What is a nested name specifier?

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I want to know what exactly is a nested name specifier? I looked up in the draft but I could understand the grammar as I haven't taken any Compiler Design classes yet.

void S(){}

struct S{
   S(){cout << 1;}
   void f(){}
   static const int x = 0;
}; 

int main(){ 
   struct S *p = new struct ::S;  
   p->::S::f();

   S::x;  

   ::S(); // Is ::S a nested name specifier?
   delete p;
} 

Answer 1

::S is a qualified-id.

In the qualified-id ::S::f, S:: is a nested-name-specifier.

In informal terms¹, a nested-name-specifier is the part of the id that

• begins either at the very beginning of a qualified-id or after the initial scope resolution operator (::) if one appears at the very beginning of the id and
• ends with the last scope resolution operator in the qualified-id.

Very informally¹, an id is either a qualified-id or an unqualified-id. If the id is a qualified-id, it is actually composed of two parts: a nested-name specifier followed by an unqualified-id.

Given:

struct  A {
    struct B {
        void F();
    };
};

• A is an unqualified-id.
• ::A is a qualified-id but has no nested-name-specifier.
• A::B is a qualified-id and A:: is a nested-name-specifier.
• ::A::B is a qualified-id and A:: is a nested-name-specifier.
• A::B::F is a qualified-id and both B:: and A::B:: are nested-name-specifiers.
• ::A::B::F is a qualified-id and both B:: and A::B:: are nested-name-specifiers.

---

[1] This is quite an inexact description. It's hard to describe a grammar in plain English...

Answer 2

A nested namespace specifier is:

nested-name-specifier :
    class-or-namespace-name::nested-name-specifier(optional)

That is, a non-empty list of namespaces and classnames, each followed by ::, representing a relative branching in the overall "namespace tree" of the program. For example, my_namespace::, my_namespace::inner_namespace::, my_namespace::my_class::, and my_class::.

Note specifically the difference from:

qualified-namespace-specifier :
    ::(optional) nested-name-specifier(optional) class-or-namespace-name

In that a nested-name-specifier may not be absolute (prefixed with :: to refer to the global scope), while a qualified-namespace-specifier can be, but doesn't end with ::.

In your example, ::S resolves to the function ::S(), and not the struct (precendence rules for that were discussed here on Stackoverflow in the question you linked to at the start of your question), so it is not a nested name specifier.