Re: Testing Program Question
"Leigh Johnston" <leigh@i42.co.uk> wrote in
news:aOWdnWeFBL2I_uXWnZ2dnUVZ8sednZ2d@giganews.com:
All that yo-yo with virtuals is one of the biggest sources of issues
in C++ so this 'no virtual interfaces' idiom should make lot of sense
if to think a bit. Also like James Kanze suggested it reduces need
for defenses all over the place by moving some of them to front. ;)
You misunderstand me, I am advocating that virtual interfaces are fine
in themselves and it is not too difficult to mutate said interfaces as
required. Obviously I am in favour of keeping public interfaces as
simple and minimalist as possible and quite often I have virtual
methods which are protected/private when they from part of a class's
implementation rather than public interface.
The following are fine for example:
class interface
{
public:
virtual void foo() = 0;
virtual void bar() = 0;
};
class widget
{
public:
virtual void draw() const;
};
It is wrong to say that virtual functions must not be public.
They can, but it would not be wise, at least not in large systems.
The main reason IMO is that over the time the system evolves and there
arise needs to change the interface. If the interface is defined in terms
of a virtual function, it is much more hassle; one has to change virtual
function declaraitons and definitions in N places, encure that no class
is overlooked, etc. Also, it would not be easy to retain the old
interface for back-compatibility. Both changing the signature of the
virtual function as well as adding a new virtual function would ruin the
binary compatibility of already compiled client code.
If the interface is formed in terms of a non-virtual function, then this
all becomes much easier. One just adds a new non-virtual function with a
new interface, calling the same virtual function behind the scenes.
So, IMO, public virtual functions are justified only in small callback
interfaces, where the functionality changes can be easily absorbed on the
call site, thus not requiring any change in the interface.
Paavo