cast<Cat*>(a)) {
        std::cout << "It's a cat\n";
    }
}

Using typeid for Type Checking

typeid() is useful when you need to examine or log type information:

void diagnose(Animal* a) {
    std::cout << "Type: " << typeid(*a).name() << "\n";

    if (typeid(*a) == typeid(Dog)) {
        std::cout << "Object is a Dog\n";
    }
}

Note: typeid(*a).name() produces compiler-specific mangled names. Use __cxa_demangle() on GCC/Clang to get readable output, or rely on the comparison directly.

Reference Casting and Exception Handling

With references, dynamic_cast throws on failure rather than returning null:

void processRef(Animal& a) {
    try {
        Dog& d = dynamic_cast<Dog&>(a);
        d.fetch();
    } catch (const std::bad_cast& e) {
        std::cerr << "Not a Dog: " << e.what() << "\n";
    }
}

When to Avoid RTTI

Modern C++ offers better alternatives for most scenarios:

Virtual Functions — If you need type-specific behavior, add a virtual method to the base class and override it in derived classes. This is faster and clearer:

class Animal {
public:
    virtual ~Animal() {}
    virtual void performSpecialAction() {}
};

class Dog : public Animal {
public:
    void performSpecialAction() override { fetch(); }
};

std::variant — For small, fixed type hierarchies where inheritance feels overkill, use std::variant with std::visit:

struct Dog { void fetch() {} };
struct Cat { void scratch() {} };
struct Bird { void sing() {} };

using Pet = std::variant<Dog, Cat, Bird>;

void interact(Pet& pet) {
    std::visit([](auto& p) {
        if constexpr (std::is_same_v<decltype(p), Dog&>) {
            p.fetch();
        } else if constexpr (std::is_same_v<decltype(p), Cat&>) {
            p.scratch();
        }
    }, pet);
}

std::variant avoids the runtime overhead of virtual function tables and provides compile-time type safety with std::visit.

Performance Considerations

RTTI adds overhead:

Virtual function table lookups for dynamic_cast
Type information stored in memory
Pointer comparison during casting

For performance-critical code (real-time systems, tight loops), prefer virtual dispatch or std::variant. For typical application code, the overhead is negligible.

RTTI Disabled Scenarios

If your compiler flag disables RTTI (e.g., -fno-rtti on GCC/Clang), typeid() and dynamic_cast won’t work. Some embedded or game engine builds disable it intentionally to reduce binary size. Check your build configuration if you encounter linker errors or unexpected null casts.

Summary

Use RTTI when you genuinely need runtime type information in a heterogeneous object hierarchy. For most new code, reach for virtual functions first, then std::variant if appropriate. Reserve RTTI for integration scenarios or when you inherit code that relies on it. Always measure performance in your actual use case before assuming RTTI is a bottleneck.

RTTI in C++: Dynamic Type Checking with Examples