问题描述：

类声明：student.h

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namespace Studenti {
  class Student : private std::string, private std::valarray<double> {
  private:
    typedef std::valarray<double> ArrayDb;
    std::ostream& arr_out(std::ostream& os) const;

  public:
    Student();

    ...
    
    friend std::ostream& operator<<(std::ostream& os, const Student& stu);
};
}

类定义：student.cpp

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#include "student.h"
...

using namespace Studenti;

ostream& operator<<(ostream& os, const Student& stu) {
  os << "Scores for " << (const string&)stu << ":\n";
  stu.arr_out(os);
  return os;
}
...

这里编译器直接给 stu.arr_out(os) 标红，说无法访问类的私有成员。很是奇怪，为什么加了给类加上命名空间后友元函数无法访问类的私有成员呢？

在这里 friend function within a namespace 找到了解决方法。修改方式有两种

一、

  1. 修改 student.h 文件中的代码为
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...
namespace Studenti {
  class Student : private std::string, private std::valarray<double> {
  private:
    typedef std::valarray<double> ArrayDb;
    std::ostream& arr_out(std::ostream& os) const;

  public:
    Student();

    ...

    friend std::ostream& operator<<(std::ostream& os, const Student& stu);
  };
  std::ostream& operator<<(std::ostream& os, const Student& stu);
}
  1. 修改 student.cpp 文件中的代码为
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#include "student.h"
...

using namespace Studenti;

ostream& Studenti::operator<<(ostream& os, const Student& stu) {
  os << "Scores for " << (const string&)stu << ":\n";
  stu.arr_out(os);
  return os;
}

...

二、

  1. 同第一种方式里的1

  2. 修改 student.cpp 文件中的代码为

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#include "student.h"
...

using namespace Studenti;

namespace Studenti {
  ostream& operator<<(ostream& os, const Student& stu) {
    os << "Scores for " << (const string&)stu << ":\n";
    stu.arr_out(os);
    return os;
  }
}

...