Using-directives facilitate the use of namespaces and types defined in other
namespaces. Using-directives
impact the name resolution process of namespace-or-type-names (§10.8) and
simple-names (§14.5.2), but
unlike declarations, using-directives do not contribute new members to the
underlying declaration spaces of
the compilation units or namespaces within which they are used.
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using-directives:
using-directive
using-directives using-directive
using-directive:
using-alias-directive
using-namespace-directive
A using-alias-directive (§16.3.1) introduces an alias for a namespace or
type.
A using-namespace-directive (§16.3.2) imports the type members of a
namespace.
The scope of a using-directive extends over the
namespace-member-declarations of its immediately
containing compilation unit or namespace body. The scope of a
using-directive specifically does not include
its peer using-directives. Thus, peer using-directives do not affect each
other, and the order in which they are
written is insignificant.
16.3.1 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
}
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204
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-aliasdirective
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 namespaceor-
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
}
Chapter 16 Namespaces
205
the last using-alias-directive results in a compile-time error because it
is not affected by the first using-aliasdirective.
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]
16.3.2 Using namespace directives
A using-namespace-directive imports the types contained in a namespace into
the immediately enclosing
compilation unit or namespace body, enabling the identifier of each type to
be used without qualification.
using-namespace-directive:
using namespace-name ;
Within member declarations in a compilation unit or namespace body that
contains a using-namespacedirective,
the types contained in the given namespace can be referenced directly.
[Example: For example:
namespace N1.N2
{
class A {}
}
namespace N3
{
using N1.N2;
class B: A {}
}
Above, within member declarations in the N3 namespace, the type members of
N1.N2 are directly
available, and thus class N3.B derives from class N1.N2.A. end example]
A using-namespace-directive imports the types contained in the given
namespace, but specifically does not
import nested namespaces. [Example: In the example
namespace N1.N2
{
class A {}
}
namespace N3
{
using N1;
class B: N2.A {} // Error, N2 unknown
}
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206
the using-namespace-directive imports the types contained in N1, but not
the namespaces nested in N1. Thus,
the reference to N2.A in the declaration of B results in a compile-time
error because no members named N2
are in scope. end example]
Unlike a using-alias-directive, a using-namespace-directive may import
types whose identifiers are already
defined within the enclosing compilation unit or namespace body. In effect,
names imported by a usingnamespace-
directive are hidden by similarly named members in the enclosing
compilation unit or
namespace body. [Example: For example:
namespace N1.N2
{
class A {}
class B {}
}
namespace N3
{
using N1.N2;
class A {}
}
Here, within member declarations in the N3 namespace, A refers to N3.A
rather than N1.N2.A. end example]
When more than one namespace imported by using-namespace-directives in the
same compilation unit or
namespace body contain types by the same name, references to that name are
considered ambiguous.
[Example: In the example
namespace N1
{
class A {}
}
namespace N2
{
class A {}
}
namespace N3
{
using N1;
using N2;
class B: A {} // Error, A is ambiguous
}
both N1 and N2 contain a member A, and because N3 imports both, referencing
A in N3 is a compile-time
error. end example] In this situation, the conflict can be resolved either
through qualification of references
to A, or by introducing a using-alias-directive that picks a particular A.
[Example: For example:
namespace N3
{
using N1;
using N2;
using A = N1.A;
class B: A {} // A means N1.A
}
end example]
Like a using-alias-directive, a using-namespace-directive does not
contribute any new members to the
underlying declaration space of the compilation unit or namespace, but,
rather, affects only the compilation
unit or namespace body in which it appears.
The namespace-name referenced by a using-namespace-directive is resolved in
the same way as the
namespace-or-type-name referenced by a using-alias-directive. Thus,
using-namespace-directives in the
same compilation unit or namespace body do not affect each other and can be
written in any order.
