Re: Get address of whole instance with multiple inheritance

From:

Greg Herlihy <greghe@mac.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 11 Dec 2008 13:12:41 CST

Message-ID:

<7bfcadc2-3062-4078-9b7e-7bc2d2c5e3f9@o40g2000prn.googlegroups.com>

On Dec 10, 5:58 am, "dustin.fri...@googlemail.com"
<dustin.fri...@googlemail.com> wrote:

is there any way to get a pointer to the "whole" instance after
casting to a base class.
i.E.:

class Interface1 {
public:
        virtual void foo() = 0;
};

class Interface2 {
public:
        virtual void bar() = 0;
};

class Interface3 {
public:
        virtual void baz() = 0;
};

class Impl : public Interface1, public Interface2, public Interface3 {
public:
        virtual void foo();
        virtual void bar();
        virtual void baz();

};

int main(int argc, char** argv) {
        Impl* impl = new Impl();

        Interface1* if1 = impl;>
        Interface2* if2 = impl;>
        Interface3* if3 = impl;

        if1->foo(); // Impl::foo() : this = 0x603010
        if2->bar(); // Impl::bar() : this = 0x603010
        if3->baz(); // Impl::baz() : this = 0x603010>
}

During a method call C++ will use the vtable (I think) but is there
any way to get the address of the whole instance (the address returned
by new) without knowing the concrete class.

Yes. A dynamic_cast applied to a polymorphic object pointer will
return a pointer to its most derived object. For example:

     void *ptr = dynamic_cast<void*>(if2);

should return a pointer to the Impl object.

Greg

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