Get address of whole instance with multiple inheritance

"" <>
Wed, 10 Dec 2008 07:58:11 CST

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

class Interface1 {
    virtual void foo() = 0;

class Interface2 {
    virtual void bar() = 0;

class Interface3 {
    virtual void baz() = 0;

class Impl : public Interface1, public Interface2, public Interface3 {
    virtual void foo() {
        printf("Impl::foo() : this = %p\n", this);

    virtual void bar() {
        printf("Impl::bar() : this = %p\n", this);

    virtual void baz() {
        printf("Impl::baz() : this = %p\n", this);

int main(int argc, char** argv) {
    Impl* impl = new Impl();
    printf("impl = %p\n", impl); // impl = 0x603010

    Interface1* if1 = impl;
    printf("if1 = %p\n", if1); // if1 = 0x603010

    Interface2* if2 = impl;
    printf("if2 = %p\n", if2); // if2 = 0x603018

    Interface3* if3 = impl;
    printf("if3 = %p\n", if3); // if3 = 0x603020

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

    return 0;

as you can see the cast from Impl to any Interface changes the
address, but if a virtual method is called
the this pointer points to the instance in which the method is

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.

What I need is:
void* ptr = some_magic(if2); // Or any other interface

and ptr should now point to impl(0x603010), not to if2(0x603018).

Is there a common name for that pointer (something I can use to search
for) or do you have idea how I can do that?



$ g++ -v
Target: x86_64-pc-linux-gnu
gcc version 4.3.2 (Gentoo 4.3.2 p1.0)

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