泛型术语：占位类型placeholder

Here’s a generic version of the same code:

1. struct Stack<Element> {
2. var items = [Element]()
3. mutating func push(_ item: Element) {
4. items.append(item)
5. }
6. mutating func pop() -> Element {
7. return items.removeLast()
8. }
9. }

Note how the generic version of Stack is essentially the same as the nongeneric version, but with a type parameter called Element instead of an actual type of Int. This type parameter is written within a pair of angle brackets (<Element>) immediately after the structure’s name.

Element defines a placeholder name for a type to be provided later. This future type can be referred to as Element anywhere within the structure’s definition. In this case, Element is used as a placeholder in three places:

• To create a property called items, which is initialized with an empty array of values of type Element
• To specify that the push(_:) method has a single parameter called item, which must be of type Element
• To specify that the value returned by the pop() method will be a value of type Element

https://docs.swift.org/swift-book/LanguageGuide/Generics.html

Generic programming is a style of computer programming in which algorithms are written in terms of types to-be-specified-later that are then instantiated when needed for specific types provided as parameters.

template<typename T>
class List
{
   /* class contents */
};

List<Animal> list_of_animals;
List<Car> list_of_cars;

Above, T is a placeholder for whatever type is specified when the list is created.

https://en.wikipedia.org/wiki/Generic_programming