[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_opt identifier ;

         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 ]

考虑上面的情况 (7.2)，其中 elaborated-type-specifier 用于 parameter-declaration-clause 的 decl-specifier-seq 在命名空间范围内定义的函数。这如何与这个 elaborated-type-specifier 必须是类在其命名空间中的第一个声明这一事实相协调？

考虑下面的示例 ( demo):

文件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);
}

文件other.cc:

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

请注意，上面的代码可以正确编译和执行，但是 parameter-declaration-clause 函数 f 不是全局命名空间中的第一个。

假设我对 [basic.scope.pdecl]/7 的解释是正确的，我想看一个例子来说明上面第 (7.2) 段的应用，其中提到的声明将是第一个 在其命名空间中。

最佳答案

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

那是因为您仍然需要在使用前声明一个名称。

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.

我不明白你在这里想表达的意思。因为它不是第一个，所以没有声明名称并且 [basic.scope.pdecl]p7 不适用。

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

关于c++ - 我在理解 [basic.scope.pdecl]/7 时遇到一些困难，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/55227455/