Using alias directives

A using-alias-directive introduces an identifier that serves as an alias for a namespace or type within the immediately enclosing compilation unit or namespace body. using-alias-directive : using identifier=namespace-or-type-name;
Within member declarations in a compilation unit or namespace body that contains a using-alias-directive, the identifier introduced by the using-alias-directive can be used to reference the given namespace or type.
[Example: For example: namespace N1.N2 { class A {} } namespace N3 { using A = N1.N2.A; class B: A {} }
Above, within member declarations in the N3 namespace, A is an alias for N1.N2.A, and thus class N3.B derives from class N1.N2.A. The same effect can be obtained by creating an alias R for N1.N2 and then referencing R.A: namespace N3 { using R = N1.N2; class B: R.A {} } end example]
The identifier of a using-alias-directive must be unique within the declaration space of the compilation unit or namespace that immediately contains the using-alias-directive. [Example: For example: namespace N3 { class A {} } namespace N3 { using A = N1.N2.A; // Error, A already exists }
Above, N3 already contains a member A, so it is a compile-time error for a using-alias-directive to use that identifier. end example]
Likewise, it is a compile-time error for two or more using-alias-directives in the same compilation unit or namespace body to declare aliases by the same name.
A using-alias-directive makes an alias available within a particular compilation unit or namespace body, but it does not contribute any new members to the underlying declaration space. In other words, a using-alias-directive is not transitive, but, rather, affects only the compilation unit or namespace body in which it occurs.
[Example: In the example namespace N3 { using R = N1.N2; } namespace N3 { class B: R.A {} // Error, R unknown } the scope of the using-alias-directive that introduces R only extends to member declarations in the namespace body in which it is contained, so R is unknown in the second namespace declaration. However, placing the using-alias-directive in the containing compilation unit causes the alias to become available within both namespace declarations: using R = N1.N2; namespace N3 { class B: R.A {} } namespace N3 { class C: R.A {} } end example]
Just like regular members, names introduced by using-alias-directives are hidden by similarly named members in nested scopes. [Example: In the example using R = N1.N2; namespace N3 { class R {} class B: R.A {} // Error, R has no member A } the reference to R.A in the declaration of B causes a compile-time error because R refers to N3.R, not N1.N2. end example]
The order in which using-alias-directives are written has no significance, and resolution of the namespace-or-type-name referenced by a using-alias-directive is not affected by the using-alias-directive itself or by other using-directives in the immediately containing compilation unit or namespace body. In other words, the namespace-or-type-name of a using-alias-directive is resolved as if the immediately containing compilation unit or namespace body had no using-directives. [Example: In the example namespace N1.N2 {} namespace N3 { using R1 = N1; // OK using R2 = N1.N2; // OK using R3 = R1.N2; // Error, R1 unknown } the last using-alias-directive results in a compile-time error because it is not affected by the first using-alias-directive. end example]
A using-alias-directive can create an alias for any namespace or type, including the namespace within which it appears and any namespace or type nested within that namespace.
Accessing a namespace or type through an alias yields exactly the same result as accessing that namespace or type through its declared name. [Example: For example, given namespace N1.N2 { class A {} } namespace N3 { using R1 = N1; using R2 = N1.N2; class B { N1.N2.A a; // refers to N1.N2.A R1.N2.A b; // refers to N1.N2.A R2.A c; // refers to N1.N2.A } } the names N1.N2.A, R1.N2.A, and R2.A are equivalent and all refer to the class whose fully qualified name is N1.N2.A. end example]