c++ - 将 friend 声明注入(inject)命名空间，Eckel，Vol1，pg :440?

Question

我试图理解将友元声明注入(inject)命名空间:

#include <iostream>
using namespace std;

namespace Z { //forward declaration
  class X;
}

class Y {
   public:
   void func(Z::X*, int);
 };

namespace Z {
  class X {
    int i;
    public:
    X() : i(999) {}
    friend void Y::func(Z::X*, int);
    void print(void) { cout << i << endl; }
  };
}


void Y::func(Z::X* ptr, int i)
{
  ptr->i = 40;
}

int main(void)
{
  Z::X zx;
  zx.print();

  Y y;
  y.func(&zx, 40);
  zx.print();

  using namespace Z;
//  func(&zx, 30); DOES NOT WORK
}

friendInject.cc:39: error: 'func' was not declared in this scope The book says: "You can inject a friend declaration into a namespace by declaring it within an enclosed class". "Now the function you() is a member of the namespace Me."

这到底是什么意思？？我试过 Y::func 但这可能只适用于静态成员函数??

Answer 1

你必须写

y.func(&zx, 30); // work well

与 y。因为基金 - 它是一个类实例方法