# What's the difference between == and ===?

{% hint style="info" %}
타 언어에서도 항상 조심하는 부분 중 하나가 두 값의 비교이다. 
{% endhint %}

* [일반 타입(string, int, float, double, boolean) 비교](#string-int-float-double-boolean)
* [컬렉션 타입(Array, Set, Dictionary) 비교](#array-set-dictionary)
* [nil 비교](#nil)
* [Custom 타입 비교](#custom)

## 일반 타입(string, int, float, double, boolean) 비교

```swift
let value1 = 42
let value2 = 42

print(value1 == value2) // true
print(value1 === value2) // error: argument type 'Int' expected to be an instance of a class or class-constrained type
```

## 컬렉션 타입(Array, Set, Dictionary) 비교

```swift
let reference1 = ["key": "value"]
let reference2 = referenceDictionary1

print(reference1 == reference2) // true
print(reference1 === reference2) // true
```

## nil 비교

> 일반 타입(string, int, float, double, boolean) 비교와 같다

```swift
let nil1: String? = nil
let nil2: String? = nil

print(value1 == value2) // true
print(value1 === value2) // error: argument type 'String' expected to be an instance of a class or class-constrained type
```

## Custom 타입 비교

> Equatable Protocol을 준수해야한다

```swift
class IntegerRef: Equatable {
    let value: Int
    init(_ value: Int) {
        self.value = value
    }

    // 아래 부분이 없다면 Compile Error
    static func == (lhs: IntegerRef, rhs: IntegerRef) -> Bool {
        return lhs.value == rhs.value
    }
}

let value1 = IntegerRef(100)
let value2 = IntegerRef(100)

print(value1 == value2) // true
```

## 출처

* <https://developer.apple.com/documentation/swift/===(_:_:)>
* <https://developer.apple.com/documentation/swift/equatable>


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