将命名空间的重定义成员放入嵌套的内联命名空间

Question

There is a quote from 7.3.1/8 of N3797:

Members of an inline namespace can be used in most respects as though they were members of the enclosing namespace.

Consider the following code snippet:

namespace M
{
    int j = 7;
    inline namespace MM
    {
        int j = 8;
    }
}

I think that the example violates the ODR . But it is not true and it is compiling successful. Can you explain that behavior?

Answer 1

Introduction

7.3p1 Namespaces [basic.namespace]

A namespace is an optionally-named declarative region. The name of a namespace can be used to access entities declared in that namespace; that is, the members of the namespace. Unlike other declarative regions, the definition of a namespace can be split over several parts of one or more translation units.

A declared entity inside a namespace belongs to that namespace, ie. it's a member of that specific namespace, no matter if the namespace is inline or not.

---

ODR VIOLATION = N0NE

Your example snippet does not violate the ODR , mainly because you have 2 different entities named j ;

namespace N {
  int j = 0;            // 1st

  inline namespace M {
    int j = 1;          // 2nd
  }
}

As pointed out further down in [namespace.def]p8 , name lookup in the enclosing namespace will include those found in any inline namespace, but the members of the nested inline namespace are still entities of their own.

7.3.1p8 Namespace definition [namespace.def]

Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lokoup (3.4.2) whenever one of them is, and a using-direction (7.3.4) that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace (7.3.1.1).

Furthermore, each member of the inline namespace can subsequently be explicitly instantiated (14.7.2) or explicitly specialized (14.7.3) as though it were a member of the enclosing namespace. Finally, lookup up a name in the enclosing namespace via explicit qualification (3.4.3.2) will include members of the inline namespace brought in by the using-directive even if there are declarations of the name in the enclosing namespace.

The added names are not treated as redeclarations of previously declared entities, they are additional names, in a nested declarative region, that are brought into the enclosing namespace during name-lookup.

---

<sup>Note : Relying on the compiler to issue a diagnostic in terms of ODR-violations is not safe, mainly because the Standard explicitly states that "no diagnostic [is] required" if an application violates the rules set up by [basic.def.odr].

Further details are avaiable in a comment by Matthieu M. on this post.</sup>