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Subject: Re: [boost] Scoped Enum Emulation
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2012-01-25 07:26:14

Vicente J. Botet wrote:
> Le 24/01/12 22:51, Beman Dawes a écrit :
> >
> > What do others think? Should
> /boost/detail/scoped_enum_emulation.hpp
> > functionality be moved to config?
> >
> > Are there any improvements that would make the emulation
> > better, without turning something simple into something
> > complex?
> I'm working with a different emulation which uses classes.
> For example
> // enum class cv_status;
> {
> no_timeout,
> timeout
> };
> The macros are defined as follows:
> struct x { \
> enum enum_type
> enum_type v_; \
> inline x() {} \
> inline x(enum_type v) : v_(v) {} \
> inline operator int() const {return v_;} \
> friend inline bool operator ==(x lhs, int rhs) \
> {return lhs.v_==rhs;} \
> friend inline bool operator ==(int lhs, x rhs) \
> {return lhs==rhs.v_;} \
> friend inline bool operator !=(x lhs, int rhs) \
> {return lhs.v_!=rhs;} \
> friend inline bool operator !=(int lhs, x rhs) \
> {return lhs!=rhs.v_;} \
> };
> #define BOOST_STRONG_ENUM_NATIVE(x) x::enum_type
> #define BOOST_DECLARE_STRONG_ENUM_BEGIN(x) enum class x
> While this is not yet a complete emulation of scoped enums,
> it has the advantage of that there is no need to use a macro to
> name the strong type.

It looks decent, but shouldn't int be a computed type based upon the size and signed-ness of enum_type? Of course, you could also provide macros to specify the underlying type and use that, instead of enum_type, as the type of v_. That would increase compatibility with strongly typed enums in C++11.

Why convert implicitly to int rather than to enum_type? You could convert to the computed type I mentioned above, but converting to enum_type would ensure the most appropriate conversions, wouldn't it?

It might also be good to put the semicolon closing the enumerated type definition into the _END macro. That would increase symmetry and more strongly tie the _END macro to the construct:


It would be nice if the macros would produce strongly typed enums, when available, and devolve to emulation when not.

It might also prove useful to have a macro to forward declare them: BOOST_FORWARD_DECLARE_STRONG_ENUM(name). That would forward declare a normal class, when emulating, and forward declare the enum class otherwise.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP


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