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find size of derived class object using base class pointer

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名媛妹妹
名媛妹妹 2020-12-09 09:32

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.

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  • 爱一瞬间的悲伤
    爱一瞬间的悲伤 (楼主)
    2020-12-09 10:06

    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 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 
#include 
#include 
#include 
#include 

using namespace std;

class ClassNotFoundException
: public exception
{};

class Register
{

    public:

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

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

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

    protected:

        static map v;

};

map 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();
    Register::reg();

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

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

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