Merge pull request Light-City#54 from xliu79/english

english

english/basic_content/abstract/README.md

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+# Pure virtual functions and abstract classes
+
+## About Author：
+
+
+
+![](../img/wechat.jpg)
+
+## 1.Pure virtual function and abstract class
+
+
+Pure virtual functions (or abstract functions) in C + + are virtual functions that we have not implemented!We just need to state it! 
+example:
+```cpp
+// abstract class
+Class A {
+public: 
+    virtual void show() = 0; // pure virtual function
+    /* Other members */
+}; 
+```
+
+ * Pure virtual function: virtual function without function body
+ * Abstract classes: classes containing pure virtual functions
+
+Abstract classes can only be used as base classes to derive new classes. Objects, pointers and references of abstract classes cannot be created->An object of a class derived from an abstract class!
+
+> Code example：[test.cpp](./test.cpp)、[pure_virtual.cpp](./pure_virtual.cpp)
+
+## 2.Implement abstract classes
+
+Abstract class：Pure virtual functions can be called within member functions.Pure virtual functions cannot be used inside constructors / destructors.
+
+If a class derives from an abstract class, it must implement all pure virtual functions in the base class to become a non abstract class.
+```cpp
+// A is abstract class
+class A {
+public:
+    virtual void f() = 0;  // pure virtual function
+    void g(){ this->f(); }
+    A(){}  // 构造函数
+};
+
+class B : public A{
+public:
+    void f(){ cout<<"B:f()"<<endl;}  // 实现了抽象类的纯虚函数
+};
+```
+
+> Code Example：[abstract.cpp](./abstract.cpp)
+
+## 3.Key point
+
+- [Pure virtual functions make a class abstract](./interesting_facts1.cpp)
+```cpp
+// Abstract class contains at least one pure virtual function
+class Base{
+public: 
+    virtual void show() = 0; // 纯虚函数
+    int getX() { return x; } // 普通成员函数
+
+private:
+     int x; 
+}; 
+```
+
+- [Pointers and references to abstract class types](./interesting_facts2.cpp)
+```cpp
+class Derived : public Base { 
+public: 
+    void show() { cout << "In Derived \n"; } // 实现抽象类的纯虚函数
+    Derived(){} // 构造函数
+}; 
+
+int main(void) 
+{ 
+    //Base b;  // error! 不能创建抽象类的对象
+    //Base *b = new Base(); error!
+
+    Base *bp = new Derived(); // 抽象类的指针和引用 -> 由抽象类派生出来的类的对象
+    bp->show();
+    return 0; 
+}
+```
+
+- [If we do not override pure virtual functions in the derived class, the derived class will also become an abstract class](./interesting_facts3.cpp)
+```cpp
+// Derived为抽象类
+class Derived: public Base 
+{ 
+public: 
+//    void show() {}
+}; 
+```
+
+- [Abstract classes can have constructors](./interesting_facts4.cpp)
+```cpp
+// abstract class
+class Base { 
+    protected: 
+        int x; 
+    public: 
+        virtual void fun() = 0; 
+        Base(int i) { x = i; }  // constructor function
+}; 
+// 派生类
+class Derived: public Base 
+{ 
+    int y; 
+public: 
+    Derived(int i, int j) : Base(i) { y = j; } // constructor function
+    void fun() { cout << "x = " << x << ", y = " << y; }
+}; 
+```
+
+- [A constructor cannot be a virtual function, and a destructor can be a virtual destructor](./interesting_facts5.cpp)
+```cpp
+// 抽象类
+class Base  {
+public:
+    Base(){ cout << "Constructor: Base" << endl; }
+    virtual ~Base(){ cout << "Destructor : Base" << endl; }
+
+    virtual void func() = 0;
+};
+
+class Derived: public Base {
+public:
+    Derived(){ cout << "Constructor: Derived" << endl; }
+    ~Derived(){ cout << "Destructor : Derived" << endl;}
+
+    void func(){cout << "In Derived.func()." << endl;}
+};
+```
+>When the base class pointer points to a derived class object and removes the object, we may want to call the appropriate destructor.
+>The destructor can only be called if it is not a virtual destructor.
+
+## 4.Complete example
+
+Abstract classes are inherited and implemented by derived classes!
+
+> Code Example：[derived_full.cpp](./derived_full.cpp)

english/basic_content/abstract/abstract.cpp

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+/**
+ * @file abstract.cpp
+ * @brief 抽象类中：在成员函数内可以调用纯虚函数，在构造函数/析构函数内部不能使用纯虚函数
+ * 如果一个类从抽象类派生而来，它必须实现了基类中的所有纯虚函数，才能成为非抽象类
+ * @author 光城
+ * @version v1
+ * @date 2019-07-20
+ */
+
+#include<iostream>
+using namespace std;
+
+class A {
+public:
+    virtual void f() = 0;  // 纯虚函数
+    void g(){ this->f(); }
+    A(){}
+};
+class B:public A{
+public:
+    void f(){ cout<<"B:f()"<<endl;}
+};
+int main(){
+    B b;
+    b.g();
+    return 0;
+}

english/basic_content/abstract/derived_full.cpp

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+/**
+ * @file derived_full.cpp
+ * @brief 完整示例！抽象类由派生类继承实现！
+ * @author 光城
+ * @version v1
+ * @date 2019-07-20
+ */
+
+#include<iostream> 
+using namespace std; 
+
+class Base 
+{ 
+    int x; 
+    public: 
+    virtual void fun() = 0; 
+    int getX() { return x; } 
+}; 
+
+class Derived: public Base 
+{ 
+    int y; 
+    public: 
+    void fun() { cout << "fun() called"; }  // 实现了fun()函数
+}; 
+
+int main(void) 
+{ 
+    Derived d; 
+    d.fun(); 
+    return 0; 
+} 

english/basic_content/abstract/interesting_facts1.cpp

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+/**
+ * @file interesting_facts1.cpp
+ * @brief 纯虚函数使一个类变成抽象类
+ * @author 光城
+ * @version v1
+ * @date 2019-07-20
+ */
+
+#include<iostream> 
+using namespace std; 
+
+/**
+ * @brief 抽象类至少包含一个纯虚函数
+ */
+class Test 
+{ 
+    int x; 
+public: 
+    virtual void show() = 0; 
+    int getX() { return x; } 
+}; 
+
+int main(void) 
+{ 
+    Test t;  //error! 不能创建抽象类的对象
+    return 0; 
+} 
+

english/basic_content/abstract/interesting_facts2.cpp

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+/**
+ * @file interesting_facts2.cpp
+ * @brief 抽象类类型的指针和引用
+ * @author 光城
+ * @version v1
+ * @date 2019-07-20
+ */
+
+#include<iostream> 
+using namespace std; 
+
+
+/**
+ * @brief 抽象类至少包含一个纯虚函数
+ */
+class Base
+{ 
+    int x; 
+public: 
+    virtual void show() = 0; 
+    int getX() { return x; } 
+
+}; 
+class Derived: public Base 
+{ 
+public: 
+    void show() { cout << "In Derived \n"; } 
+    Derived(){}
+}; 
+int main(void) 
+{ 
+    //Base b;  //error! 不能创建抽象类的对象
+    //Base *b = new Base(); error!
+    Base *bp = new Derived(); // 抽象类的指针和引用 -> 由抽象类派生出来的类的对象
+    bp->show();
+    return 0; 
+} 
+

english/basic_content/abstract/interesting_facts3.cpp

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+/**
+ * @file interesting_facts3.cpp
+ * @brief 如果我们不在派生类中覆盖纯虚函数，那么派生类也会变成抽象类。
+ * @author 光城
+ * @version v1
+ * @date 2019-07-20
+ */
+
+#include<iostream> 
+using namespace std; 
+
+class Base
+{ 
+    int x; 
+public: 
+    virtual void show() = 0; 
+    int getX() { return x; } 
+}; 
+class Derived: public Base 
+{ 
+public: 
+//    void show() { } 
+}; 
+int main(void) 
+{ 
+    Derived d;  //error! 派生类没有实现纯虚函数，那么派生类也会变为抽象类，不能创建抽象类的对象
+    return 0; 
+} 
+

english/basic_content/abstract/interesting_facts4.cpp

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+/**
+ * @file interesting_facts4.cpp
+ * @brief 抽象类可以有构造函数
+ * @author 光城
+ * @version v1
+ * @date 2019-07-20
+ */
+
+#include<iostream> 
+using namespace std; 
+
+// An abstract class with constructor 
+class Base 
+{ 
+    protected: 
+        int x; 
+    public: 
+        virtual void fun() = 0; 
+        Base(int i) { x = i; } 
+}; 
+
+class Derived: public Base 
+{ 
+    int y; 
+    public: 
+    Derived(int i, int j):Base(i) { y = j; } 
+    void fun() { cout << "x = " << x << ", y = " << y; } 
+}; 
+
+int main(void) 
+{ 
+    Derived d(4, 5); 
+    d.fun(); 
+    return 0; 
+} 

english/basic_content/abstract/interesting_facts5.cpp

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+
+/**
+ * @file interesting_facts5.cpp
+ * @brief 构造函数不能是虚函数，而析构函数可以是虚析构函数。
+ * 例如：当基类指针指向派生类对象并删除对象时，我们可能希望调用适当的析构函数。如果析构函数不是虚拟的，则只能调用基类析构函数。
+ * @author 光城
+ * @version v1
+ * @date 2019-07-20
+ */
+#include<iostream>
+using namespace std;
+
+class Base  {
+    public:
+        Base()    { cout << "Constructor: Base" << endl; }
+        virtual ~Base()   { cout << "Destructor : Base" << endl; }
+};
+
+class Derived: public Base {
+    public:
+        Derived()   { cout << "Constructor: Derived" << endl; }
+        ~Derived()   { cout << "Destructor : Derived" << endl; }
+};
+
+int main()  {
+    Base *Var = new Derived();
+    delete Var;
+    return 0;
+}
+