使用声明（派生类） [英] Using declaration (Derived class)

问题描述

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（）被视为 B2 :: fb（B2 *）？也就是说，隐式参数是否有助于区分它们？

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-

$13.3.1/4-

对于通过使用声明将
引入派生的
类的非转换函数，该函数被视为
是
的派生类的成员。定义隐式对象参数
的类型。

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.

推荐答案

C ++标准（C ++ 03§7.3.3/ 12）解释：

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

当使用声明出现时名称从基类到派生类范围中，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（而不是冲突）。

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).

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

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

至于为什么两个 u都格式不正确请在 B2 的定义中使用B1 :: d; 和 int d; C ++标准（C ++ 03§7.3.3/ 10）解释：

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:

由于 using-declaration 是声明，对同一声明区域中同名声明的限制也适用于 using-declaration。

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.

因此，它的格式错误是由于以下原因导致的格式错误：它在单个声明性区域中导致两个名称相同的对象：

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:

struct S { int d; int d; };

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