C++多态，模板与泛型编程
0 引言
本文探讨一些C++的基本语法，包括C++多态(polymorphism)，模板(template)与泛型编程(generic programming)。
（1）basic C++:
https://www.learncpp.com/
（2）C++ 多态：
https://www.runoob.com/cplusplus/cpp-polymorphism.html
https://zhuanlan.zhihu.com/p/37340242
（3）模板与泛型编程
https://www.programiz.com/cpp-programming/class-templates
1 C++多态
多态性可以简单概括为“一个接口， 多种方法”。 它是面向对象编程领域的核心问题。
C++ 支持两种多态：
1. 编译时多态性（静态多态）：通过重载函数实现：先期联编 early binding
(1) 函数重载（function overloading）：通过重载使得函数具备不同的输入类型/输入变量个数，或者执行不同的操作。
(2) 运算符重载 （operator overloading）：通过重载使得运算符在当前类对象的运算中具备独特的含义
(3) 函数覆盖（function overriding): 子类覆盖父类的实现
2. 运行时多态性（动态多态）：通过虚函数实现 ：滞后联编 late binding
#include <iostream>
using namespace std;
class Shape {
protected:
int width, height;
public:
Shape( int a=0, int b=0)
{
width = a;
height = b;
}
virtual int area()
{
cout << "Parent class area :" <<endl;
return 0;
}
};
class Rectangle: public Shape{
public:
Rectangle( int a=0, int b=0):Shape(a, b) { }
virtual int area ()
{
cout << "Rectangle class area :" <<endl;
return (width * height);
}
};
class Triangle: public Shape{
public:
Triangle( int a=0, int b=0):Shape(a, b) { }
virtual int area ()
{
cout << "Triangle class area :" <<endl;
return (width * height / 2);
}
};
// 程序的主函数
int main( )
{
Shape *shape;
Rectangle rec(10,7);
Triangle tri(10,5);
// 存储矩形的地址
shape = &rec;
// 调用矩形的求面积函数 area
shape->area();
// 存储三角形的地址
shape = &tri;
// 调用三角形的求面积函数 area
shape->area(); ///< before virtual: all call base->area();
///< after virtual: all call derived->area, which is polymorphism
return 0;
}
２模板与泛型编程
这里有一些最基本的例子可以帮助理解模板类和模板函数。
3 模板类的多态
目前不支持将模板类中的template function设为virtual, https://stackoverflow.com/questions/2354210/can-a-class-member-function-template-be-virtual
#include <iostream>
template <typename T>
class Geometry {
public:
Geometry(const T& area) {
d_area = area;
std::cout << "construct Geometry object by area value!\n";
}
Geometry() {
d_area = 0.0d;
std::cout << "construct Geometry object by empty param!\n";
}
~Geometry() {
std::cout << "deconstruct Geometry object!\n";
}
/* no virtual
T getArea() {
return d_area;
}
*/
template <typename T1>
virtual T1 getArea() {
return d_area;
}
private:
T d_area;
};
template <typename T>
class Triangle: public Geometry<T> {
public:
Triangle(const T& area): Geometry<T>(area) {
d_width = 0.0d;
d_height = 0.0d;
std::cout << "construct Triangle object by area value!\n";
}
Triangle(const T& width, const T& height): Geometry<T>(0.0d) {
d_width = width;
d_height = height;
std::cout << "construct Triangle object by width and height value!\n";
}
~Triangle() {
std::cout << "deconstruct Triangle object by area value!\n";
}
/* no virtual
T getArea() {
d_area = 0.5d * d_width * d_height;
return d_area;
}
*/
template <typename T1>
virtual T1 getArea() {
d_area = 0.5d * d_width * d_height;
return d_area;
}
private:
T d_width;
T d_height;
T d_area;
};
int main()
{
Geometry<double>* geom;
Triangle<double> tri(7.0d, 8.0d);
geom = &tri;
std::cout << "area of triangle = " << geom->getArea<double>() << std::endl; ///< when no virtual function, geom will call
return 0;
}
compiling result: template_virtual.cpp:24:5: error: templates may not be ‘virtual’
Solution for this usage;
#include <iostream>
template <typename T, typename T1>
class Geometry {
public:
Geometry(const T& area) {
d_area = area;
std::cout << "construct Geometry object by area value!\n";
}
Geometry() {
d_area = 0.0d;
std::cout << "construct Geometry object by empty param!\n";
}
~Geometry() {
std::cout << "deconstruct Geometry object!\n";
}
/* no virtual
T getArea() {
return d_area;
}
*/
virtual T1 getArea() {
return d_area;
}
private:
T d_area;
};
template <typename T, typename T1>
class Triangle: public Geometry<T, T1> {
public:
Triangle(const T& area): Geometry<T, T1>(area) {
d_width = 0.0d;
d_height = 0.0d;
std::cout << "construct Triangle object by area value!\n";
}
Triangle(const T& width, const T& height): Geometry<T, T1>(0.0d) {
d_width = width;
d_height = height;
std::cout << "construct Triangle object by width and height value!\n";
}
~Triangle() {
std::cout << "deconstruct Triangle object by area value!\n";
}
/* no virtual
T getArea() {
d_area = 0.5d * d_width * d_height;
return d_area;
}
*/
virtual T1 getArea() {
d_area = 0.5d * d_width * d_height;
return d_area;
}
private:
T d_width;
T d_height;
T d_area;
};
int main()
{
Geometry<double, double>* geom;
Triangle<double, double> tri(7.0d, 8.0d);
geom = &tri;
std::cout << "area of triangle = " << geom->getArea() << std::endl; ///< when no virtual function, geom will call
return 0;
}
it compiles good.
4 对比
