C++模板：函数模板、类模板、模板与继承

C++模板：描述

C++提供一种模板的机制来减少代码重复。比如：对于同一样函数使用不同的数据类型，int,double,char等。C++模板属于“元编程”的范畴。

C++ 模板函数

1.支持不同数据类型的函数重载：

#include <iostream>
using namespace std;

int square (int x)
{
  return x * x;
};

float square (float x)
{
  return x * x;
};

double square (double x)
{
  return x * x;
};

main()
{
   int    i, ii;
   float  x, xx;
   double y, yy;

   i = 2;
   x = 2.2;
   y = 2.2;

   ii = square(i);
   cout << i << ": " << ii << endl;

   xx = square(x);
   cout << x << ": " << xx << endl;

   yy = square(y);
   cout << y << ": " << yy << endl;
}

2.支持所有数据类型的函数模板

#include <iostream>
using namespace std;

template <class T>
inline T square(T x)
{
   T result;
   result = x * x;
   return result;
};



main()
{
   int    i, ii;
   float  x, xx;
   double y, yy;

   i = 2;
   x = 2.2;
   y = 2.2;

   ii = square<int>(i);
   cout << i << ": " << ii << endl;

   xx = square<float>(x);
   cout << x << ": " << xx << endl;

   // Explicit use of template
   yy = square<double>(y);// 显式使用模板
   cout << y << ": " << yy << endl;

   yy = square(y);//隐含的方式使用模板
   cout << y << ": " << yy << endl;
}

注明：模板的关键字可以用class或者typename.

template<class T>
template<typename T>

两者表达的意思是一样的，但是我更喜欢使用后者。

可以采用两种方式使用模板函数square<int>(value) or square(value).

在模板函数的定义中，T代表数据类型。

模板的声明和定义必须在同一个文件中，如头文件中。

C语言的宏定义也可以实现函数模板的功能，#define square(x) (x * x) 但是宏没有类型检查，函数模板有类型检查。

C++ 模板特例化

下面的例子字符串类型需要特殊处理，采用模板的特例化

#include <iostream>
using namespace std;

template <class T>
inline T square(T x)
{
   T result;
   result = x * x;
   return result;
};

// 模板特殊化
template <>
string square<string>(string ss)
{
   return (ss+ss);
};

main()
{
   int i = 2, ii;
   string ww("Aaa");

   ii = square<int>(i);
   cout << i << ": " << ii << endl;

    cout << square<string>(ww) << endl;
}

注明：模板特例化用于当一个数据类型需要进行不同的处理和实现的情况。

C++ 模板无类型参数

#include <iostream>
using namespace std;

template <typename T, int count>
void loopIt(T x)
{
   T val[count];

   for(int ii=0; ii<count; ii++)
   { 
       val[ii] = x++;
       cout <<  val[ii] << endl;
   }
};

main()
{
   float xx = 2.1;

   loopIt<float,3>(xx);
}

C++ 模板默认类型参数以及无类型参数

#include <iostream>
using namespace std;

template <typename T=float, int count=3>
T multIt(T x)
{
   for(int ii=0; ii<count; ii++)
   {
       x = x * x;
   }
   return x;
};

main()
{
   float xx = 2.1;

   cout << xx << ": " << multIt<>(xx) << endl;;
}

注明：multIt<>没有指定参数类型，默认为float;

C++ 类模板

类模板定义：template <class T> class MyTemplateClass { ... };

类模板特例化：template <> class MyTemplateClass <specific-data-type> { ... };

File: Matrix2x2.hpp

#ifndef MATRIX_2X2_HPP__
#define MATRIX_2X2_HPP__

using namespace std;

/**
    m(11)  m(12)
    m(21)  m(22)
*/

template <class T>
class Matrix2x2
{
public:
   Matrix2x2(T m11, T m12, T m21, T m22);    //constructor
   Matrix2x2(T m[2][2]);
   Matrix2x2();

   int Add(Matrix2x2 x)
   int Multiply(Matrix2x2 x)
   void Print();
   T m[2][2];
};

template <class T>
Matrix2x2<T>::Matrix2x2(T _m11, T _m12, T _m21, T _m22)
{
   m[0][0] = _m11;
   m[0][1] = _m12;
   m[1][0] = _m21;
   m[1][1] = _m22;
}

template <class T>
Matrix2x2<T>::Matrix2x2(T _m)
{
   m[0][0] = _m[0][0];
   m[0][1] = _m[0][1];
   m[1][0] = _m[1][0];
   m[1][1] = _m[1][1];
}

template <class T>
Matrix2x2<T>::Matrix2x2()
{
   m[0][0] = 0;
   m[0][1] = 0;
   m[1][0] = 0;
   m[1][1] = 0;
}

template <class T>
Matrix2x2<T>::Add(Matrix2x2 _x)
{
    Matrix2x2<T> sum;
    sum.m[0][0] = m[0][0] + _x.m[0][0];
    sum.m[0][1] = m[0][1] + _x.m[0][1];
    sum.m[1][0] = m[1][0] + _x.m[1][0];
    sum.m[1][1] = m[1][1] + _x.m[1][1];
    return sum;
}

template <class T>
Matrix2x2<T>::Multiply(Matrix2x2 _x)
{
    Matrix2x2<T> sum;
    sum.m[0][0] = m[0][0] * _x.m[0][0] + m[0][1] * _x.m[1][0];
    sum.m[0][1] = m[0][0] * _x.m[0][1] + m[0][1] * _x.m[1][1];
    sum.m[1][0] = m[1][0] * _x.m[0][0] + m[1][1] * _x.m[1][0];
    sum.m[1][1] = m[1][0] * _x.m[0][1] + m[1][1] * _x.m[1][1];
    return sum;
}

template <class T>
Matrix2x2<T>::Print()
{
    cout << "|" << m[0][0] << "  " <<  m[0][1] << "|" << endl;
    cout << "|" << m[1][0] << "  " <<  m[1][1] << "|" << endl;
}

#endif

TestMatrix2x2.cpp

#include <iostream>

#include "Matrix2x2.hpp"

using namespace std;

int main(int argc, char* argv[])
{
    Matrix2x2<int> X(1,2,3,4);
    Matrix2x2<int> Y(5,6,7,8);

    cout << "X:" << endl;
    X.Print();

    cout << "Y:" << endl;
    Y.Print();

    Matrix2x2<int> A = X.Add(Y);
    cout << "A:" << endl;
    A.Print();

    Matrix2x2<int> B = X.Add(Y);
    cout << "B:" << endl;
    B.Print();
}

C++ 普通类和类模板的静态成员变量

普通类的静态成员函数：

#include <iostream>

using namespace std;


class XYZ
{
public:
    void putPri();
    static int ipub;
private:
    static int ipri;
};


void XYZ::putPri()
{
    cout << ipri++ << endl;
}

// 静态成员变量初始化:
int XYZ::ipub = 1;
int XYZ::ipri = 1;

main()
{
    XYZ aaa;
    XYZ bbb;

    aaa.putPri();
    cout << aaa.ipub << endl;
    bbb.putPri();
}

类模板的静态成员：

#include <iostream>

using namespace std;

template <class T> 
class XYZ
{
public:
    void putPri();
    static T ipub;
private:
    static T ipri;
};

template <class T> 
void XYZ<T>::putPri()
{
    cout << ipri++ << endl;
}

// 静态成员初始化:
template <class T> T XYZ<T>::ipub = 1;
template <class T> T XYZ<T>::ipri = 1.2;

main()
{
    XYZ<int> aaa;
    XYZ<float> bbb;

    aaa.putPri();
    cout << aaa.ipub << endl;
    bbb.putPri();
}

C++ 模板的模板参数

#include <iostream>
using namespace std;

template <template <typename T> typename U>
class Xyz
{
    ....
};

C++ 类模板和继承

Color.hpp (无模板的基类)

#ifndef COLOR_HPP__
#define COLOR_HPP__
#include <string>
enum eColor { none = 0, red, white, blue, yellow, green, black };

class Color
{
public:
    Color(eColor color);
    void setColor(eColor color);
    eColor getColor() { return mColor; };
    std::string getStrColor();

protected:
    eColor mColor;
};

Color::Color(eColor _color)
{
   mColor = _color;
}

void Color::setColor(eColor _color)
{
    mColor = _color;
}

std::string Color::getStrColor()
{
    switch(mColor)
    {
       case red:
           return "red";
       case white:
           return "white";
       case blue:
           return "blue";
       case yellow:
           return "yellow";
       case green:
           return "green";
       case black:
           return "black";
       case none:
       default:
           return "none";
    }
}
#endif

File: Circle.hpp (模板基类)

#ifndef CIRCLE_HPP__
#define CIRCLE_HPP__
#include <math.h>
#include <string>

#include "Color.hpp"

template <typename T>
class Circle : public Color
{
public:
    Circle(T centerX, T centerY, T radius, eColor color);
    Circle(T centerX, T centerY, T radius);
    Circle(T radius);

    T area();
    T circumference();
    T getX();
    T getY();
    T getRadius();

protected:
    T x;
    T y;
    T radius;
};

template <typename T>
Circle<T>::Circle(T _x, T _y, T _radius, eColor _color)
: Color(_color)
{
    x = _x;
    y = _y;
    radius = _radius;
}

template <typename T>
Circle<T>::Circle(T _x, T _y, T _radius)
: Color(none)
{
    x = _x;
    y = _y;
    radius = _radius;
}

template <typename T>
Circle<T>::Circle(T _radius)
: Color(none)
{
    x = const_cast<T>(0);
    y = const_cast<T>(0);
    radius = _radius;
}

template <typename T>
T Circle<T>::area()
{
    return M_PI * radius * radius;
}

template <typename T>
T Circle<T>::circumference()
{
    return const_cast<T>(2) * M_PI * radius;
}
#endif

File: testCircle.cpp

#include <iostream>
#include "Circle.hpp"

using namespace std;

int main(int argc, char* argv[])
{
    Circle<float> circleA(0.0, 0.0, 10.0, white);
    cout << "Area: "  << circleA.area() << endl;
    cout << "Color: " << circleA.getStrColor() << endl;
}

一个模板类继承另外一个模板类：

File: Sphere.hpp (派生类)

#ifndef SPHERE_HPP__
#define SPHERE_HPP__

#include "Circle.hpp"

template <typename T>
class Sphere : public Circle<T>
{
public:
    Sphere(T centerZ, T centerX, T centerY, T radius, eColor color);
    Sphere(T radius);
    Sphere();

    T surfaceArea();
    T volume();
    T getZ();

private:
    T z;
};

template <typename T>
Sphere<T>::Sphere(T _x, T _y, T _z, T _radius, eColor _color)
: Circle<T>::Circle (_x, _y, _radius, _color)
{
    this->z = _z;
}

template <typename T>
Sphere<T>::Sphere(T _radius)
: Circle<T>::Circle (_radius)
{
    this->x = const_cast<T>(0);
    this->y = const_cast<T>(0);
    this->z = const_cast<T>(0);
    this->radius = _radius;
}

template <typename T>
Sphere<T>::Sphere()
{
    this->x = const_cast<T>(0);
    this->y = const_cast<T>(0);
    this->z = const_cast<T>(0);
    this->radius = const_cast<T>(1);
}

template <typename T>
T Sphere<T>::surfaceArea()
{
    return const_cast<T>(4) * M_PI * this->radius * this->radius;
}

template <typename T>
T Sphere<T>::volume()
{
    T three = 3;
    T four  = 4;
    return four * M_PI * this->radius * this->radius * this->radius / three;
}
#endif

注明：用this来显示类的依赖

File: testSphere.cpp

#include <iostream>
#include "Sphere.hpp"

using namespace std;

int main(int argc, char* argv[])
{
    Sphere<float> sphereA(0.0, 0.0, 0.0,10.0, blue);
    cout << "Volume: " << sphereA.volume() << endl;
    cout << "Color: "  << sphereA.getStrColor() << endl;
}

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