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| 1 | +--- |
| 2 | +title: Curiously Recurring Template Pattern |
| 3 | +language: zh |
| 4 | +category: Structural |
| 5 | +tag: |
| 6 | +- Extensibility |
| 7 | +- Instantiation |
| 8 | +--- |
| 9 | + |
| 10 | +## 名称/分类 |
| 11 | + |
| 12 | +Curiously Recurring Template Pattern,CRTP,奇异递归模板模式 |
| 13 | + |
| 14 | +## 别名 |
| 15 | + |
| 16 | +递归类型绑定,递归泛型 |
| 17 | + |
| 18 | +## 目的 |
| 19 | + |
| 20 | +允许派生组件从与派生类型兼容的基本组件继承某些功能。 |
| 21 | + |
| 22 | +## 解释 |
| 23 | + |
| 24 | +真实世界的例子 |
| 25 | + |
| 26 | +> 对于正在策划赛事的综合格斗推广活动来说,确保在相同重量级的运动员之间组织比赛至关重要。这样可以防止体型明显不同的拳手之间的不匹配,例如重量级拳手与雏量级拳手的对决。 |
| 27 | +
|
| 28 | +用通俗的话来讲 |
| 29 | + |
| 30 | +> 使类型中的某些方法接受特定于其子类型的参数。 |
| 31 | +
|
| 32 | +维基百科介绍 |
| 33 | + |
| 34 | +> 奇异递归模板模式(curiously recurring template pattern,CRTP)是C++模板编程时的一种惯用法:其中类X派生自使用X本身作为模板参数的类模板实例化。 |
| 35 | +
|
| 36 | +**程序示例** |
| 37 | + |
| 38 | +让我们来定义通用接口Fighter |
| 39 | + |
| 40 | +```java |
| 41 | +public interface Fighter<T> { |
| 42 | + |
| 43 | + void fight(T t); |
| 44 | + |
| 45 | +} |
| 46 | +``` |
| 47 | + |
| 48 | +MmaFighter类用于实例化按重量级别区分的拳手 |
| 49 | + |
| 50 | +``` Java |
| 51 | +public class MmaFighter<T extends MmaFighter<T>> implements Fighter<T> { |
| 52 | + |
| 53 | + private final String name; |
| 54 | + private final String surname; |
| 55 | + private final String nickName; |
| 56 | + private final String speciality; |
| 57 | + |
| 58 | + public MmaFighter(String name, String surname, String nickName, String speciality) { |
| 59 | + this.name = name; |
| 60 | + this.surname = surname; |
| 61 | + this.nickName = nickName; |
| 62 | + this.speciality = speciality; |
| 63 | + } |
| 64 | + |
| 65 | + @Override |
| 66 | + public void fight(T opponent) { |
| 67 | + LOGGER.info("{} is going to fight against {}", this, opponent); |
| 68 | + } |
| 69 | + |
| 70 | + @Override |
| 71 | + public String toString() { |
| 72 | + return name + " \"" + nickName + "\" " + surname; |
| 73 | + } |
| 74 | +``` |
| 75 | + |
| 76 | +以下是 MmaFighter 的一些子类型 |
| 77 | + |
| 78 | +```Java |
| 79 | +class MmaBantamweightFighter extends MmaFighter<MmaBantamweightFighter> { |
| 80 | + |
| 81 | + public MmaBantamweightFighter(String name, String surname, String nickName, String speciality) { |
| 82 | + super(name, surname, nickName, speciality); |
| 83 | + } |
| 84 | + |
| 85 | +} |
| 86 | + |
| 87 | +public class MmaHeavyweightFighter extends MmaFighter<MmaHeavyweightFighter> { |
| 88 | + |
| 89 | + public MmaHeavyweightFighter(String name, String surname, String nickName, String speciality) { |
| 90 | + super(name, surname, nickName, speciality); |
| 91 | + } |
| 92 | + |
| 93 | +} |
| 94 | +``` |
| 95 | + |
| 96 | +允许拳手与相同重量级的对手交手,如果对手是不同重量级,则会出现错误 |
| 97 | + |
| 98 | +``` Java |
| 99 | +MmaBantamweightFighter fighter1 = new MmaBantamweightFighter("Joe", "Johnson", "The Geek", "Muay Thai"); |
| 100 | +MmaBantamweightFighter fighter2 = new MmaBantamweightFighter("Ed", "Edwards", "The Problem Solver", "Judo"); |
| 101 | +fighter1.fight(fighter2); // This is fine |
| 102 | + |
| 103 | +MmaHeavyweightFighter fighter3 = new MmaHeavyweightFighter("Dave", "Davidson", "The Bug Smasher", "Kickboxing"); |
| 104 | +MmaHeavyweightFighter fighter4 = new MmaHeavyweightFighter("Jack", "Jackson", "The Pragmatic", "Brazilian Jiu-Jitsu"); |
| 105 | +fighter3.fight(fighter4); // This is fine too |
| 106 | + |
| 107 | +fighter1.fight(fighter3); // This will raise a compilation error |
| 108 | +``` |
| 109 | + |
| 110 | +## 类图 |
| 111 | + |
| 112 | + |
| 113 | + |
| 114 | +## 适用性 |
| 115 | + |
| 116 | +在以下情况下使用CRTP |
| 117 | + |
| 118 | +* 在对象层次结构中链接方法时存在类型冲突 |
| 119 | +* 您想使用一个参数化的类方法,该方法可以接受类的子类作为参数,从而可以应用于继承自类的对象 |
| 120 | +* 您希望某些方法仅适用于相同类型的实例,例如实现相互比较。 |
| 121 | + |
| 122 | +## 教程 |
| 123 | + |
| 124 | +* [The NuaH Blog](https://nuah.livejournal.com/328187.html) |
| 125 | +* Yogesh Umesh Vaity answer to [What does "Recursive type bound" in Generics mean?](https://stackoverflow.com/questions/7385949/what-does-recursive-type-bound-in-generics-mean) |
| 126 | + |
| 127 | +## 已知用途 |
| 128 | + |
| 129 | +* [java.lang.Enum](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Enum.html) |
| 130 | + |
| 131 | +## 鸣谢 |
| 132 | + |
| 133 | +* [How do I decrypt "Enum<E extends Enum\<E>>"?](http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeParameters.html#FAQ106) |
| 134 | +* Chapter 5 Generics, Item 30 in [Effective Java](https://www.amazon.com/gp/product/0134685997/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0134685997&linkCode=as2&tag=javadesignpat-20&linkId=4e349f4b3ff8c50123f8147c828e53eb) |
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