本文分享自华为云社区《多语言编程 返回多个不同类型的方法样例》，作者： 张俭 。
背景
你可能会在一些场景下碰到需要返回多个不同类型的方法。比如协议解析读取报文时，更具体地像kubernetes在开始解析Yaml的时候，怎么知道这个类型是属于Deployment还是Service？
C
C语言通常通过使用Struct（结构体）和Union（联合体）的方式来实现这个功能，如下文例子
#include #include #include <string.h>typedef enum { MONKEY, COW, UNKNOWN} AnimalType;typedef struct { char* description;} Monkey;typedef struct { char* description;} Cow;typedef struct { AnimalType type; union { Monkey monkey; Cow cow; };} Animal;Animal createAnimal(const char* animalType) { Animal animal; if (strcmp(animalType, "Monkey") == 0) { animal.type = MONKEY; animal.monkey.description = "I am a monkey!"; } else if (strcmp(animalType, "Cow") == 0) { animal.type = COW; animal.cow.description = "I am a cow!"; } else { animal.type = UNKNOWN; } return animal;}int main() { Animal animal1 = createAnimal("Monkey"); if (animal1.type == MONKEY) { printf("%s\n", animal1.monkey.description); } Animal animal2 = createAnimal("Cow"); if (animal2.type == COW) { printf("%s\n", animal2.cow.description); } Animal animal3 = createAnimal("Dog"); if (animal3.type == UNKNOWN) { printf("Unknown animal type\n"); } return 0;}
C++
在C++中，我们可以使用基类指针来指向派生类的对象。可以使用动态类型识别（RTTI）来在运行时确定对象的类型
#include #include class Animal {public: virtual std::string toString() const = 0;};class Monkey : public Animal {public: std::string toString() const override { return "I am a monkey!"; }};class Cow : public Animal {public: std::string toString() const override { return "I am a cow!"; }};Animal* createAnimal(const std::string& animalType) { if (animalType == "Monkey") { return new Monkey(); } if (animalType == "Cow") { return new Cow(); } throw std::runtime_error("Unknown animal type: " + animalType);}int main() { try { Animal* animal1 = createAnimal("Monkey"); if (Monkey* monkey = dynamic_cast(animal1)) { std::cout <toString() << std::endl; } delete animal1; Animal* animal2 = createAnimal("Cow"); if (Cow* cow = dynamic_cast(animal2)) { std::cout <toString() << std::endl; } delete animal2; } catch (const std::runtime_error& e) { std::cerr << e.what() << std::endl; } return 0;}
Go
Go的常见处理方式，是返回一个接口或者**interface{}**类型。调用者使用Go语言类型断言来检查具体的类型
package mainimport ( "fmt")type Animal interface { String() string}type Monkey struct{}func (m Monkey) String() string { return "I am a monkey!"}type Cow struct{}func (c Cow) String() string { return "I am a cow!"}func createAnimal(typeName string) (Animal, error) { switch typeName { case "Monkey": return Monkey{}, nil case "Cow": return Cow{}, nil default: return nil, fmt.Errorf("Unknown animal type: %s", typeName) }}func main() { animal1, err := createAnimal("Monkey") if err != nil { fmt.Println(err) return } if monkey, ok := animal1.(Monkey); ok { fmt.Println(monkey) } animal2, err := createAnimal("Cow") if err != nil { fmt.Println(err) return } if cow, ok := animal2.(Cow); ok { fmt.Println(cow) }}
Java
Java语言的常见处理方式，是返回Object类型或者一个基础类型。然后由调用方在进行instance of判断。或者Java17之后，可以使用模式匹配的方式来简化转型
public class MultiTypeReturnExample { static class Monkey { @Override public String toString() { return "I am a monkey!"; } } static class Cow { @Override public String toString() { return "I am a cow!"; } } public static Object createAnimal(String type) throws IllegalArgumentException { switch (type) { case "Monkey": return new Monkey(); case "Cow": return new Cow(); default: throw new IllegalArgumentException("Unknown animal type: " + type); } } public static void main(String[] args) throws Exception { Object animal1 = createAnimal("Monkey"); // java8 写法，后面如果明确用做精确的类型，需要强制转换 if (animal1 instanceof Monkey) { System.out.println(animal1); } Object animal2 = createAnimal("Cow"); if (animal2 instanceof Cow) { System.out.println(animal2); } // java17 写法，不需要强制转换 if (createAnimal("Monkey") instanceof Monkey animal3) { System.out.println(animal3); } if (createAnimal("Cow") instanceof Cow animal4) { System.out.println(animal4); } }}
Javascript
动态类型语言，使用instanceof运算符判断
class Animal { toString() { return 'I am an animal'; }}class Monkey extends Animal { toString() { return 'I am a monkey'; }}class Cow extends Animal { toString() { return 'I am a cow'; }}function createAnimal(animalType) { switch (animalType) { case 'Monkey': return new Monkey(); case 'Cow': return new Cow(); default: throw new Error(`Unknown animal type: ${animalType}`); }}try { const animal1 = createAnimal('Monkey'); if (animal1 instanceof Monkey) { console.log(animal1.toString()); } const animal2 = createAnimal('Cow'); if (animal2 instanceof Cow) { console.log(animal2.toString()); } const animal3 = createAnimal('Dog');} catch (error) { console.error(error.message);}
Kotlin
Kotlin可以使用Sealed Class(密封类)和Any类型两种方式。使用Any的场景，与Java返回Object类似。Sealed Class更加安全、更方便一些。
使用Any类型
open class Animalclass Monkey: Animal() { override fun toString(): String { return "I am a monkey!" }}class Cow: Animal() { override fun toString(): String { return "I am a cow!" }}fun createAnimal(type: String): Any { return when (type) { "Monkey" -> Monkey() "Cow" -> Cow() else -> throw IllegalArgumentException("Unknown animal type: $type") }}fun main() { val animal1 = createAnimal("Monkey") when (animal1) { is Monkey -> println(animal1) is Cow -> println(animal1) } val animal2 = createAnimal("Cow") when (animal2) { is Monkey -> println(animal2) is Cow -> println(animal2) }}
使用SealedClass
sealed class Animal { data class Monkey(val info: String = "I am a monkey!") : Animal() data class Cow(val info: String = "I am a cow!") : Animal()}fun createAnimal(type: String): Animal { return when (type) { "Monkey" -> Animal.Monkey() "Cow" -> Animal.Cow() else -> throw IllegalArgumentException("Unknown animal type: $type") }}fun main() { val animal1 = createAnimal("Monkey") when (animal1) { is Animal.Monkey -> println(animal1.info) is Animal.Cow -> println(animal1.info) } val animal2 = createAnimal("Cow") when (animal2) { is Animal.Monkey -> println(animal2.info) is Animal.Cow -> println(animal2.info) }}
Python
Python是动态类型的语言，可以简单基于一些条件返回不同类型的对象，然后在接收到返回值之后使用type()函数或isinstance()函数来确定其类型
class Animal: def __str__(self): return "I am an animal"class Monkey(Animal): def __str__(self): return "I am a monkey"class Cow(Animal): def __str__(self): return "I am a cow"def create_animal(animal_type): if animal_type == "Monkey": return Monkey() elif animal_type == "Cow": return Cow() else: raise ValueError(f"Unknown animal type: {animal_type}")def main(): animal1 = create_animal("Monkey") if isinstance(animal1, Monkey): print(animal1) animal2 = create_animal("Cow") if isinstance(animal2, Cow): print(animal2)if __name__ == "__main__": main()
Ruby
Ruby也较为简单，在方法内部直接返回不同类型的对象。然后，可以使用is_a方法或class方法来确定返回对象的实际类型。
class Animal def to_s "I am an animal" endendclass Monkey < Animal def to_s "I am a monkey" endendclass Cow < Animal def to_s "I am a cow" endenddef create_animal(animal_type) case animal_type when "Monkey" Monkey.new when "Cow" Cow.new else raise "Unknown animal type: #{animal_type}" endendbegin animal1 = create_animal("Monkey") if animal1.is_a? Monkey puts animal1 end animal2 = create_animal("Cow") if animal2.is_a? Cow puts animal2 endend
Rust
在Rust中，可以使用enum（枚举）来创建一个持有多种不同类型的数据结构。然后使用match语句来做模式匹配。
use std::fmt;enum Animal { Monkey, Cow,}impl fmt::Display for Animal { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { match self { Animal::Monkey => write!(f, "I am a monkey!"), Animal::Cow => write!(f, "I am a cow!"), } }}fn create_animal(animal_type: &str) -> Result { match animal_type { "Monkey" => Ok(Animal::Monkey), "Cow" => Ok(Animal::Cow), _ => Err(format!("Unknown animal type: {}", animal_type)), }}fn main() { match create_animal("Monkey") { Ok(animal) => match animal { Animal::Monkey => println!("{}", animal), _ => (), }, Err(e) => println!("{}", e), } match create_animal("Cow") { Ok(animal) => match animal { Animal::Cow => println!("{}", animal), _ => (), }, Err(e) => println!("{}", e), } match create_animal("Dog") { Ok(_) => (), Err(e) => println!("{}", e), }}
Scala
scala中，可以使用sealed trait和case class来创建一个能够返回多种不同类型的方法。Sealed trait可以定义一个有限的子类集合，可以确保类型安全
sealed trait Animal { def info: String}case class Monkey() extends Animal { val info: String = "I am a monkey!"}case class Cow() extends Animal { val info: String = "I am a cow!"}object MultiTypeReturnExample { def createAnimal(animalType: String): Animal = { animalType match { case "Monkey" => Monkey() case "Cow" => Cow() case _ => throw new IllegalArgumentException(s"Unknown animal type: $animalType") } } def main(args: Array[String]): Unit = { try { val animal1 = createAnimal("Monkey") animal1 match { case Monkey() => println(animal1.info) case _ => } val animal2 = createAnimal("Cow") animal2 match { case Cow() => println(animal2.info) case _ => } } catch { case e: IllegalArgumentException => println(e.getMessage) } }}
TypeScript
总得来说，和javascript区别不大
abstract class Animal { abstract toString(): string;}class Monkey extends Animal { toString(): string { return 'I am a monkey'; }}class Cow extends Animal { toString(): string { return 'I am a cow'; }}function createAnimal(animalType: string): Animal { switch (animalType) { case 'Monkey': return new Monkey(); case 'Cow': return new Cow(); default: throw new Error(`Unknown animal type: ${animalType}`); }}try { const animal1 = createAnimal('Monkey'); if (animal1 instanceof Monkey) { console.log(animal1.toString()); } const animal2 = createAnimal('Cow'); if (animal2 instanceof Cow) { console.log(animal2.toString()); } const animal3 = createAnimal('Dog');} catch (error) { console.error(error.message);}
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