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Generic Classes

Generic Interfaces introduced the definition and usage of generic interfaces. This section covers the definition and usage of generic classes. For example, the key-value pairs in Map are defined using generic classes.

The Node type for key-value pairs in Map can be defined using a generic class:

Since the types of keys and values may differ and can be any type that meets certain conditions, the Node class requires two type parameters K and V. The constraint K <: Hashable, K <: Equatable specifies that the key type K must implement both the Hashable and Equatable interfaces, which are the conditions K must satisfy. For more details on generic constraints, refer to the Generic Constraints section.

Because static member variables of generic classes share memory, the type declarations and expressions of static member variables or properties cannot reference type parameters or contain uninstantiated generic type expressions. Additionally, static variable or property initialization expressions cannot call static member functions or properties of generic classes.

cangjie

open class Node<K, V> where K <: Hashable & Equatable<K> {
    var key: Option<K> = Option<K>.None
    var value: Option<V> = Option<V>.None

    init() {}

    init(key: K, value: V) {
        this.key = Option<K>.Some(key)
        this.value = Option<V>.Some(value)
    }
}

cangjie

class A<T> {}
class B<T> {
    static func foo() {1}
    static var err1: A<T> = A<T>() // Error, static member cannot depend on generic parameter 'Generics-T'
    static prop err2: A<T> { // Error, static member cannot depend on generic parameter 'Generics-T'
        get() {
            A<T>() // Error, static member cannot depend on generic parameter 'Generics-T'
        }
    }
    static var vfoo = foo() // Error, it's equal to 'static var vfoo = B<T>.foo()', implicit reference to generic 'T'.
    static var ok: Int64 = 1
}

main() {
    B<Int32>.ok = 2
    println(B<Int64>.ok) // 2
}