“override”限定符在哪里与尾返回类型？ [英] Where does the 'override' qualifier go with trailing return types?

问题描述

C ++ 11具有新的 override 限定符，可以应用于成员函数，以声明它们覆盖基类中的虚函数。 C ++ 11也允许尾随返回类型，所以函数可以声明为 auto f（） - > return_type 。当我结合这两个特性，我不知道 override 是在 - > 之前还是之后。 / p>

例如，假设我们有以下基类：

  struct Base {
 virtual auto f（）const  - > int = 0; 
}; 
  

派生类的两种可能性是：

  struct派生：public Base {
 virtual auto f（）const override  - > int {return 0; } //编译g ++ 4.7.1 
}; 
  

或

  struct Derived：public Base {
 virtual auto f（）const  - > int override {return 0; } //编译clang ++ 4.0 
}; 
  

g ++ 4.7.1编译第一个版本，但第二个版本失败，

$ b $成员声明结束时，b

  test.cpp：6：30：error：expected';'
 test.cpp：6：34：error： 'override'没有命名类型
  

，而clang ++ 4.0编译第二个，但第一个失败

  test.cpp：6：11：error：'auto'return without trailing return type 
 virtual auto f ）const override  - > int {return 0; } 
 ^ 
 test.cpp：6：3：错误：只有virtual成员函数可以标记为'override'
 virtual auto f（）const override  - > int {return 0; } 
 ^ ~~~~~~~~ 
 test.cpp：6：35：error：expected';'在声明列表的结尾
 virtual auto f（）const override  - > ; int {return 0; } 
  

这些编译器中哪些正在根据标准做正确的事？

修改：Kerrek SB注意到，这是

解决方案

根据标准8.4.1，函数的声明符包括 trailing-return-type ，类函数定义包含 declarator virt-specifier-seq _opt 。第二个 virt-specifier-seq 是 final 或 override 因此这些字符在尾随返回类型之后。 （即Clang得到正确的结果。）

C++11 has the new override qualifier which can be applied to member functions to assert that they override a virtual function in the base class. C++11 also allows trailing return types so functions can be declared as auto f() -> return_type. When I combine both these features, I don't know whether override goes before or after the ->.

For example, suppose we have the following base class:

struct Base {
    virtual auto f () const -> int = 0;
};

The two possibilities for a derived class are:

struct Derived : public Base {
    virtual auto f () const override -> int { return 0; } // Compiles on g++ 4.7.1
};

or

struct Derived : public Base {
    virtual auto f () const -> int override { return 0; } // Compiles on clang++ 4.0
};

g++ 4.7.1 compiles the first version but fails on the second with

test.cpp:6:30: error: expected ';' at end of member declaration
test.cpp:6:34: error: 'override' does not name a type

whereas clang++ 4.0 compiles the second one but fails on the first with

test.cpp:6:11: error: 'auto' return without trailing return type
  virtual auto f () const override -> int { return 0; }
          ^
test.cpp:6:3: error: only virtual member functions can be marked 'override'
  virtual auto f () const override -> int { return 0; }
  ^                       ~~~~~~~~
test.cpp:6:35: error: expected ';' at end of declaration list
  virtual auto f () const override -> int { return 0; }

Which of these compilers is actually doing the right thing according to the standard?

Edit: As Kerrek SB notes, this is a bug in gcc (Bugzilla link).

解决方案

According to the standard, 8.4.1, a declarator for a function includes the trailing-return-type, and a class function definition contains "declarator virt-specifier-seq_opt". The second one, virt-specifier-seq, is one of final or override, so those come after the trailing return type. (I.e. Clang gets it right.)

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