Re: How can I use unqualified names? (Possibly hard or impossible?)

"Alf P. Steinbach" <>
Mon, 20 Jul 2009 04:06:32 +0200
* Alf P. Steinbach:

* Alf P. Steinbach:

Problem: long qualified option names are impractical and unreadable,
but short good names like 'x' can easily conflict with client code.
Especially where that client code cretating an Options instance is a
constructor initializer list a 'using namespace...' isn't practical
(as far as I can see, but I may be wrong). I started thinking in terms
of ADL and such, but no dice: my brain cannot come up with good
solution, even now after having slept on it!

First, thanks to Jonathan Lee (else-thread) for a different tack on the
option pack problem, the problem that led to the above. Yeah, I'm using
that and'ing scheme for simpler options, those that are encoded as
bit-fields. But I don't quite see how it helps with the above.

Anyway, I found a sort of almost-solution, shown below.

It doesn't get rid of qualification as I wanted, but it reduces
qualification to only one qualifier, which can be freely named within a
usage class.

It's sort of like if one could have 'using namespace foo=' within a
class. :-)

Might be a useful technique on its own.

Uh oh, I should have tested with MSVC.

MSVC 7.1 doesn't accept the 'typedef'. But it does accept a class derivation.
However, results are incorrect relative to what I expect: I expect the output
that compilation with g++ produces.

Results g++ versus MSVC are posted below after the new adjusted-for-MSVC code.

template< typename OptionValues >
class Options_: public OptionValues
     template< typename OptionType >
     Options_& operator<<( OptionType const& option )
         //STATIC_ASSERT( IS_DERIVED_AND_BASE( Options_, OptionType ) );
         static_cast<OptionType&>(*this) = option;
         return *this;

     template< typename OptionType >
     OptionType const& as() const
         //STATIC_ASSERT( IS_DERIVED_AND_BASE( Options_, OptionType ) );
         return static_cast<OptionType const&>( *this );

template< typename T, typename Unique >
struct OptionsMember_
     T member;
     OptionsMember_(): member() {}
     OptionsMember_( T const& aValue ): member( aValue ) {}
     operator T& () { return member; }
     operator T const& () const { return member; }

struct Nix {};

     class T,
     class U = Nix,
     class V = Nix,
     class W = Nix,
     class X = Nix,
     class Y = Nix,
     class Z = Nix
     struct WithMembers_;

template< class T, class U, class V, class W, class X, class Y, class Z >
struct WithMembers_: T, U, V, W, X, Y, Z {};

template< class T, class U, class V, class W, class X, class Y >
struct WithMembers_<T, U, V, W, X, Y>: T, U, V, W, X, Y {};

template< class T, class U, class V, class W, class X >
struct WithMembers_<T, U, V, W, X>: T, U, V, W, X {};

template< class T, class U, class V, class W >
struct WithMembers_<T, U, V, W>: T, U, V, W {};

template< class T, class U, class V >
struct WithMembers_<T, U, V>: T, U, V {};

template< class T, class U >
struct WithMembers_<T, U>: T, U {};

template< class T >
struct WithMembers_<T>: T {};

//------------------------ Usage:

#include <iostream>

namespace blahblah {
namespace reallyLongName {
namespace california {
namespace pos2 {
     typedef OptionsMember_<double, struct UniqueTypeFor_x> x;
     typedef OptionsMember_<double, struct UniqueTypeFor_y> y;
     typedef Options_< WithMembers_<x, y> > Options;

     struct Names
         typedef pos2::Options Options;
         typedef pos2::x x;
         typedef pos2::y y;
}}}} // namespace blahblah::reallyLongName::california::pos2

struct UserClass1
     //using namespace pos2 = blahblah::reallyLongName::california::pos2;
     struct pos2: ::blahblah::reallyLongName::california::pos2::Names {};

     UserClass1( pos2::Options const& options )
         using namespace std;
         cout << options.pos2::x::member << endl;
         cout << options.pos2::y::member << endl;

namespace blahblah {
namespace reallyLongName {
namespace california {
namespace pos3 {
     typedef OptionsMember_<double, struct UniqueTypeFor_z> z;
     typedef Options_< WithMembers_<pos2::Options, z> > Options;

     struct Names: pos2::Names
         typedef pos3::Options Options;
         typedef pos3::z z;
}}}} // namespace blahblah::reallyLongName::california::pos3

struct UserClass2: UserClass1
     //using namespace pos3 = blahblah::reallyLongName::california::pos3;
     struct pos3: ::blahblah::reallyLongName::california::pos3::Names {};

     UserClass2( pos3::Options const& options )
         : UserClass1( options )
         using namespace std;
         cout << std::endl;
         cout << options.pos3::x::member << endl;
         cout << options.pos3::y::member << endl;
         cout << options.pos3::z::member << endl;

int main()
     //using namespace pos3 = blahblah::reallyLongName::california::pos3;
     typedef UserClass2::pos3 pos3;
     UserClass2( pos3::Options() << pos3::x(3.14) << pos3::z(42) );

C:\temp> gnuc x.cpp -o a && a


C:\temp> msvc x.cpp -o b && b



Does this mean that the code above has UB?

And if not, can one conclude that MSVC 7.1 is wrong?

Cheers & TIA.,

- Alf

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