Inline namespaces and usage of the “using” directive inside namespaces
Prerequisite : Namespaces in C++
An inline namespace is a namespace that uses the optional keyword inline in its original-namespace-definition.
We can see from above example that members of an inline namespace are treated as if they are members of the enclosing namespace in many situations (listed below). This property is transitive: if a namespace N contains an inline namespace M, which in turn contains an inline namespace O, then the members of O can be used as though they were members of M or N.
The inline specifier makes the declarations from the nested namespace appear exactly as if they had been declared in the enclosing namespace. This means it drags out the declaration (“var” in the above example) from a nested namespace to the containing namespace.
Advantages of using inline namespaces:
- Avoid verbose :Consider the above code, if you want to print “var”, you write:
cout << ns1::ns2::ns3::var;
This looks good only if namespace's names are short as in the above example. But by using inline with namespaces there is no need to type the entire namespace as given above or use the "using" directive.
- Support of Library :The inline namespace mechanism is intended to support library evolution by providing a mechanism that supports a form of versioning. Refer this for details.
This same behavior (same as inline namesapces) can also be achieved by using the "using" declarative inside namespaces. A using-directive that names the inline namespace is implicitly inserted in the enclosing namespace (similar to the implicit using-directive for the unnamed namespace). Consider the following C++ code:
Here again, the using directive makes the declarations from the nested namespace appear exactly as if they had been declared in the enclosing namespace.
Also see: Nesting of namespaces
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