Lou*_*ole 11 c++ alias namespaces global-scope c++11
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 = :: ;
上面的裸体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中读到了这一点
... [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::,即使在C++中,全局命名空间是未命名的.
PS EDIT:对于迄今为止的受访者,我添加了一个痛苦的长篇表来解决以下长篇形式后的一些问题,因为它可能会澄清一些事情.和追随者,如果你看到我们而不是彼此说话,那就深入挖掘.
长形式:其潜在用途的一个例子如下.如果不满意,那么是的,这是一个学术问题; 看到这个之后痛苦的长形.
想象一下,你的老板在一天内闯入并说:"我刚读了一本关于后实证主义的书.摆脱它namespace ObjectiveReality." 在下面的代码,你需要做的"一切"是忽略我喜欢标线/* -> */这样/* <- */.你可以为目前的中间嵌套级别做到这一点; 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"的一些答案的回答,请注意'HOME!' 是不可访问的,我可能没有在团队中手工挑选名称空间的奢侈品.
//
// 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!
*/
我不认为你可以给全局命名空间起别名。
该标准定义了一个命名空间别名，并使用 :: 显式调用，而不是将其视为另一个命名空间名称：
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），但名称空间名称不是。
但是，您难道不能仅使用全局名称空间以外的其他名称作为名称空间堆栈的开头，并且仍然遵循您概述的模式吗？
另请注意，当您删除 ObjectiveReality 时，您的示例实际上需要编辑几行：
namespace current_authority = ObjectiveReality ;
和：
<< ObjectiveReality::EntangledObserver::cat_is_alive::value
以下是您的示例使用名称空间“something”而不是全局名称空间作为名称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
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