c++ - 运算符<<无法访问类的私有(private)整数

Question

我在尝试编译时得到了这个:

../Monster.h:26:9: error: ‘int ProjectIV::Monster::con’ is private

`int con;`
     ^

../Monster.cpp:17:39: error: within this context

cout << "Constitution: " << monster.con << endl; ^

make: * [Monster.o] Error 1

据我了解制作运算符<<一个 friend 应该允许它访问int con。我没看到什么。

怪物.h:

#ifndef MONSTER_H_
#define MONSTER_H_

#include <iostream>
using std::cout;
using std::endl;
using std::ostream;
#include <string>
using std::string;

namespace ProjectIV
{
  class Monster
  {
  friend ostream &operator<< (ostream &out, const Monster &monster);
  public:
    Monster(int con);
  private:
    int con;
  };
} /* namespace ProjectIV */

#endif /* MONSTER_H_ */

怪物.cpp:

#include "Monster.h"

ostream &operator<< (ostream &out, const ProjectIV::Monster &monster)
{
  cout << "Constitution: " << monster.con << endl;

  return out;
}

ProjectIV::Monster::Monster(int con): con(con)
{}

主要.cpp:

#include "Monster.h"
using namespace ProjectIV;

int main()
{
  Monster Gojira(140);
  cout << Gojira << endl;
  return 0;
}

Answer 1

这个:

ostream& operator<<(ostream& out, const ProjectIV::Monster& monster)

应该是:

ostream& ProjectIV::operator<<(ostream& out, const ProjectIV::Monster& monster)

Here你的不工作例子，和here是工作的。

---

此外，根据 AndreyT的评论，你应该在 friend 声明之前添加一个函数声明:

namespace ProjectIV {
    class Monster {
    friend ostream& operator<<(ostream& out, const Monster& monster);
    public:
        Monster(int con);
    private:
        int con;
    };
    ostream& operator<<(ostream& out, const Monster& monster);
    // ^^^ this
}