How to alias the global namespace in C++11


Answers

I'm not aware of a way to directly alias the default namespace, but as a workaround you might use a macro (assuming you're willing to live with the macro's name colliding with unrelated symbols).

#define root_namespace

Usage:

root_namespace::cat_is_alive

As Mooing Duck correctly points out, this hack won't work with using clauses.

Question

Short Form: How can I define an alias for the root (global) namespace in C++11 ? It might look like

namespace root_namespace = :: ;

where the scope resolution operator in its naked form above is a place holder for some handle of the global namespace. I read in the GCC Internals Manual that

...The root of the [compiler's] entire intermediate representation is the variable global_namespace. This is the namespace specified with :: in C++ source code... The name of the global namespace is ::, even though in C++ the global namespace is unnamed.

PS EDIT: to respondents to-date, I appended a painfuly long form to address some concerns after the following long form, since it may clarify some things. And followers, if you see us talking at rather than to each other, dig in.


Long Form: An example of its potential use follows. If unsatisfactory, then yes, this is an academic question ; see the painfully long form that follows this one.

Imagine that your boss barges in one day and says, "I just read a book about post-positivism. Get rid of namespace ObjectiveReality." In the code below, 'all' you have to do is omit the lines I've marked like /* -> */ this /* <- */. You can do that for intermediate levels of nesting currently ; however, I am uncertain how to define the global-scope namespace current_authority to allow simple elision of the first non-global namespace.

#include <iostream>
#include <iomanip>


// ...
using cat_is_alive = std::true_type ; // because I like cats
using cat_is_alive = ::cat_is_alive ; // seems to work, see `g++ -v` below
// ...


// ASIDE: I originally had `namespace higher_authority = COMPILER ;` as a comment, but changed it for simpler chaining closure

// The next two lines are the crux of my question...

namespace higher_authority = global_namespace ;
namespace current_authority = global_namespace ; // a.k.a. the naked ::

// If the above two lines defined aliases for the (unnamed) global namespace, then the suggested elisions/replacements work...

/* -> */
namespace ObjectiveReality {
/* <- */
// Simplest fix: replace with `using ObjectiveReality = current_authority ;`
//   (Otherwise, a few other changes are necessary)

    namespace higher_authority = current_authority ;
    namespace current_authority = ObjectiveReality ;

    using cat_is_alive = higher_authority::cat_is_alive ;

    namespace EntangledObserver {

        namespace higher_authority = current_authority ;
        namespace current_authority = EntangledObserver ;

        using cat_is_alive = higher_authority::cat_is_alive ;
    } ;
/* -> */
} ;
/* <- */


int main( int argc, char** argv ) {

    std::cout
        << "It is "
        << ObjectiveReality::EntangledObserver::cat_is_alive::value
       << " that the cat is alive." << std::endl ;

    return 0 ;
}


// EOF

In case compiler info is needed:

$ g++ -std=c++11 -v

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-linux-gnu/4.7/lto-wrapper
Target: i686-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.7.2-11precise2' 
--with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs 
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.7 
--enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib 
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.7 --libdir=/usr/lib
--enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug 
--enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin
--enable-objc-gc --enable-targets=all --disable-werror --with-arch-32=i686
--with-multilib-list=m32,m64 --with-tune=generic --enable-checking=release 
--build=i686-linux-gnu --host=i686-linux-gnu --target=i686-linux-gnu
Thread model: posix
gcc version 4.7.2 (Ubuntu/Linaro 4.7.2-11precise2) 

PAINFULLY LONG FORM: as a response to some answers about "start from a nested namespace," note that 'HOME!' is inaccessible, and that I may not have the luxury of hand-picking namespaces in a team.

//
// alice.cpp
//


#include <iostream>
#include <type_traits>


/////
///// The Setup
/////

//
// One-and-a-half macros
//

// BOO! Get rid of this case!
#define ENABLE_SUBSPACE_1( namespace_name ) \
    namespace namespace_name { \
    namespace last_namespace = ::namespace_name ; \
    namespace this_namespace = ::namespace_name ;

// YEAH! Instead, define 'namespace this_namespace = :: ;' and then...
#define ENABLE_SUBSPACE( namespace_name ) \
    namespace namespace_name { \
    namespace last_namespace = this_namespace ; \
    namespace this_namespace = last_namespace::namespace_name ;

//
// Some characters
//

struct dorothy {
    static constexpr auto obvious_statement = "There's no place like " ;
} ;

struct rabbit {
    template< typename T >
    static constexpr char const* says( T ) {
        return T::value ? "I'm late!" : "I'm late, but I ditched that addled girl." ;
    }
} ;

struct alice {

    using blue_pill = std::false_type ;
    static constexpr auto where_am_i = "HOME!" ;
} ;


/////
///// The Central Structure
/////

ENABLE_SUBSPACE_1( oxford_england ) // {

    using has_strangers_with_candy = std::true_type ;

    struct alice {
        using blue_pill = this_namespace::has_strangers_with_candy ;
        static constexpr auto where_am_i = "Uncle Charles' picnic blanket." ;
    } ;

ENABLE_SUBSPACE( rabbit_hole ) // {
    struct rabbit { using is_late = typename alice::blue_pill ; } ;
ENABLE_SUBSPACE( rabbit_hole ) // {
    struct rabbit { using is_late = typename alice::blue_pill ; } ;
ENABLE_SUBSPACE( rabbit_hole ) // {
    struct rabbit { using is_late = typename alice::blue_pill ; } ;
    using has_strangers_with_candy = std::false_type ; ///// Different...
ENABLE_SUBSPACE( rabbit_hole ) // {
    struct rabbit { using is_late = typename alice::blue_pill ; } ;
ENABLE_SUBSPACE( rabbit_hole ) // {
    struct rabbit { using is_late = typename alice::blue_pill ; } ;
    struct alice { ///// Different...
        using blue_pill = has_strangers_with_candy ;
        static constexpr auto where_am_i = "needing a fix." ;
    } ;
ENABLE_SUBSPACE( rabbit_hole ) // {
    struct rabbit { using is_late = typename alice::blue_pill ; } ;
    struct alice : last_namespace::alice { ///// Different...
        static constexpr auto where_am_i = "an empty rabbit hole." ;
    } ;
} ; // END rabbit_hole
} ; // END rabbit_hole
} ; // END rabbit_hole
} ; // END rabbit_hole
} ; // END rabbit_hole
} ; // END rabbit_hole
} ; // END oxford_england


/////
///// Snarky Output
/////

int main( int argc, char** argv ) {

    std::cout << std::endl
        << dorothy::obvious_statement
        << alice::where_am_i
        << std::endl ; // There's no place like HOME!

    std::cout
        << dorothy::obvious_statement
        << oxford_england::rabbit_hole::rabbit_hole::rabbit_hole::rabbit_hole::rabbit_hole::alice::where_am_i
        << std::endl ; // There's no place like needing a fix.

    std::cout
        << dorothy::obvious_statement
        << oxford_england::rabbit_hole::rabbit_hole::rabbit_hole::rabbit_hole::rabbit_hole::rabbit_hole::alice::where_am_i
        << std::endl ; // There's no place like an empty rabbit hole.

    std::cout << std::endl
        << rabbit::says(
            oxford_england::
                rabbit_hole::
                    rabbit_hole::
                        rabbit_hole::
                            rabbit_hole::
                                rabbit_hole::rabbit::is_late()
        ) << std::endl ; // I'm late!
    std::cout
        << rabbit::says(
            oxford_england::
                rabbit_hole::
                    rabbit_hole::
                        rabbit_hole::
                            rabbit_hole::
                                rabbit_hole:: // NOTE : alice::blue_pill is false_type
                                    rabbit_hole::rabbit::is_late() // ... not this time ; Alice is crashing.
        ) << std::endl ; // I'm late, but I ditched that addled girl.

    std::cout << std::endl
        << dorothy::obvious_statement
        << oxford_england::
                rabbit_hole:: // 1
                    rabbit_hole:: // 2
                        rabbit_hole:: // 3
                            rabbit_hole:: // 4
                                rabbit_hole:: // 5
                                    rabbit_hole:: // rabbit_hole #6
                                last_namespace:: // rabbit_hole #5
                            last_namespace:: // rabbit_hole #4
                        last_namespace:: // rabbit_hole #3
                    last_namespace:: // rabbit_hole #2
                last_namespace:: // rabbit_hole #1
            last_namespace::alice::where_am_i // oxford_england
        << std::endl ; // There's no place like Uncle Charles' picnic blanket.
    std::cout
        << dorothy::obvious_statement
        << oxford_england::
                rabbit_hole::
                    rabbit_hole::
                        rabbit_hole::
                            rabbit_hole::
                                rabbit_hole::
                                    rabbit_hole::
                                last_namespace::
                            last_namespace::
                        last_namespace:: // 3
                    last_namespace:: // 2
                last_namespace:: // 1
            last_namespace:: // oxford
        last_namespace::alice::where_am_i // not the global namespace!
        << ".. but I'd rather be " << ::alice::where_am_i // the global namespace.
        << std::endl ; // There's no place like Uncle Charles' picnic blanket... but I'd rather be HOME!

    std::cout << std::endl ;
    return 0 ;
}


/////
///// EOF
/////


/* Compiled with:
    `g++ -std=c++11 -o alice alice.cpp`
*/

/* Output of `alice` :

There's no place like HOME!
There's no place like needing a fix.
There's no place like an empty rabbit hole.

I'm late!
I'm late, but I ditched that addled girl.

There's no place like Uncle Charles' picnic blanket.
There's no place like Uncle Charles' picnic blanket... but I'd rather be HOME!

*/



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