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好友无法使用命名空间访问私有成员

friend operator-overloading namespaces c++

Thomas Matthews · 技术社区 · 2 年前

在使用MS Visual Studio 2019进行重载时,我遇到了以下错误 operator<< :

Severity    Code    Description Project File    Line    Suppression State
Error   C2248   'Instruction::Device_Impedance::m_minimum_ohms': cannot access private member declared in class 'Instruction::Device_Impedance' friend_ostream_namespace    device_impedance.cpp    13  
Severity    Code    Description Project File    Line    Suppression State
Error   C2248   'Instruction::Device_Impedance::m_maximum_ohms': cannot access private member declared in class 'Instruction::Device_Impedance' friend_ostream_namespace    device_impedance.cpp    14

设备阻抗hpp:

#ifndef INSTRUCTION_DEVICE_IMPEDANCE_HPP
#define INSTRUCTION_DEVICE_IMPEDANCE_HPP

#include <string>
#include <iostream>


namespace Instruction
{

class Device_Impedance
{
public:
    Device_Impedance();

    Device_Impedance(const unsigned int min_ohms,
                     const unsigned int max_ohms);
       
    //! Constructor -- Copy
    Device_Impedance(const Device_Impedance&  di);
    
    //! Destructor
    ~Device_Impedance();

public:
    Device_Impedance&
    operator=(const Device_Impedance& di);

    friend std::ostream& operator<< (std::ostream& out, const Instruction::Device_Impedance&   di);
   

    //--------------------------------------------------------------------------
    //  Public Methods
    //--------------------------------------------------------------------------
public:
    const std::string&      get_name() const override;
    
    //--------------------------------------------------------------------------
    //  Private Members
    //--------------------------------------------------------------------------
private:
    unsigned int    m_minimum_ohms;
    unsigned int    m_maximum_ohms;
};


}   // End namespace Instruction

/*! @}  // End Doxygen Group
 */

#endif // INSTRUCTION_DEVICE_IMPEDANCE_HPP

设备阻抗.cpp

#include "device_impedance.hpp"

using namespace Instruction;


/*!
 *  \details    Output the instruction name to the stream.
 */
std::ostream&
operator<< (std::ostream& out, const Instruction::Device_Impedance&   di)
{
        out << di.get_name()
//***** The next two lines are causing the error.
            << "    " << di.m_minimum_ohms
            << ", "   << di.m_maximum_ohms
            << "\n";

    return out;
}


const std::string&
Device_Impedance ::
get_name() const
{
    static std::string  instruction_name{"DEVICE_IMPEDANCE"};
    return instruction_name;
}

我是否有正确的命名空间语法来实现 操作员<&书信电报; ?

2 回复 | 直到 2 年前

user17732522 2 年前

不,你的语法不正确。 using namespace 不会导致以下声明/定义禁止该命名空间。

要么使用 namespace Instruction { /*definition of operator<< here*/ } 就像你在标题中做的那样,或者在定义中限定名称:

std::ostream&
Instruction::operator<< (std::ostream& out, const Instruction::Device_Impedance&   di)

否则,您将定义全局 ::operator<< 超载,而不是 friend ed一个在封闭类的命名空间中禁止的类,即 Instruction .

什么 使用命名空间 然而,这样做的目的是,您不必为了查找而限定名称,例如。 Instruction:: 函数中的参数是多余的。类中的声明也是如此。

此外,正如问题评论中所指出的那样 override 在 get_name 这也是错误的,因为这个函数实际上并不是一个覆盖。

3CxEZiVlQ 2 年前

namespace Instruction {

class Device_Impedance {
 public:
  friend std::ostream& operator<<(std::ostream& out,
                                  const Instruction::Device_Impedance& di);

 private:
  unsigned int m_minimum_ohms;
  unsigned int m_maximum_ohms;
};

}  // namespace Instruction

上述最小化类声明朋友 operator<< 在命名空间中 Instruction .

using namespace Instruction;

std::ostream& operator<<(std::ostream& out,
                         const Instruction::Device_Impedance& di) {
  out << di.get_name() << "    " << di.m_minimum_ohms << ", "
      << di.m_maximum_ohms << "\n";
  return out;
}

上述代码定义了 操作员<&书信电报; 在全局命名空间中,它没有定义朋友 操作员<&书信电报; 在命名空间中 说明书 .

定义必须在命名空间中

namespace Instruction {

std::ostream& operator<<(std::ostream& out,
                         const Instruction::Device_Impedance& di) {
  out << di.get_name() << "    " << di.m_minimum_ohms << ", "
      << di.m_maximum_ohms << "\n";
  return out;
}

}  // namespace Instruction

或者在课堂上

namespace Instruction {

class Device_Impedance {
 public:
  friend std::ostream& operator<<(std::ostream& out,
                                  const Instruction::Device_Impedance& di) {
    out << di.get_name() << "    " << di.m_minimum_ohms << ", "
        << di.m_maximum_ohms << "\n";
    return out;
  }

 private:
  unsigned int m_minimum_ohms;
  unsigned int m_maximum_ohms;
};

}  // namespace Instruction