我有一个称为Item的基类:
#ifndef ITEM_H
#define ITEM_H
#include <ostream>
class Item {
public:
virtual ~Item() {}
virtual void print(std::ostream& out) const {}
friend std::ostream& operator << (std::ostream& out, Item& item){
item.print(out);
return out;
}
};
#endif
我有一个派生类Tower:
#ifndef TOWER_H
#define TOWER_H
#include <iostream>
#include <iostream>
#include <exception>
#include "item.h"
#include "Card.h"
class Tower : public Item {
unsigned height;
void print(std::ostream& o) const;
public:
Tower(const Card& card);
int demolish(Card& card) const;
unsigned getHeight() const;
};
#endif
塔的源代码:
#include "tower.h"
Tower::Tower(const Card& card){
height = card.getDis();
}
void Tower::print(std::ostream& o) const {
o << height;
}
int Tower::demolish(Card& card) const{
try {
if(height != card.getDis()){
throw std::exception ();
} else {
return height;
}
} catch (std::exception e){
cout<< "Card's value not enough to demolish the tower." << endl;
}
}
unsigned Tower::getHeight() const {
return height;
}
现在,我正在尝试测试代码,以查看运算符重载是否正常工作:
void test() {
Card card (Card::SOUTH, 3);
Tower tower(card);
std::cout << "Printing tower: " << tower << std::endl; //PRINTS OUT 3 AS EXPECTED
Card one (Card::NORTH, 2);
Card two (Card::WEST, 3);
std::cout << "Expecting to receive an error: " <<endl;
tower.demolish(one);
std::cout << "Expecting to have the tower demolished: ";
std::cout << tower.demolish(two) <<std::endl;
std::cout << "Height of the tower: " << tower.getHeight() <<std::endl;
std::vector<Item> items; //creating an Item vector
items.push_back(Tower(one));
Item items2[1]; //creating an array of Items
items[0]= tower;
std::cout << "Printing out an Item: ";
std::cout << items.back()<<std::endl; //EXPECTING TO GET 2 BUT IT PRINTS NOTHING, WHY IS THAT?
std::cout << "OR: " << items2[0]<<std::endl; //SAME ISSUE HERE, EXPECTING TO GET 3
}
从代码可以理解,Card拥有整数值距离和枚举值方向。如果我也包含该代码,那将是一团糟。我在最后一段代码test()中评论了我的问题。感谢您的帮助。
最佳答案
std::vector<Item> items; //creating an Item vector
items.push_back(Tower(one));
这里发生的事情称为“切片”。由于您不是存储指针,而是存储实际的对象,因此仅将类的
Tower
部分切掉,并且仅将Item
部分推入向量中。要使用虚函数和多态性,您需要一个指向基类的引用或指针。std::vector<Item*> items; //creating an Item vector
items.push_back(new Tower(one));
// ...
// at the end of main:
for(int i=0; i < items.size(); ++i)
delete items[i];
或者使用Boost或C ++ 11库中的智能指针
std::vector<shared_ptr<Item>> items;
items.push_back(make_shared<Tower>(one));
// nothing further needs to be done
对于打印,您现在显然需要取消引用指针:
std::cout << "Printing out an Item: ";
std::cout << *items.back()<<std::endl;
std::cout << "OR: " << *items2[0]<<std::endl;
}