c++ - 对“base_rec::showme()”的 undefined reference 错误

Question

所以我正在为类做调试作业。我们不允许进行任何严重的代码更改。我的代码是:

#include <string>
#include <iostream>
#include <sstream>


class base_rec
{public:
  base_rec (std::string contentstr):str(contentstr){};
  void showme();
  std::string str;};

class u_rec:public base_rec
{public:
  u_rec():base_rec("undergraduate records"){};
  void showme()
  {std::cout<<"showme() function of u_rec class\t" <<str<<std::endl;};};


class g_rec:public base_rec
{public:
  g_rec():base_rec("graduate records"){};
  void showme()
  {std::cout << "showme() function of a g_rec class\t"<<str<<std::endl;};};



  int main()
  { base_rec *brp[2];
    brp[1] = new u_rec;
    brp[2] = new g_rec;

    for (int i=0;i<1;i++)
      {brp[i]->showme ();}

    return 0;
}

但是，每当我尝试编译它时，都会收到错误消息:

/tmp/ccFm7Xvz.o: In function main':
quiz2.cpp:(.text+0x54): undefined reference tobase_rec::showme()' collect2: error: ld returned 1 exit status

我不完全确定这里的问题是什么。有什么建议吗？

Answer 1

首先:

brp[2] = new g_rec;

将超出范围，因为数组索引从 0

开始

其次:

showme 在base_rec 中没有定义。如果真的要调用派生类的方法，需要声明为virtual。

第三，你的代码中有几个语法错误:

 u_rec():base_rec("undergraduate records"){}; //redundant ;
 void showme()
 {std::cout<<"showme() function of u_rec class\t" <<str<<std::endl;};
                                                     //redundant ; again
 //You can find several others.

您可以执行以下操作:

#include <string>
#include <iostream>
#include <sstream>


class base_rec
{
  public:
     base_rec (std::string contentstr):str(contentstr){}
     void showme(){ std::cout << "base class showme";}
     std::string str;
};

class u_rec: public base_rec
{
public:
    u_rec():base_rec("undergraduate records"){}
    void showme()
   {
      std::cout<<"showme() function of u_rec class\t" <<str<<std::endl;
   }
};


class g_rec:public base_rec
{
  public:
   g_rec():base_rec("graduate records"){};
     void showme()
     {
        std::cout << "showme() function of a g_rec class\t"<<str<<std::endl;
     }
};



int main()
{ 
    base_rec *brp[2];
    brp[0] = new u_rec;
    brp[1] = new g_rec;  //index starting from 0

   for (int i=0;i<1;i++)
   {
      brp[i]->showme ();
   }

   return 0;
}

如果你真的想看看多态是如何工作的，你需要在base_rec类中将showme声明为virtual。然后当你通过基类指针调用它时，它会向你展示多态行为。例如:

 class base_rec
 {
  public:
     base_rec (std::string contentstr):str(contentstr){}
     virtual void showme(){ std::cout << "base class showme";}
     std::string str;
 };