[問題] 參照變數為什麼不能宣告為const?

[phdch (好好的生活)]

遇到的問題： (題意請描述清楚)
在螞蟻本5版p719(原文書)的參照parameter為什麼不能宣告成const
我們知道pointer可以宣告成const
為什麼reference varable不能宣告成const?


希望得到的正確結果：


程式跑出來的錯誤結果：
若問題------->改為void virtualViaReference( const Employee & const);
多了後面的const,會出現error message如下
23 C:\dev\fig13_23.cpp `const' qualifiers cannot be applied to `const
Employee&'


開發平台： (例: VC++ or gcc/g++ or Dev-C++, Windows or Linux)
Dev-C++

有問題的code： (請善用置底文標色功能)

// Fig. 13.23: fig13_23.cpp
// Processing Employee derived-class objects individually
// and polymorphically using dynamic binding.
#include <iostream>
using std::cout;
using std::endl;
using std::fixed;

#include <iomanip>
using std::setprecision;

#include <vector>
using std::vector;

// include definitions of classes in Employee hierarchy
#include "Employee.h"
#include "SalariedEmployee.h"
#include "HourlyEmployee.h"
#include "CommissionEmployee.h"
#include "BasePlusCommissionEmployee.h"

void virtualViaPointer( const Employee * const ); // prototype
*[1m問題---------> void virtualViaReference( const Employee &); // prototype*[m

int main()
{
   // set floating-point output formatting
   cout << fixed << setprecision( 2 );

   // create derived-class objects
   SalariedEmployee salariedEmployee(
      "John", "Smith", "111-11-1111", 800 );
   HourlyEmployee hourlyEmployee(
      "Karen", "Price", "222-22-2222", 16.75, 40 );
   CommissionEmployee commissionEmployee(
      "Sue", "Jones", "333-33-3333", 10000, .06 );
   BasePlusCommissionEmployee basePlusCommissionEmployee(
      "Bob", "Lewis", "444-44-4444", 5000, .04, 300 );

   cout << "Employees processed individually using static binding:\n\n";

   // output each Employee旧 information and earnings using static binding
   salariedEmployee.print();
   cout << "\nearned $" << salariedEmployee.earnings() << "\n\n";
   hourlyEmployee.print();
   cout << "\nearned $" << hourlyEmployee.earnings() << "\n\n";
   commissionEmployee.print();
   cout << "\nearned $" << commissionEmployee.earnings() << "\n\n";
   basePlusCommissionEmployee.print();
   cout << "\nearned $" << basePlusCommissionEmployee.earnings()
      << "\n\n";

   // create vector of four base-class pointers
   vector < Employee * > employees( 4 );

   // initialize vector with Employees
   employees[ 0 ] = &salariedEmployee;
   employees[ 1 ] = &hourlyEmployee;
   employees[ 2 ] = &commissionEmployee;
   employees[ 3 ] = &basePlusCommissionEmployee;

   cout << "Employees processed polymorphically via dynamic binding:\n\n";

   // call virtualViaPointer to print each Employee's information
   // and earnings using dynamic binding
   cout << "Virtual function calls made off base-class pointers:\n\n";

   for ( size_t i = 0; i < employees.size(); i++ )
      virtualViaPointer( employees[ i ] );

   // call virtualViaReference to print each Employee's information
   // and earnings using dynamic binding
   cout << "Virtual function calls made off base-class references:\n\n";

   for ( size_t i = 0; i < employees.size(); i++ )
      virtualViaReference( *employees[ i ] ); // note dereferencing
   system("pause");
   return 0;
} // end main

// call Employee virtual functions print and earnings off a
// base-class pointer using dynamic binding
void virtualViaPointer( const Employee * const baseClassPtr )
{
   baseClassPtr->print();
   cout << "\nearned $" << baseClassPtr->earnings() << "\n\n";
} // end function virtualViaPointer

// call Employee virtual functions print and earnings off a
// base-class reference using dynamic binding
void virtualViaReference( const Employee & const baseClassRef )
{
   baseClassRef.print();
   cout << "\nearned $" << baseClassRef.earnings() << "\n\n";
} // end function virtualViaReference


補充說明：
一般的reference variable是不是也不能宣告成const呢?

thanks a lot

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◆ From: 114.137.29.80
→ phdch:剛才驗證一般的reference variable也不能宣告成const了!!!
推 littleshan:因為 reference 本來就不能改變它指涉的對象
→ phdch:若int a=1; int &A=a; A=10; 這是可以把a指派為10啊!
推 littleshan:我說的是不能改變「指涉的對象」而非不能改變「內容」
推 littleshan:想象一下 int a,b; int* p = &a; p = &b;
→ littleshan:上面的寫法若要改用 reference 該怎麼寫？
推 HudsonE:簡單的說加那個 const 是多餘的