重复的限定标识符C ++的extern声明

Question

Why is it that qualified names cannot be re-declared inside functions?

The following code fails to compile (in MSVC2015 and clang)

int i;

namespace N
{
int j;
}

void foo()
{
    extern int i;
    extern int i;
    extern int N::j;
    extern int N::j;
}

int main()
{
    return 0;
}

However, if we move the two lines extern int N::j; to just before void foo() then the code compiles fine.

---

UPDATE It is noteworthy that

1. repeated declarations of unqualified names do work,
2. both ::i and ::N::j were already defined in their respective declarations,
3. ::N::j is not visible to foo
4. the grammar does allow extern int ::N::j and it is not a definition
5. the following code also fails (thank to TC for pointing this out)

.

int i;
void foo()
{
    extern int ::i;
}

6. the following code works in MSVC but it gives a warning in clang

.

int i;
extern int ::i;

Answer 1

TC directed me to [dcl.meaning]p1

… When the declarator-id is qualified, the declaration shall refer to a previously declared member of the class or namespace to which the qualifier refers (or, in the case of a namespace, of an element of the inline namespace set of that namespace (7.3.1)) or to a specialization thereof …

It seems to me that this is a case where the standard is not faithfully represented by the implementations.

Answer 2

It is the namespace member definition vs. declaration issue here. Please see C++11 spec:

7.3.1.2 Namespace member definitions [namespace.memdef]

1 Members (including explicit specializations of templates (14.7.3)) of a namespace can be defined within that namespace. [ Example: namespace X { void f() { / ... / } } —end example ]

2 Members of a named namespace can also be defined outside that namespace by explicit qualification (3.4.3.2) of the name being defined, provided that the entity being defined was already declared in the namespace and the definition appears after the point of declaration in a namespace that encloses the declaration's namespace.

Therefore, the revised code below compiles.

int i;

namespace N
{
    int j;

    void foo()
    {
        extern int i;
        extern int j;
    }
}

void foo()
{
    extern int i;

    using namespace N;
    extern int j;
}

int main()
{
    return 0;
}

Here is a generic example of using extern with namespace : In one CPP file:

namespace N
{
    int j;
}

In its header file:

namespace N
{
    extern int j;
}

---

Update:

More on extern, namespace, declaration and definition

The :: scope resolution operator cannot be use for declaration. It may be used for definition. Using :: for extern declaration would be ill-formed. Please see C++11 spec:

7.5 Linkage specifications [dcl.link] ... ...
4 Linkage specifications nest. When linkage specifications nest, the innermost one determines the language linkage. A linkage specification does not establish a scope. A linkage-specification shall occur only in namespace scope (3.3). In a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names with external linkage, and variable names with external linkage declared within the linkage-specification.

3.3.6 Namespace scope [basic.scope.namespace]

1 The declarative region of a namespace-definition is its namespace-body. …

// Compiled with VC2013.
// extern int ::i;   // error C2039: 'i' : is not a member of '`global namespace''
extern int i;     // declaration  
int i;  // definition

namespace N
{
    extern int j;  // declaration
}
int N::j;  // definition
namespace N
{
     // int j;  // definition
}

void foo()
{
    // extern int ::i;  // fatal error C1506: unrecoverable block scoping error
    extern int i;     // declaration  
    // extern int N::j;  // error C2086: 'int N::j' : redefinition
}

int main()
{
    return 0;
}