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 <stdio.h>
#include <stdlib.h>
#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 <iostream>
#include <stdexcept>
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<Monkey*>(animal1)) {
std::cout << monkey->toString() << std::endl;
}
delete animal1;
Animal* animal2 = createAnimal("Cow");
if (Cow* cow = dynamic_cast<Cow*>(animal2)) {
std::cout << cow->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 main
import (
"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 Animal
class 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"
end
end
class Monkey < Animal
def to_s
"I am a monkey"
end
end
class Cow < Animal
def to_s
"I am a cow"
end
end
def create_animal(animal_type)
case animal_type
when "Monkey"
Monkey.new
when "Cow"
Cow.new
else
raise "Unknown animal type: #{animal_type}"
end
end
begin
animal1 = create_animal("Monkey")
if animal1.is_a? Monkey
puts animal1
end
animal2 = create_animal("Cow")
if animal2.is_a? Cow
puts animal2
end
end
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<Animal, String> {
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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