Stack allocation skips vtable, optimalisation option?

Stack allocation skips vtable, optimalisation option?

[Daan Oosterveld]

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Hi all,

I have a problem finding the right switch to eliminate the compiler to 
jump over the vtable and calling the wrong virtual  function. When 
allocating a class on the stack and copying a subclass over it virtual 
functions don't behave as expected. As a pointer the code calls the 
virtual function of the subclass, but when called directly it always 
calls the function in the base class.

The compiler assumes the pointer to the vtable is to Foo, and then 
assumes to call Foo::foo() in stead of looking in the object and see the 
vtable is pointing at Bar and calling Bar::foo.

The output of this is:

$ ./a.out
Foo!
Bar!

It should be in my opinion:

$ ./a.out
Bar!
Bar!

The source is listed below. It is compiled without optimalisations (-O0)

Thanks

Daan Oosterveld


---- source:
#include <stdio.h>

class Foo
{
protected:
    char aap[12];
public:
    Foo() {}
    Foo(const Foo & foo) {
        char * d = (char*)this;
        char * s = (char*)&foo;
        
        for(int i = 0; i < sizeof(Foo); i ++) {
            d[i] = s[i];
        }
    }

    virtual void foo() { printf("Foo!\n"); }
};

class Bar : public Foo
{
public:
    virtual void foo() { printf("Bar!\n"); }
};

int main()
{
    Foo foo = Bar();
    Foo * bar = &foo;
    foo.foo();
    bar->foo();
}


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begin:vcard
fn:Daan Oosterveld
n:Oosterveld;Daan
email;internet:d.oosterveld@wanadoo.nl
tel;home:053-4323699
tel;cell:06-24732100
version:2.1
end:vcard

Re: Stack allocation skips vtable, optimalisation option?

[Eljay Love-Jensen]

Hi Daan,

Even with -O0, the compiler *KNOWS* that foo is a Foo (regardless of your
copy constructor shenanigans), and hence can call Foo::foo directly in main,
without using the vfntbl lookup.

HTH,
--Eljay