[Development] Is overriding an existing virtual method 'BC' in Qt 4?

Thiago Macieira thiago.macieira at intel.com
Tue Oct 9 18:50:59 CEST 2012

On terça-feira, 9 de outubro de 2012 18.19.19, Marc Mutz wrote:
> > It would still break forward compatibility. Assume you do the change in
> > 5.0.1, and an app gets compiled against 5.0.1 using the new symbol (by not
> > reimplementing it in a derived class, or explicitly calling it). Your app
> > would crash when run against 5.0.0.
> Ok, so a pure virtual doesn't create a symbol. Understood.

A new pure virtual doesn't create a new symbol. Adding a virtual changes the 
vtable layout.

That means you can *override* an existing virtual with a new pure virtual, 
thus forcing the class to be abstract. That's binary compatible. 

But you cannot override the virtual with a proper function. That breaks the 
forwards compatibility.

Note that overriding with a pure virtual might not be *source* compatible. If 
the class is non-final (non-leaf), then something might be calling that 
function and will now start calling the pure virtual.

struct Base
    virtual ~Base();
    virtual void f();

struct Derived : public Base /* before */

struct UserClass : public Derived
    void f();

void UserClass::f() { Derived::f(); }
/* Assembly:
        jmp     _ZN4Base1fEv

But if we now override the Derived class's f with a pure virtual
struct Derived : public Base /* after */
    virtual void f() = 0;

Then UserClass::f() becomes:
        jmp     _ZN7Derived1fEv

Note that it's calling the new symbol, whether it exists or not. That means 
the change was not source-compatible.

> Now, what's the forward-compatibility policy on the 4.8 branch?

Any program compiled with Qt 4.8.x will run with Qt 4.8.y, whatever x and y 
are (modulo bugs, of course).

Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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