特征模板技术
是一种将与某种类型相关联的所有声明绑定在一起的实现方式
本质上说,使用特征技术
可以以一种灵活的方法从它们的语境中将这些类型和值进行混合与匹配
//: C05:BearCorner.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
#ifndef BEARCORNER_H
#define BEARCORNER_H
#include <iostream>
using std::ostream;
// Item classes (traits of guests):
class Milk {
public:
friend ostream& operator<<(ostream& os, const Milk&) {
return os << "Milk";
}
};
class CondensedMilk {
public:
friend ostream&
operator<<(ostream& os, const CondensedMilk &) {
return os << "Condensed Milk";
}
};
class Honey {
public:
friend ostream& operator<<(ostream& os, const Honey&) {
return os << "Honey";
}
};
class Cookies {
public:
friend ostream& operator<<(ostream& os, const Cookies&) {
return os << "Cookies";
}
};
// Guest classes:
class Bear {
public:
friend ostream& operator<<(ostream& os, const Bear&) {
return os << "Theodore";
}
};
class Boy {
public:
friend ostream& operator<<(ostream& os, const Boy&) {
return os << "Patrick";
}
};
// Primary traits template (empty-could hold common types)
template<class Guest> class GuestTraits;
// Traits specializations for Guest types
template<> class GuestTraits<Bear> {
public:
typedef CondensedMilk beverage_type;
typedef Honey snack_type;
};
template<> class GuestTraits<Boy> {
public:
typedef Milk beverage_type;
typedef Cookies snack_type;
};
#endif // BEARCORNER_H ///:~
//: C05:BearCorner.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Illustrates traits classes.
#include <iostream>
#include "BearCorner.h"
using namespace std;
// A custom traits class
class MixedUpTraits {
public:
typedef Milk beverage_type;
typedef Honey snack_type;
};
// The Guest template (uses a traits class)
template<class Guest, class traits = GuestTraits<Guest> >
class BearCorner {
Guest theGuest;
typedef typename traits::beverage_type beverage_type;
typedef typename traits::snack_type snack_type;
beverage_type bev;
snack_type snack;
public:
BearCorner(const Guest& g) : theGuest(g) {}
void entertain() {
cout << "Entertaining " << theGuest
<< " serving " << bev
<< " and " << snack << endl;
}
};
int main() {
Boy cr;
BearCorner<Boy> pc1(cr);
pc1.entertain();
Bear pb;
BearCorner<Bear> pc2(pb);
pc2.entertain();
BearCorner<Bear, MixedUpTraits> pc3(pb);
pc3.entertain();
getchar();
} ///:~
输出
Entertaining Patrick serving Milk and Cookies
Entertaining Theodore serving Condensed Milk and Honey
Entertaining Theodore serving Milk and Honey
在这个程序中
为招待作为客人的类Boy和类Bear的实例,提供了适合他们的口味的食物
Boy喜欢牛奶和小甜点
Bear喜欢浓缩的牛奶和蜂蜜