Re: [boost] A little utility class I'd like to contribute to boost

From: Luis Pedro Coelho (deepblack9_at_[hidden])
Date: 2001-06-14 09:26:03

Em Quarta, 13 de Junho de 2001 23:43, escreveste:
<snip>
> > template <typename T, T init = T()>
>
> Given the intended usage for this class, I think that the 'init' value
> should be always given.
> This removes the 'default constructible' requirement on T imposed by the
> default initialization of 'init'.

My usage of this has always been with builtin types so that has never been a 
problem. In general, I think value classes should have default constructors, 
so I do not see this as a problem.

> > struct auto_init {
> >         auto_init():object(init) { }
> >         auto_init(const T& other):object(other) { }
> >         operator T& () { return object; }
> >         operator const T& () const { return object; }
> >         T& operator = (const T& other) { object = other; return object; }
> >         void reset() { object = init; }
> >     private:
> >         T object;
> > };
>
> Since you are providing conversion operators, the constructor for T should
> be explicit.
> Therefore, auto_init's own copy constructor and assigment should also be
> present.
>
> But there are other important issues to be addressed:
> Ideally, auto_init<T> objects should inherit the semantics of the inner
> type T,
> such as other operators (==,+,&, void const*, etc...) but that's tricky,
> because T might not support some of the them.
> Otherwise I won't be able to write expressions like:  if ( data == 3 ), 
> int a = data + 1 , int* p = &data, if ( data ) ..., etc...
<snip>

There except for the taking of the address, all other cases are handled by 
the conversion to T operator. For example, "if (data == 3)" gets transformed 
into "if (data.operator int() == 3)", so no problems there.

Luis