使用声明（派生类）

发布于 2024-09-15 04:07:08 字数 484 浏览 13 评论 0原文

struct B1{
  int d;
  void fb(){};
};

struct B2 : B1{
  using B1::d;
  using B1::fb;

  int d;               // why this gives error?
  void fb(){}          // and this does not?
};

int main(){}

是因为， B1::fb() 被视为 B1::fb(B1*) 和 B2::fb() 对待作为B2::fb(B2*)？也就是说，隐式参数是否有助于区分这些？

$13.3.1/4-
对于非转换函数的引入通过使用声明到派生类，该函数被认为是是派生类的成员定义类型的目的隐式对象参数。

原文

struct B1{
  int d;
  void fb(){};
};

struct B2 : B1{
  using B1::d;
  using B1::fb;

  int d;               // why this gives error?
  void fb(){}          // and this does not?
};

int main(){}

Is it because, B1::fb() is treated as B1::fb(B1*) and B2::fb() treated as B2::fb(B2*)? That is, does the implicit parameter, help in distinguishing these?

$13.3.1/4-
For nonconversion functions introduced
by a using-declaration into a derived
class, the function is considered to
be a member of the derived class for
the purpose of defining the type of
the implicit object parameter.

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海风掠过北极光 2024-09-22 04:07:08

C++ 标准 (C++03 §7.3.3/12) 解释道：

当 using-declaration 将基类中的名称带入派生类作用域时，派生类中的成员函数会覆盖和/或隐藏基类中具有相同名称和参数类型的成员函数类（而不是冲突）。

在您的示例中，B2::fb() 隐藏了 using 声明引入的 B1::fb()。

至于为什么在 B2 的定义中同时使用 using B1::d; 和 int d; 是格式错误的，C++标准 (C++03 §7.3.3/10) 解释：

由于using-declaration是一个声明，因此对同一声明区域中同名声明的限制也适用于using-declarations。

因此，它是格式错误的出于同样的原因，以下内容格式不正确：它会在单个声明区域中产生两个具有相同名称的对象：

struct S { int d; int d; };

The C++ standard (C++03 §7.3.3/12) explains:

When a using-declaration brings names from a base class into a derived class scope, member functions in the derived class override and/or hide member functions with the same name and parameter types in a base class (rather than conflicting).

In your example, B2::fb() hides the B1::fb() introduced by the using declaration.

As for why it is ill-formed to have both using B1::d; and int d; in the definition of B2, the C++ standard (C++03 §7.3.3/10) explains:

Since a using-declaration is a declaration, the restrictions on declarations of the same name in the same declarative region also apply to using-declarations.

So, it is ill-formed for the same reason that the following is ill-formed: it results in two objects with the same name in a single declarative region: