c++ - 关于从派生类在基类中实现的C++调用虚函数的问题

Question

下面的代码有什么问题？

struct A {
  virtual int hash() const = 0;
  virtual int hash(int x) const = 0;
};

struct B : public A {
  int hash() const final {
    return 10;
  };

  int hash(int x) const override {
    return 10;
  };
};

struct C : public B {
  int hash(int x) const override {
    return x;
  }
};

#include <iostream>

int main() {
  C a;
  std::cout << a.hash() << std::endl;
  std::cout << a.hash(20) << std::endl;
  return 0;
}

我遇到编译错误，错误信息如下

xx.cc:26:23: error: too few arguments to function call, single argument 'x' was
      not specified
  std::cout << a.hash() << std::endl;
               ~~~~~~ ^
xx.cc:17:3: note: 'hash' declared here
  int hash(int x) const override {
  ^
1 error generated.

Answer 1

这是名称隐藏问题。根据name lookup的规则,

(强调我的)

name lookup examines the scopes as described below, until it finds at least one declaration of any kind, at which time the lookup stops and no further scopes are examined.

因此 C::hash 对基类隐藏了名称。

您可以应用 using 将名称引入类 C 作用域。

struct C : public B {
  using B::hash;
  int hash(int x) const override {
    return x;
  }
};