I'm having some difficulty understanding [basic.scope.pdecl]/7

Question

[basic.scope.pdecl]/7:

The point of declaration of a class first declared in an elaborated-type-specifier is as follows:

(7.1)   for a declaration of the form

          class-key attribute-specifier-seq_optidentifier ;

         the identifier is declared to be a class-name in the scope that contains the declaration,          otherwise

(7.2)   for an elaborated-type-specifier of the form

          class-key identifier

         if the elaborated-type-specifier is used in the decl-specifier-seq or
         parameter-declaration-clause of a function defined in namespace scope, the identifier is          declared as a class-name in the namespace that contains the declaration; otherwise,          except as a friend declaration, the identifier is declared in the smallest namespace or          block scope that contains the declaration. [ Note: These rules also apply within templates.           — end note ] [ Note: Other forms of elaborated-type-specifier do not declare a new          name, and therefore must refer to an existing type-name. See [basic.lookup.elab] and          [dcl.type.elab].— end note ]

Consider the case (7.2) above where the elaborated-type-specifier is used in the decl-specifier-seq of a parameter-declaration-clause of a function defined in namespace scope. How would that conciliate with the fact that this elaborated-type-specifier must be the first declaration of the class in its namespace?

Consider the example (demo) below:

File prog.cc:

struct S;
extern S s;
int S;
void f(struct S&);     // The elaborated-type-specififer `struct S` is not
                       // the first declaration in the global namespace and
                       // if we eliminate the first declaration `struct S;` 
                       // on the top, the code doesn't compile !!
int main(){
    f(s);
}

File other.cc:

#include<iostream>
struct S{
    int i = 1;
};
void f(struct S& s) { std::cout << s.i << '\n'; }
S s;

Note that the code above compiles and executes correctly, but the elaborated-type-specififer in the parameter-declaration-clause of function f is not the first in the global namespace.

Assuming that my interpretation about [basic.scope.pdecl]/7 is correct, I would like to see an example showing the application of paragraph (7.2) above, where the alluded declaration would be the first in its namespace.

Answer 1

I would like to see an example showing the application of paragraph (7.2) above, where the alluded declaration would be the first in its namespace.

Simply:

namespace ns {
    // declares S into ns as per [basic.scope.pdecl]
    void f(struct S&);
}

extern ns::S s;
//extern ::S s; // not declared

Here struct S is first declared in the elaborated-type-specifier in a parameter-declaration-clause of a function defined in namespace scope, with the form class-key identifier, and therefore [basic.scope.pdecl]/7.2 applies and struct S is declared in the namespace ns where the function is declared.

you'll have to use an object of class S in your function f

Here is an example:

// ... continuing from previous example ...
namespace ns {
    struct S {
        int i;
    };

    void f(S& s) {
        std::cout << s.i;
    }
}

---

As a bonus, an example where the class is not first declared in the elaborated-type-specifier, and so the quoted rule does not apply:

struct S;
namespace ns {
    void f(struct S&); // refers to ::S
}

//extern ns::S s; // not declared
extern ::S s;

Here, the elaborated-type-specifier is not the first declaration of struct S, so [basic.scope.pdecl]/7 does not apply, and no class is declared into the namespace.

Answer 2

if we eliminate the first declaration struct S; on the top, the code doesn't compile !!

Well that's because you still need to declare a name before you use it.

int S;
void f(struct S&); 
extern struct S s; // If you write this line before then it
                   // will not compile.
                   // The name still needs to be declared
                   // before you use it.
// If you drop the `int S` above, then the following
// will also compile, since S has already been declared
// extern S s2;

int main(){
    f(s);
}

Note that the code above compiles and executes correctly, but the elaborated-type-specififer in the parameter-declaration-clause of function f is not the first in the global namespace.

I don't understand the point you're trying to make here. Since it's not the first, no name is declared and [basic.scope.pdecl]p7 doesn't apply.

I would like to see an example showing the application of paragraph (7.2) above, where the alluded declaration would be the first in its namespace.

auto addrof(struct S& s) { // First declaration
    return &s;
}
int get(struct T&); // First declaration