c++ - 没有显式初始化父类的虚拟继承

Question

我有两个类 Machine 和 Human，它们实际上是从 atom 继承的。现在我编写了一个名为“Cyborg”的类，并希望从 Cyborg 初始化 Atom。代码是

#include <iostream>

using namespace std;

class Atom {
public:
  Atom(size_t t): transformium(t) {};
private:
  const size_t transformium;
};

class Human : public virtual Atom {
public:
  Human(string name) : Atom(-1), name(name) {}
protected:
  string name;
};

class Machine : public virtual Atom {
public:
  Machine() {}
  Machine(int id) : id(id) {}
protected:
  int id;
};

class Cyborg : public Human, public Machine {
public:
  Cyborg(string name, int id) : Atom(0), Human(name) {}
};

int main() {
  Cyborg cyborg("robocup", 911);
  return 0;
}

但是，由于常量成员“transformium”，CXX 编译器需要 Machine 来初始化 Atom。

error: constructor for 'Machine' must explicitly initialize the base class 'Atom' which does not have a default constructor

Answer 1

正如编译器错误提示的那样，您必须在 Machine 的构造函数中显式初始化基类 Atom。

这样做的原因是您可以创建 Machine 的实例。对于这样的对象，必须有一种方法来正确初始化 Machine 的 Atom 部分。

更新

Atom 的初始化方式不同，具体取决于您创建的是 Human、Machine 还是 Cyborg 的实例。这是您的代码的更新版本，其中包含一些解释。

#include <iostream>

using namespace std;

class Atom {
   public:
      Atom(size_t t): transformium(t) { std::cout << "Came to Atom::Atom()\n"; }
   private:
      const size_t transformium;
};

class Human : public virtual Atom {
   public:
      Human(string name) : Atom(-1), name(name) {}
      // Atom(-1) is ignored when an instance of Cyborg is created
      // but not when an instance of Human is created.
   protected:
      string name;
};

class Machine : public virtual Atom {
   public:
      Machine(int id) : id(id), Atom(-1) {}
      // Atom(-1) is ignored when an instance of Cyborg is created
      // but not when an instance of Machine is created.
   protected:
      int id;
};

class Cyborg : public Human, public Machine {
   public:
      Cyborg(string name, int id) : Atom(0), Human(name), Machine(id) {}
      // Atom needs to be intialized here since it won't be initialized 
      // in either Human or Machine.
};

int main() {
   Cyborg cyborg("robocup", 911); // Calls Atom::Atom() only once.
   std::cout << "------------------\n";
   Human human("robocup");        // Calls Atom::Atom() once.
   std::cout << "------------------\n";
   Machine machine(911);          // Calls Atom::Atom() once.
   return 0;
}

输出:

Came to Atom::Atom()------------------Came to Atom::Atom()------------------Came to Atom::Atom()