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简短形式:如何在 C++11 中为根(全局)命名空间定义别名?它可能看起来像

namespace root_namespace = :: ;

其中,上面裸形式的范围解析运算符是全局命名空间的某个句柄的占位符。我在GCC Internals Manual中读到

...[编译器]整个中间表示的根是变量 global_namespace。这是::在 C++ 源代码中指定的命名空间... 全局命名空间的名称是::,尽管在 C++ 中全局命名空间是未命名的。

PS 编辑:对于迄今为止的受访者,我在以下长表格之后附加了一个痛苦的长表格来解决一些问题,因为它可能会澄清一些事情。追随者,如果你看到我们在谈论而不是互相交谈,请深入了解。


长格式:以下是其潜在用途的一个示例。如果不满意,那么是的,这是一个学术问题;看看下面这个令人痛苦的长表格

想象一下,有一天你的老板闯入说:“我刚读了一本关于后实证主义的书。摆脱…… namespace ObjectiveReality”。在下面的代码中,你所要做的就是省略我这样标记的/* -> *//* <- */。您目前可以对中间级别的嵌套执行此操作;但是,我不确定如何定义全局范围namespace current_authority以允许简单地省略第一个非全局命名空间。

#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

如果需要编译器信息:

$ 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:作为对“从嵌套命名空间开始”的一些答案的回应,请注意“HOME!” 是不可访问的,而且我可能没有在团队中手工挑选命名空间的奢侈。

//
// 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!

*/
4

2 回答 2

2

我不认为你可以给全局命名空间取别名。

该标准定义了一个名称空间别名,其中 :: 显式调用,而不是将其视为另一个名称空间名称:

7.3.2   Namespace alias
A namespace-alias-definition declares an alternate name for a namespace according to the following grammar:
namespace-alias:
    identifier 
namespace-alias-definition:
    namespace identifier = qualified-namespace-specifier ;
qualified-namespace-specifier:
    ::_opt nested-name-specifier_opt namespace-name

请注意 :: 是 opt(可选),但 namespace-name 不是。

但是,您不能只使用全局名称空间以外的东西作为名称空间堆栈的开始,并且仍然遵循您概述的模式吗?

另请注意,当您删除 ObjectiveReality 时,您的示例实际上确实需要编辑几行:

namespace current_authority = ObjectiveReality ;

和:

<< ObjectiveReality::EntangledObserver::cat_is_alive::value

这是您的示例使用命名空间“something”而不是全局命名空间作为命名空间堆栈的根的样子。

#include <iostream>
#include <iomanip>


// ...                                                                                                                                                                                                                                                                                                                                                                    
namespace something {
  using cat_is_alive = std::true_type ; // because I like cats                                                                                                                                                                                                                                                                                                            
}
using namespace something;
// ...                                                                                                                                                                                                                                                                                                                                                                    


// 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 = something;
namespace current_authority = something;                                                                                                                                                                                                                                                                                                         

// 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
于 2013-02-08T00:14:42.837 回答
1

我不知道直接别名默认命名空间的方法,但作为一种解决方法,您可以使用宏(假设您愿意忍受宏的名称与不相关的符号冲突)。

#define root_namespace

用法:

root_namespace::cat_is_alive

正如 Mooing Duck 正确指出的那样,此 hack 不适用于using子句。

于 2013-02-08T21:12:57.990 回答