Determining size of a polymorphic C++ class

Question

Using the sizeof operator, I can determine the size of any type – but how can I dynamically determine the size of a polymorphic class at runtime?

For example, I have a pointer to an Animal, and I want to get the size of the actual object it points to, which will be different if it is a Cat or a Dog. Is there a simple way to do this, short of creating a virtual method Animal::size and overloading it to return the sizeof of each specific type?

Answer 1

If you know the set of possible types, you can use RTTI to find out the dynamic type by doing dynamic_cast. If you don't, the only way is through a virtual function.

Answer 2

Or you can use typeid, which might be faster than dynamic_cast (also with dynamic_cast you can cast to intermediary types in the hierarchy).

It looks rather bad:

#include <iostream>
#include <typeinfo>

class Creature
{
    char x[4];
public:
    virtual ~Creature() {}
};

class Animal: public Creature { char x[8];};

class Bird: public Creature { char x[16]; };

class Dog: public Animal { char x[32]; };

class Cat: public Animal { char x[64]; };

class Parrot: public Bird { char x[128]; };

unsigned creature_size(const Creature& cr)
{
    if (typeid(cr) == typeid(Animal)) {
        return sizeof (Animal);
    }
    else if (typeid(cr) == typeid(Dog)) {
        return sizeof(Dog);
    }
    else if (typeid(cr) == typeid(Cat)) {
        return sizeof(Cat);
    }
    else if (typeid(cr) == typeid(Bird)) {
        return sizeof(Bird);
    }
    else if (typeid(cr) == typeid(Parrot)) {
        return sizeof(Parrot);
    }
    else if (typeid(cr) == typeid(Creature)){
        return sizeof(Creature);
    }
    assert(false && "creature_size not implemented for this type");
    return 0;
}

int main()
{
    std::cout << creature_size(Creature()) << '\n'
    << creature_size(Animal()) << '\n'
    << creature_size(Bird()) << '\n'
    << creature_size(Dog()) << '\n'
    << creature_size(Cat()) << '\n'
    << creature_size(Parrot()) << '\n' ;
}

For each new type you'll need to add code to the creature_size function. With a virtual size function you'll need to implement this function in each class as well. However, this function will be significantly simpler (perfectly copy-n-pasteable, which shows there might be both a limitation in the language and a problem with your code design):

virtual unsigned size() const { return sizeof(*this); }

And you can make it abstract in the base class which means that it will be a compiler error if you forget to override this method.

Edit: this is naturally assuming that given any Creature you want to know its size. If you have a strong reason to believe that you are dealing with a Dog - or a subclass of Dog (and you don't care if it is a subclass), then naturally you can use dynamic_cast for an ad hoc test.

Answer 3

If you are able to change source classes' design, you can totally replace dynamic polymorphism (which uses virtual functions) with static polymorphism and use the CRTP idiom:

template <class TDerived>
class Base
{
public:
    int getSize()
    { return sizeof(TDerived); }

    void print()
    {
          std::cout
             << static_cast<TDerived*>(this)->getSize()
             << std::endl;
    }

    int some_data;
};

class Derived : public Base<Derived>
{
public:
    int some_other_data1;
    int some_other_data2;
};

class AnotherDerived : public Base<AnotherDerived>
{
public:
    int getSize()
    { return some_unusual_calculations(); }
    // Note that the static_cast above is required for this override to work,
    //  because we are not using virtual functions
};

int main()
{
    Derived d;
    d.print();

    AnotherDerived ad;
    ad.print();

    return 0;
}

You can do this when the needed polymorphic behavior of program can be determined at compile-time (like the sizeof case), since the CRTP has not the flexibility of dynamic polymorphism to resolve the desired object at run-time.

The static polymorphism also has the advantage of higher performance by removing virtual-function-call overhead.

If you don't want to templatize Base class or you need to hold different derived instances of Base class in a same location (like an array or a vector), you can use CRTP on a middle class and move the polymorphic behavior to that class (similar to the Polymorphic copy construction example in the Wikipedia):

class Base
{
public:
    virtual int getSize() = 0;

    void print()
    {
        std::cout << getSize() << std:endl;
    }

    int some_data;
};

template <class TDerived>
class BaseCRTP: public Base
{
public:
    virtual int getSize()
    { return sizeof(TDerived); }
};

class Derived : public BaseCRTP<Derived>
{
    // As before ...
};

class AnotherDerived : public BaseCRTP<AnotherDerived>
{
    // As before ...

    // Note that although no static_cast is used in print(),
    //  the getSize() override still works due to virtual function.
};

Base* obj_list1[100];
obj_list1[0] = new Derived();
obj_list1[2] = new AnotherDerived();

std::vector<Base*> obj_list2;
obj_list2.push_back(new Derived());
obj_list2.push_back(new AnotherDerived());

--
Update: I now found a similar but more detailed answer on stackoverflow which explains that if we further derive from the derived classes above (e.g. class FurtherDerived : public Derived {...}), the sizeof will not report correctly. He gives a more complex variant of the code to overcome this.

Answer 4

I can't believe that somebody's invented type_id() instead of implementing proper traits ....