如果派生类具有私有变量,则派生类的数组不起作用

Question

我有一个抽象基类和2个不同的类,它们从基类实现虚函数.

我把它们放在一个数组中,对于"derived1"类,这个有效.但是,如果我创建一个具有一些额外私有变量的"derived2"类数组,则代码将在运行时编译但出现错误.

#include <iostream>
class base{
protected:
  int inner_a;
  int inner_b;
public:
  void setInner(int a,int b){inner_a=a;inner_b=b;};
  virtual int doStuff()=0;
};


class derived1: public base{
public:
  virtual int doStuff();
};



class derived2: public base{
private:
  int tmpVar;//works if I remove
public:
  int doStuff();
};


int derived2::doStuff(){

  return inner_a-inner_b;
}

int derived1::doStuff(){
    return inner_a+inner_b;
}



int main(){
  base *classAry1 = new derived1[3];//this works
  base *classAry2 = new derived2[2];//derived2 has extra private variables


  classAry1[0].setInner(1,3);
  classAry1[1].setInner(10,7);
  std::cout <<classAry1[0].doStuff() <<std::endl;;
  std::cout <<classAry1[1].doStuff() <<std::endl;


  classAry2[0].setInner(1,3);
  classAry2[1].setInner(10,7);
  std::cout <<classAry2[0].doStuff() <<std::endl;;
  std::cout <<classAry2[1].doStuff() <<std::endl;


  return 0;
}

任何人都可以帮助我,如何将衍生类放在std数组中？

谢谢

编辑:

代码段错误,valgrind告诉我

-2

==25096== Use of uninitialised value of size 8
==25096==    at 0x400AC9: main (abc.cpp:52)
==25096== 
==25096== Invalid read of size 8
==25096==    at 0x400AC9: main (abc.cpp:52)
==25096==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==25096== 
==25096== 
==25096== Process terminating with default action of signal 11 (SIGSEGV)
==25096==  Access not within mapped region at address 0x0
==25096==    at 0x400AC9: main (abc.cpp:52)

Answer 1

一个阵列的derived1对象(或derived2对象)不能被解释为一个阵列的base对象.数组不是多态的.只有独立对象可以是多态的,即derived1具有IS-A关系base.但是array of derived1没有IS-A的关系array of base.你的阵列(既不是classAry1也不是classAry2)都不能真正"起作用".

换句话说,这个

base *classAry1 = new derived1[3];
base *classAry2 = new derived2[2];

已经没有任何意义,即使它是正式格式良好的代码.

第一个阵列"似乎工作"只是纯粹的意外.即使您使用,代码的行为也是未定义的classAry1.

如果要拥有存储多态实体的数组(或容器),则必须存储指向该数组中实际对象的指针,而不是存储实际对象本身.

在您的特定情况下,可以通过以下方式重写代码.(它看起来不太好,我只是这样做来说明原理,因为不知道你的全部意图,很难选择最好的方法)

int main(){
  derived1 *d1s = new derived1[2];
  base **classAry1 = new base *[2];
  classAry1[0] = &d1s[0];
  classAry1[1] = &d1s[1];

  derived2 *d2s = new derived2[2];
  base **classAry2 = new base *[2];
  classAry2[0] = &d2s[0];
  classAry2[1] = &d2s[1];

  classAry1[0]->setInner(1,3);
  classAry1[1]->setInner(10,7);
  std::cout << classAry1[0]->doStuff() << std::endl;;
  std::cout << classAry1[1]->doStuff() << std::endl;

  classAry2[0]->setInner(1,3);
  classAry2[1]->setInner(10,7);
  std::cout << classAry2[0]->doStuff() << std::endl;;
  std::cout << classAry2[1]->doStuff() << std::endl;

  delete[] classAry2;
  delete[] d2s;
  delete[] classAry1;
  delete[] d1s;

  return 0;
}