问题描述
如果我删除行delete Jim;或delete DiamondWeapon;中的任何一个，它都可以工作，但是为什么呢？我的意思是
这是，如果我只使用delete DiamondWeapon;也会起作用，但是如果我也只使用delete Jim;并删除delete DiamondWeapon;也会起作用。我很难包裹我的
顺其自然，任何帮助将不胜感激
#include<iostream>
#include<string>
// Strategy pattern IBehavIoUr (Iweapon)
// Context Character ( will access this.strategy_.Function() ) ( his.strategy will be Iweapon )
// Concrete Strategies ( Each weapon will have a unique Algorithms for each weapon ).
class Iweapon // ( interface IbehavIoUr strategy )
{
public:
virtual ~Iweapon() { }
virtual void Use() const = 0; /* Search Results Featured snippet from the web
The const member functions are the functions which are declared as constant in the program.
The object called by these functions cannot be modified.It is recommended
to use const keyword so that accidental changes to object are avoided.A const member function can be called by any type of object.*/
};
class Character // its Context as per UML diagram
{
private:
Iweapon* iweapon_Strat;
public:
Character(Iweapon* iweaponStrat = nullptr) :iweapon_Strat(iweaponStrat) { }
/*{
this->iweapon_Strat = iweaponStrat;
}*/
~Character() {
delete this->iweapon_Strat;
}
void SetStrategy(Iweapon* iweaponstrat) {
delete this->iweapon_Strat;
this->iweapon_Strat = iweaponstrat;
}
void Attack() const //DoSomeLogic()
{
this->iweapon_Strat->Use();
}
};
// following are ConcreteStrategies(A,B,C ... )
class Sword :public Iweapon {
public:
void Use() const override {
std::cout << "\n\nSword is Attacking\n\n";
}
};
class Axe : public Iweapon {
public:
void Use() const override {
std::cout << "\n\nAxe is Attacking\n\n";
}
};
int main() {
//this work
{
Character* Mike = new Character(new Sword);
Mike->Attack();
Mike = new Character(new Axe);
Mike->Attack();
delete Mike;
}
//this work too
{
Character* Ryan = new Character();
Iweapon* SilverWeapon = NULL;
SilverWeapon = new Sword;
Ryan->SetStrategy(SilverWeapon);
Ryan->Attack();
SilverWeapon = new Axe;
Ryan->SetStrategy(SilverWeapon);
Ryan->Attack();
delete Ryan;
}
//this doesn't work but if I remove either one of the line *** delete Jim; *** or *** delete DiamondWeapon; *** it work,But Why
// i meant here is : Alone if I just have delete DiamondWeapon; it will work
// but if Alone i also just have delete Jim; and I remove delete DiamondWeapon; it will work too.
//I am having hard time wraping my head around it any help will really be appreciated
{
Iweapon* DiamondWeapon = new Sword();
Character* Jim = new Character();
DiamondWeapon = new Sword();
Jim->SetStrategy(DiamondWeapon);
Jim->Attack();
DiamondWeapon = new Axe();
Jim->SetStrategy(DiamondWeapon);
Jim->Attack();
delete DiamondWeapon;
delete Jim;
}
//This also doesn't work
{
Iweapon* WoodenWepon = new Sword();
Character Kim;
//WoodenWepon = new Sword();
Kim.SetStrategy(WoodenWepon);
Kim.Attack();
WoodenWepon = new Axe();
Kim.SetStrategy(WoodenWepon);
Kim.Attack();
delete WoodenWepon;
//delete Kim;
}
//this Work
{
Character* Denzel = new Character();
Iweapon* weapon = new Sword;
Denzel->SetStrategy(weapon);
Denzel->Attack();
Denzel->SetStrategy(new Axe);
Denzel->Attack();
//delete weapon;
delete Denzel;
}
// This also work
{
Iweapon* GoldWeapon = NULL;
Character* Jhon = new Character();
GoldWeapon = new Sword;
Jhon->SetStrategy(GoldWeapon);
Jhon->Attack();
GoldWeapon = new Axe;
Jhon->SetStrategy(GoldWeapon);
Jhon->Attack();
delete GoldWeapon;
//delete Jhon;
}
//following is just for system pause
std::string end;
std::cin >> end;
return 0;
}
解决方法
操作时：
void Character::SetStrategy(Iweapon* iweaponstrat)
{
delete this->iweapon_Strat;
this->iweapon_Strat = iweaponstrat;
}
字符获取指针的所有权。您的代码将处理iweaponstrat的删除。目前尚不清楚它是出于设计还是偶然，但由于delete在那儿，因此可以说它确实是所有者。
现在，当您这样做时：
DiamondWeapon = new Axe();
Jim->SetStrategy(DiamondWeapon); // allows deletes DiamondWeapon
Jim->Attack();
delete DiamondWeapon; // deletes DiamondWeapon
您将Axe交给吉姆，授予吉姆所有权，您也可以自行删除它。这是自找麻烦，最终会发生。
您从此处有两条路径：
- 强烈建议使用
shared_ptr或unique_ptr。 - 定义清晰的所有权，尊重它，仅删除拥有的对象。很难完成。 C程序员必须始终处理这些问题，它节省了智能指针的开销，但在您的水平上确实不值得。
您遇到的问题是相同指针值的多个副本。
当你
delete DiamondWeapon;
delete Jim;
析构函数~Character试图删除DiamondWeapon也指向的对象。这是不正确的C ++，并且您的程序行为未定义。
您应该使用*，而不是传递原始（std::unique_ptr）指针，这样您才知道只有一个指针指向每个Weapon和Character
class Character // its Context as per UML diagram
{
private:
std::unique_ptr<Iweapon> iweapon_Strat;
public:
Character(std::unique_ptr<Iweapon> iweaponStrat) :iweapon_Strat(iweaponStrat) {}
void SetStrategy(std::unique_ptr<Iweapon> iweaponstrat)
{
iweapon_Strat = iweaponstrat;
}
void Attack() const
{
iweapon_Strat->Use();
}
};
然后，当您保留额外的指针时，将出现编译时错误。另外，定义指针的作用域结束时，将清除动态存储持续时间（“堆”）对象。
int main()
{
//this work
{
auto Mike = std::make_unique<Character>(std::make_unique<Sword>());
Mike->Attack();
Mike = std::make_unique<Character>(std::make_unique<Axe>());
Mike->Attack();
}
//this work too
{
auto Ryan = std::make_unique<Character>Character();
std::unique_ptr<Iweapon> SilverWeapon;
SilverWeapon = std::make_unique<Sword>();
// Ryan->SetStrategy(SilverWeapon); // Error,can't copy SilverWeapon
Ryan->SetStrategy(std::move(SilverWeapon)); // SilverWeapon is now null
Ryan->Attack();
SilverWeapon = std::make_unique<Axe>();
// Ryan->SetStrategy(SilverWeapon); // Error,can't copy SilverWeapon
Ryan->SetStrategy(std::move(SilverWeapon)); // SilverWeapon is now null
Ryan->Attack();
}
{
std::unique_ptr<Iweapon> DiamondWeapon = std::make_unique<Sword>();
auto Jim = std::make_unique<Character>Character();
DiamondWeapon = std::make_unique<Sword>();
// Jim->SetStrategy(DiamondWeapon); // Error,can't copy DiamondWeapon
Jim->SetStrategy(std::move(DiamondWeapon)); // DiamondWeapon is now null
Jim->Attack();
DiamondWeapon = std::make_unique<Axe>();
// Jim->SetStrategy(DiamondWeapon); // Error,can't copy DiamondWeapon
Jim->SetStrategy(std::move(DiamondWeapon)); // DiamondWeapon is now null
Jim->Attack();
}
{
std::unique_ptr<Iweapon> WoodenWepon = std::make_unique<Sword>();
Character Kim;
// Kim.SetStrategy(WoodenWepon); // Error,can't copy WoodenWeapon
Kim.SetStrategy(std::move(WoodenWepon)); // WoodenWeapon is now null
Kim.Attack();
WoodenWepon = std::make_unique<Axe>();
// Kim.SetStrategy(WoodenWepon); // Error,can't copy WoodenWeapon
Kim.SetStrategy(std::move(WoodenWepon)); // WoodenWeapon is now null
Kim.Attack();
}
// etc.
//following is just for system pause
std::string end;
std::cin >> end;
return 0;
}