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