Re: [boost] Formal review or Variant Library (Ed B.)

From: Peter Dimov (pdimov_at_[hidden])
Date: 2003-02-18 12:19:08

Eric Friedman wrote:
>
> In the past we sought to support the following...
>
>   variant<...> var;
>   T* p = extract<T>(&var);
>   T& r = extract<T>(var);
>
> ...but it had to be dropped because (essentially) ambiguity exists
> between the following:
>
> template <typename T, typename Extractable>
>   T& extract(Extractable & operand);
> template <typename T, typename Extractable>
>   T* extract(Extractable * const operand);

This is only ambiguous in its most general form given above. If you rewrite
it as

template <class T, ...> T & extract(variant<...> & operand);
template <class T, ...> T const & extract(variant<...> const & operand);
template <class T, ...> T * extract(variant<...> * operand);
template <class T, ...> T const * extract(variant<...> const * operand);

it's no longer ambiguous.

The other option is to support a dynamic_cast-style extract:

T * p = extract<T*>(&var);
T & r = extract<T&>(var);

but it would likely require partial ordering/specialization.

"extract" is probably not the best name in both cases since it doesn't
extract, it returns a reference to the internal representation. The
documentation needs to describe when this reference is invalidated.