多态的摸索之路( 甲 )
C++ 中的多態(tài)是極為復雜、難纏、龐大的，就像深海里的鯊魚，兇猛而強大。那么到底如何才能游刃有余地駕于好這頭潛伏在深海的遠古猛獸呢？！別著急，咱們慢慢來，一點一點地摸索清它的性格和脾氣，再控制就容易啦。
例甲：
#include<iostream>
using namespace std;
class TradesPerson {
public:
?????? virtual void sayHi() { cout << "Just Hi, and nothing." << endl; }
};
class Tinker : public TradesPerson {
public:
?????? virtual void sayHi() { cout << "Hi, I'm a Tinker." << endl; }
};
class Tailor : public TradesPerson {
public:
?????? virtual void sayHi() { cout << "Hi, I'm a Tailor." << endl; }
};
?
int main() {
?????? TradesPerson* p;
?????? int which;
?????? do{
????????????? cout << "1 = TradesPerson, 2 = Tinker, 3 = Tailor, 4 = quit; /n? ";
????????????? _input: cin >> which;
?????? }while( which < 1 || which >4 );
?????? switch( which ) {
????????????? case 1: p = new TradesPerson; break;
????????????? case 2: p = new Tinker; break;
????????????? case 3: p = new Tailor; break;
????????????? case 4: exit( 0 );
?????? }
?????? p->sayHi();
?????? goto _input;
?????? delete p;
?????? return 0;
}
?
模型一:
class TradesPerson {
public:
?????? virtual void sayHi() { cout << "Just Hi, and nothing." << endl; }
};
class Tinker : public TradesPerson {
public:
?????? virtual void sayHi() { cout << "Hi, I'm a Tinker." << endl; }
};
class Tailor : public TradesPerson {
public:
?????? virtual void sayHi() { cout << "Hi, I'm a Tailor." << endl; }
};
?
模型二:
class TradesPerson {
public:
?????? virtual void sayHi() { cout << "Just Hi, and nothing." << endl; }
};
class Tinker : public TradesPerson {
public:
?????? ?void sayHi() { cout << "Hi, I'm a Tinker." << endl; }
};
class Tailor : public TradesPerson {
public:
?????? ?void sayHi() { cout << "Hi, I'm a Tailor." << endl; }
};
?
模型三:
class TradesPerson {
public:
?????? void sayHi() { cout << "Just Hi, and nothing." << endl; }
};
class Tinker : public TradesPerson {
public:
?????? ?void sayHi() { cout << "Hi, I'm a Tinker." << endl; }
};
class Tailor : public TradesPerson {
public:
?????? ?void sayHi() { cout << "Hi, I'm a Tailor." << endl; }
};
?
模型四:
class TradesPerson {
public:
?????? void sayHi() { cout << "Just Hi, and nothing." << endl; }
};
class Tinker : public TradesPerson {
public:
?????? virtual void sayHi() { cout << "Hi, I'm a Tinker." << endl; }
};
class Tailor : public TradesPerson {
public:
?????? virtual void sayHi() { cout << "Hi, I'm a Tailor." << endl; }
};
?
實驗結(jié)果：模型一和模二相等，模三和模四相等。那么也就是說，當聲明了基類的一個成員函數(shù)為虛后，那么如果該函數(shù)在派生類中再次出現(xiàn)并得到新定義，它將自動成為虛函數(shù)！可如果在基類沒有聲明為虛，那么到派生類再怎么聲明是虛的都變得毫無意義了——根本起不到多態(tài)的神奇妙用。
說到多態(tài)，咱們在此再重復一下多態(tài)的概念：多態(tài)性也稱后約束或動態(tài)約束，它常用虛函數(shù)來實現(xiàn)。在C++中，多態(tài)是指C++的代碼可以根據(jù)運行情況的不同而執(zhí)行不同的操作。C++的多態(tài)性就是為同一個函數(shù)和運算符定義幾個版本。C++支持兩種多態(tài)性，編譯時的多態(tài)性和運行時的多態(tài)性。編譯時的多態(tài)性通過使用重載函數(shù)獲得，運行時的多態(tài)性通過使用繼承和虛函數(shù)獲得。要獲得多態(tài)性的對象，必須建立一個類等級，然后在派生類中重定義基類函數(shù)，該函數(shù)可以被定義為重載函數(shù)或虛函數(shù)，以獲得編譯時的多態(tài)性對象或運行時的多態(tài)性對象。
?
?
一次非常成功而快樂的多態(tài)實驗!趕快來先睹為快吧！
例甲：
class A {
public:
?????? void disp() { cout << "This is A speaking!/n"; }
};
class B : public A {
public:
?????? void disp() { cout << "This is B speaking!/n"; }
};
?
int main() {
?????? A* pa = new B;
?????? pa->disp(); ???????? //輸出為This is A speaking!
?????? delete pa;
?????? return 0;
}
如果為了讓其輸出"This is B speaking!/n"的話，你可能會把那個關(guān)鍵句改成pa->B::disp();，可實驗證明，這么做編譯為報錯的。而且顯示三個錯誤，什么“'B' : is not a member of 'A'”、“ 'B' : is not a class or namespace name”等一派胡言亂語！那好，既然讓其輸出B speaking已經(jīng)不可能了，接下來就virtual伺候吧！
例乙：
class A {
public:
?????? virtual void disp() { cout << "This is A speaking!/n"; }
};
class B : public A {
public:
?????? void disp() { cout << "This is B speaking!/n"; }
};
?
int main() {
?????? A* pa = new B;
?????? pa->disp(); ???????? //輸出為This is B speaking!
?????? delete pa;
?????? return 0;
}
很明顯，無非多了個關(guān)鍵字兒virtual，世界競?cè)绱舜笞?#xff01;指針pa調(diào)用了B類里的成員函數(shù)，這也是應該的、應得的、正常的！因為它pa是B類給的它初始化！雖然A類是pa的親生母親，但是，是B類一把屎一把尿養(yǎng)活了pa!所以當pa長大成人、能做事能掙錢時，它理應贍養(yǎng)孝順B類！！這沒什么好商量的！！
如果想讓其輸出This is A speaking!,也可以，直接修改關(guān)鍵句為: pa->A::disp(); 就行了！
對比例甲與例乙，我們看到,當沒有用虛函數(shù)時，只能輸出A speaking，不能企圖通過用域解析符::來改變函數(shù)調(diào)用輸出B speaking；但使用了virtual后，代碼好像自己長眼睛了，知道程序員心里真正想調(diào)用的是誰并把程序員的心聲正確付諸于實踐，而且函數(shù)調(diào)用也變得來去自如，想調(diào)誰調(diào)誰！（欲調(diào)其他類的同名成員函數(shù)，只需加個域解析符即可！）。
不過也別高興的太早，這兩道例子看起來簡單無比，可我硬是花了幾個星期地功夫才摸索出來。難的、使我在大門外急得直跺腳就是進不來的東西是：你能否正確實驗并探究出虛函數(shù)的精要！也就是說，你很容易做出錯誤的實驗！廢話少說，來看看一條“不該地實驗，不該地愛……”
例甲：
int main() {
?????? B* pb = new A;
?????? pb->disp();
?????? delete pb;
?????? return 0;
}
用派生類B創(chuàng)建指針對象！用基類A給它初始化！結(jié)果一編譯呢——錯！“cannot convert from 'class A *' to 'class B *'”。所以呢，我估且得出一個結(jié)論：如果不想跟編譯器過不去，就不能用派生類創(chuàng)建對象然后基類初始化。只能是 A* pa = new B; ，謹記謹記！
?
?
?
以下三個例子中，主函均為：
int main() {
?????? A* pa = new B;
?????? pa->disp();
?????? delete pa;
?????? return 0;
}
因而全部省略。
?
例甲：
class A {
public:
?????? A() { cout << "A constructed,yeah!/n"; }?
?????? virtual void disp() { cout << "This is A speaking!/n"; }
};
?
class B : public A {
public:
?????? B() { cout << "B constructed,yeah!/n"; }?
?????? virtual void disp() { cout << "This is B speaking!/n"; }
};
輸出：
A constructed,yeah!
B constructed,yeah!
This is B speaking!
?
例乙：
class A {
public:
?????? ~A() { cout << "A destructed/n"; }
?????? virtual void disp() { cout << "This is A speaking!/n"; }
};
?
class B : public A {
public:
?????? ~B() { cout << "B destructed/n"; }
?????? virtual void disp() { cout << "This is B speaking!/n"; }
};
輸出：
This is B speaking!
A destructed
?
例丙:
class A {
public:
?????? A() { cout << "A constructed,yeah!/n"; }
?????? ~A() { cout << "A destructed/n"; }
?????? virtual void disp() { cout << "This is A speaking!/n"; }
};
class B : public A {
public:
?????? B() { cout << "B constructed,yeah!/n"; }
?????? ~B() { cout << "B destructed/n"; }
?????? virtual void disp() { cout << "This is B speaking!/n"; }
};
輸出：
A constructed,yeah!
B constructed,yeah!
This is B speaking!
A destructed
?
來個簡單明晰點的：
class A {
public:
?????? ~A() { cout << "A destructed/n"; }
};
class B : public A {
public:
?????? ~B() { cout << "B destructed/n"; }
};
?
?
int main() {
?????? A* pa = new B;
?????? delete pa;????? // 輸出 A destructed
?????? return 0;
}
接下來我不禁要問了，為什么析構(gòu)函數(shù)不像構(gòu)造函數(shù)那樣兩個類的統(tǒng)統(tǒng)都調(diào)用呢？
答案明日揭曉！！
?
?
?
總結(jié)
以上是生活随笔為你收集整理的多态的摸索之路( 甲 )的全部內(nèi)容，希望文章能夠幫你解決所遇到的問題。
- 上一篇： 让工程师沉思的68个经典小故事
- 下一篇： 我的2018新年计划