定义封闭命名空间 - 这对 clang 和 g++ 有什么影响（编译时间、内存使用等）

Question

给出示例代码段：

namespace project_namespace::my_math
{
    namespace enclosing_namespace = my_math;

    struct sphere {};
    
    struct sphere_stuff {
        using sphere = enclosing_namespace::sphere;
    };
    
    struct line {};
    
    struct line_stuff {
        using line = enclosing_namespace::line;
    } ;
}


int main()
{
    using sphere_stuff = project_namespace::my_math::sphere_stuff;
    sphere_stuff::sphere sphere;

    using line_stuff = project_namespace::my_math::line_stuff;
    line_stuff::line line;
    
    return 0;
}

当我看到上面的代码时，我心里想：

1. 是的，enclosure_namespace/super_namespace，我希望 c++ 有这个！我刚刚知道该怎么做吗？
2. 我想知道这是不是一个可怕的主意？

所以我的问题有两个部分：

1. 当 clang 和 g++ 编译器遇到这个“命名空间 enclusion_namespace = my_math”时，它们如何处理这个问题（c++17 和 c++20）？在上面的片段中（如果在大型代码库中使用），我是否创建了一种将命名空间复制到命名空间到命名空间等的情况，这对于编译器来说是一场噩梦？或者这对于编译器来说是微不足道的吗？

2. 有什么理由不应该这样做（在 2023 年，c++17 或 c++20）吗？

编辑：最初代码使用“super_namespace”而不是“enleading_namespace” - 下面的一些评论提到了这一点。

Answer 1

他们为我提供了一个在线编译器的链接，该编译器生成 AST 转储，并且还澄清了基本上每个别名只插入一个节点。

我提供给他们的例子是：

namespace project {
    namespace enclosing_namespace = project;
    
    namespace namespace_1 {
        namespace parent_namespace = project;
        namespace enclosing_namespace = namespace_1;
    
        namespace b {
            namespace parent_namespace = namespace_1;
            namespace enclosing_namespace = b;
            
            struct X {};
            struct Y {
                using Z = enclosing_namespace::X;
            } ;
        }
    }
    
    namespace namespace_2 {
        namespace parent_namespace = project;
        namespace enclosing_namespace = namespace_2;
    
        namespace a {
            namespace parent_namespace = namespace_2;
            namespace enclosing_namespace = a;
    
            struct X {};
            struct Y {
                using Z = enclosing_namespace::X;
            } ;
        }
    }
}

int main()
{
    project::namespace_1::parent_namespace::namespace_2::a::Y::Z z;

    return 0;
}

转储看起来像这样：

TranslationUnitDecl
|-NamespaceDecl <line:1:1, line:33:1> line:1:11 project
| |-NamespaceAliasDecl <line:2:5, col:37> col:15 enclosing_namespace
| | `-Namespace 'project'
| |-NamespaceDecl <line:4:5, line:17:5> line:4:15 namespace_1
| | |-NamespaceAliasDecl <line:5:9, col:38> col:19 parent_namespace
| | | `-Namespace 'project'
| | |-NamespaceAliasDecl <line:6:9, col:41> col:19 enclosing_namespace
| | | `-Namespace 'namespace_1'
| | `-NamespaceDecl <line:8:9, line:16:9> line:8:19 b
| |   |-NamespaceAliasDecl <line:9:13, col:42> col:23 parent_namespace
| |   | `-Namespace 'namespace_1'
| |   |-NamespaceAliasDecl <line:10:13, col:45> col:23 enclosing_namespace
| |   | `-Namespace 'b'
| |   |-CXXRecordDecl <line:12:13, col:23> col:20 referenced struct X definition
| |   | |-DefinitionData pass_in_registers empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init
| |   | | |-DefaultConstructor exists trivial constexpr needs_implicit defaulted_is_constexpr
| |   | | |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param
| |   | | |-MoveConstructor exists simple trivial needs_implicit
| |   | | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param
| |   | | |-MoveAssignment exists simple trivial needs_implicit
| |   | | `-Destructor simple irrelevant trivial needs_implicit
| |   | `-CXXRecordDecl <col:13, col:20> col:20 implicit struct X
| |   `-CXXRecordDecl <line:13:13, line:15:13> line:13:20 struct Y definition
| |     |-DefinitionData pass_in_registers empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init
| |     | |-DefaultConstructor exists trivial constexpr needs_implicit defaulted_is_constexpr
| |     | |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param
| |     | |-MoveConstructor exists simple trivial needs_implicit
| |     | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param
| |     | |-MoveAssignment exists simple trivial needs_implicit
| |     | `-Destructor simple irrelevant trivial needs_implicit
| |     |-CXXRecordDecl <col:13, col:20> col:20 implicit struct Y
| |     `-TypeAliasDecl <line:14:17, col:48> col:23 Z 'enclosing_namespace::X':'project::namespace_1::b::X'
| |       `-ElaboratedType 'enclosing_namespace::X' sugar
| |         `-RecordType 'project::namespace_1::b::X'
| |           `-CXXRecord 'X'
| `-NamespaceDecl <line:19:5, line:32:5> line:19:15 namespace_2
|   |-NamespaceAliasDecl <line:20:9, col:38> col:19 parent_namespace
|   | `-Namespace 'project'
|   |-NamespaceAliasDecl <line:21:9, col:41> col:19 enclosing_namespace
|   | `-Namespace 'namespace_2'
|   `-NamespaceDecl <line:23:9, line:31:9> line:23:19 a
|     |-NamespaceAliasDecl <line:24:13, col:42> col:23 parent_namespace
|     | `-Namespace 'namespace_2'
|     |-NamespaceAliasDecl <line:25:13, col:45> col:23 enclosing_namespace
|     | `-Namespace 'a'
|     |-CXXRecordDecl <line:27:13, col:23> col:20 referenced struct X definition
|     | |-DefinitionData pass_in_registers empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init
|     | | |-DefaultConstructor exists trivial constexpr defaulted_is_constexpr
|     | | |-CopyConstructor simple trivial has_const_param implicit_has_const_param
|     | | |-MoveConstructor exists simple trivial
|     | | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param
|     | | |-MoveAssignment exists simple trivial needs_implicit
|     | | `-Destructor simple irrelevant trivial needs_implicit
|     | |-CXXRecordDecl <col:13, col:20> col:20 implicit struct X
|     | |-CXXConstructorDecl <col:20> col:20 implicit used constexpr X 'void () noexcept' inline default trivial
|     | | `-CompoundStmt <col:20>
|     | |-CXXConstructorDecl <col:20> col:20 implicit constexpr X 'void (const X &)' inline default trivial noexcept-unevaluated 0xbb9e218
|     | | `-ParmVarDecl <col:20> col:20 'const X &'
|     | `-CXXConstructorDecl <col:20> col:20 implicit constexpr X 'void (X &&)' inline default trivial noexcept-unevaluated 0xbb9e418
|     |   `-ParmVarDecl <col:20> col:20 'X &&'
|     `-CXXRecordDecl <line:28:13, line:30:13> line:28:20 struct Y definition
|       |-DefinitionData pass_in_registers empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init
|       | |-DefaultConstructor exists trivial constexpr needs_implicit defaulted_is_constexpr
|       | |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param
|       | |-MoveConstructor exists simple trivial needs_implicit
|       | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param
|       | |-MoveAssignment exists simple trivial needs_implicit
|       | `-Destructor simple irrelevant trivial needs_implicit
|       |-CXXRecordDecl <col:13, col:20> col:20 implicit struct Y
|       `-TypeAliasDecl <line:29:17, col:48> col:23 referenced Z 'enclosing_namespace::X':'project::namespace_2::a::X'
|         `-ElaboratedType 'enclosing_namespace::X' sugar
|           `-RecordType 'project::namespace_2::a::X'
|             `-CXXRecord 'X'
`-FunctionDecl <line:35:1, line:40:1> line:35:5 main 'int ()'
  `-CompoundStmt <line:36:1, line:40:1>
    |-DeclStmt <line:37:5, col:67>
    | `-VarDecl <col:5, col:66> col:66 z 'project::namespace_1::parent_namespace::namespace_2::a::Y::Z':'project::namespace_2::a::X' callinit
    |   `-CXXConstructExpr <col:66> 'project::namespace_1::parent_namespace::namespace_2::a::Y::Z':'project::namespace_2::a::X' 'void () noexcept'
    `-ReturnStmt <line:39:5, col:12>
      `-IntegerLiteral <col:12> 'int' 0

您还可以在这里使用在线编译器：

https://godbolt.org/z/G4beW7YnP

看来创建这些命名空间别名几乎是免费的。