find size of derived class object using base class pointer

Question

Is it possible to find the size of a derived class object using a base class pointer, when you don't know the derived type.

Thank you.

Answer 1

There's no direct way, but you can write a virtual size() method child classes can implement. An intermediary templates class can automate the leg work.

struct base {
  virtual size_t size() const =0;
  virtual ~base() { }
};

template<typename T> 
struct intermediate : base {
  virtual size_t size() const { return sizeof(T); }
};

struct derived : intermediate<derived> 
{ };

This does require your hierarchy be polymorphic... however, requesting behavior based on the dynamic type of an object rather than its static type is part of the definition of polymorphic behavior. So this won't add a v-table to the average use case, since at the very least you probably already have a virtual destructor.

This particular implementation does limit your inheritance tree to a single level without getting into multiple inheritance [ie, a type derived from derived will not get its own override of size]. There is a slightly more complex variant that gets around that.

struct base { /*as before */ };

template<typename Derived, typename Base>
struct intermediate : Base {
  virtual size_t size() const { return sizeof(Derived); }
};

struct derived : intermediate<derived, base>
{ };

struct further_derived : intermediate<further_derived, derived>
{ };

Basically, this inserts an intermediate in between each actual layer of your hierarchy, each overriding size with the appropriate behavior, and deriving from the actual base type. Repeat ad nauseum.

//what you want
base >> derived 
     >> more_deriveder
     >> most_derivedest

//what you get
base >> intermediate<derived, base> 
     >> derived >> intermediate<more_deriveder, derived> 
     >> more_deriveder >> intermediate<most_derivedest, more_deriveder> 
     >> most_derivedest

Several mixin-type libraries make use of such a scheme, such that the mixins can be added to an existing hierarchy without introducing multiple inheritance. Personally, I rarely use more than a single level of inheritance, so I don't bother with the added complexity, but your mileage may vary.

Answer 2

I don't think it can be done, because sizeof works on compile time types. You could define a virtual Size function in the base class and override it for each derived class.

Answer 3

Considering the nice answer of @Dennis Zickefoose, there's a case where you can implement multiple levels of inheritance which requires you neither to have virtual functions nor an intermediate class between each layer of inheritance and the added complexity.

And that's when all the intermediate (non-leaf) classes in the inheritance hierarchy are abstract classes, that is, they are not instantiated.

If that's the case, you can write the non-leaf abstract classes templated (again) on derived concrete types.

The example below demonstrates this:

template <class TDerived>
class Shape     // Base
{
public:
    float centerX;
    float centerY;

    int getSize()
    { return sizeof(TDerived); }

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

class Circle : public Shape<Circle>
{
public:
    float radius;
};

class Square : public Shape<Square>
{
    // other data...
};

template <class TDerived>
class Shape3D : public Shape<TDerived>
    // Note that this class provides the underlying class the template argument
    //   it receives itself, and note that Shape3D is (at least conceptually)
    //   abstract because we can't directly instantiate it without providing it
    //   the concrete type we want, and because we shouldn't.
{
public:
    float centerZ;
};

class Cube : public Shape3D<Cube>
{
    // other data...
};

class Polyhedron : public Shape3D<Polyhedron>
{
public:
    typedef float Point3D[3];

    int numPoints;
    Point3D points[MAX_POINTS];

    int getSize()   // override the polymorphic function
    { return sizeof(numPoints) + numPoints * sizeof(Point3D); }
    // This is for demonstration only. In real cases, care must be taken about memory alignment issues to correctly determine the size of Polyhedron.
};

Sample usage:

Circle c;
c.demo();

Polyhedron p;
p.numPoints = 4;
p.demo();

Answer 4

Due to lack of reflection in C++, this is not generally possible with arbitrary classes at a whim. There are some workarounds however. You can write a virtual size() method as others have suggested. You can also use the Curiously Recurring Template Pattern, aka inheriting from Register<T> as well but I wouldn't recommend it, vtable costs 4 bytes per object, subclasses of T report incorrect size and correcting it results in multiple inheritance.

The best way would be to use a class to register, store and query dynamic size information, without modifying the class you want to query:

EDIT: As it turns out, due to the inconsistent semantics of typeid, it still needs classes with vtables, see the comments.

#include <cstddef>
#include <exception>
#include <iostream>
#include <map>
#include <typeinfo>

using namespace std;

class ClassNotFoundException
: public exception
{};

class Register
{

    public:

        template <class T>
        static void reg (T* = NULL)
        {
            //  could add other qualifiers
            v[&typeid(T)] = sizeof(T);
            v[&typeid(const T)] = sizeof(T);
            v[&typeid(T*)] = sizeof(T);
            v[&typeid(const T*)] = sizeof(T);
        }

        template <class T>
        static int getSize (const T& x)
        {
            const type_info* id = &typeid(x);
            if( v.find(id) == v.end() ){
                throw ClassNotFoundException();
            }
            return v[id];
        }

        template <class T>
        static int getSize (T* x)
        {
            return getSize(*x);
        }

        template <class T>
        static int getSize (const T* x)
        {
            return getSize(*x);
        }

    protected:

        static map<const type_info*, int> v;

};

map<const type_info*, int> Register::v;

class A
{
    public:
        A () : x () {}
        virtual ~A () {}
    protected:
        int x;
};

class B
: public A
{
    public:
        B() : y () {}
        virtual ~B () {}
    protected:
        int y;
};

int main ()
{
    Register::reg<A>();
    Register::reg<B>();

    A* a = new B();
    const A* b = new B();

    cout << Register::getSize(a) << endl;
    cout << Register::getSize(b) << endl;
}