2nd/10_Wiki/Topics/Other/Inheritance-and-Polymorphism.md

---
id: wiki-2026-0508-inheritance-and-polymorphism
title: Inheritance and Polymorphism
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [OOP Inheritance, Subtype Polymorphism, Method Dispatch]
duplicate_of: none
source_trust_level: A
confidence_score: 0.95
verification_status: applied
tags: [oop, programming-language, type-theory, polymorphism]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: multi
  framework: oop
---

# Inheritance and Polymorphism

## 매 한 줄
> **"매 inheritance 의 mechanism — polymorphism 의 outcome"**. 매 inheritance 의 code reuse + 매 is-a 의 subtype relationship; 매 polymorphism 의 same interface 의 different behavior. 매 modern 2026 의 view 의 "favor composition over inheritance" 의 default — Go/Rust 의 inheritance-free, but interface-polymorphism 의 universal. 매 LLM-generated code 의 over-inheritance 의 common antipattern.

## 매 핵심

### 매 inheritance 의 종류
- **Single inheritance**: 매 one parent class (Java, Kotlin, C#).
- **Multiple inheritance**: 매 N parents — diamond problem (C++, Python via MRO).
- **Mixins / traits**: 매 horizontal composition (Rust traits, Scala traits, Python mixins).
- **Prototype-based**: 매 object → object delegation (JavaScript, Lua).

### 매 polymorphism 의 종류
- **Subtype polymorphism**: 매 subclass 의 parent 의 substitute (Liskov LSP).
- **Parametric polymorphism (generics)**: 매 type parameter — `List<T>`.
- **Ad-hoc polymorphism (overloading)**: 매 same name 의 different signatures.
- **Row polymorphism**: 매 structural — TypeScript / OCaml 의 records.

### 매 응용
1. **Domain hierarchy**: 매 `Animal → Dog/Cat`. Often overused.
2. **Plugin architecture**: 매 `Plugin` interface + N implementations.
3. **AST transformation**: 매 `Visitor` pattern + node hierarchy.
4. **Strategy pattern**: 매 interchangeable algorithms via interface.

## 💻 패턴

### 매 subtype polymorphism (Python ABC + Liskov)
```python
from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self) -> float: ...

class Circle(Shape):
    def __init__(self, r: float): self.r = r
    def area(self) -> float: return 3.14159 * self.r ** 2

class Rectangle(Shape):
    def __init__(self, w: float, h: float): self.w, self.h = w, h
    def area(self) -> float: return self.w * self.h

# Polymorphic use — caller knows nothing about concrete type
def total_area(shapes: list[Shape]) -> float:
    return sum(s.area() for s in shapes)

print(total_area([Circle(2), Rectangle(3, 4)]))  # 24.566...
```

### 매 composition over inheritance (modern preferred)
```python
# ❌ Inheritance-heavy — fragile
class Animal:
    def move(self): print("move")
class Bird(Animal):
    def fly(self): print("fly")
class Penguin(Bird):  # Penguin 의 fly X — Liskov violation
    def fly(self): raise NotImplementedError

# ✅ Composition-based — flexible
from dataclasses import dataclass
from typing import Protocol

class Mover(Protocol):
    def move(self) -> str: ...

@dataclass
class Walker:
    def move(self) -> str: return "walk"

@dataclass
class Flyer:
    def move(self) -> str: return "fly"

@dataclass
class Animal2:
    name: str
    mover: Mover  # plug in capability — no inheritance

penguin = Animal2("Penguin", Walker())
sparrow = Animal2("Sparrow", Flyer())
print(penguin.mover.move(), sparrow.mover.move())
```

### 매 generic parametric polymorphism (TypeScript)
```typescript
// Same code for any T
function head<T>(xs: T[]): T | undefined {
    return xs[0];
}

const n: number | undefined = head([1, 2, 3]);
const s: string | undefined = head(["a", "b"]);

// Bounded generic — T must be Comparable
interface Comparable<T> { compareTo(other: T): number; }

function max<T extends Comparable<T>>(xs: T[]): T {
    return xs.reduce((acc, x) => x.compareTo(acc) > 0 ? x : acc);
}
```

### 매 trait-based polymorphism (Rust — no inheritance)
```rust
trait Area {
    fn area(&self) -> f64;
}

struct Circle { r: f64 }
struct Square { side: f64 }

impl Area for Circle {
    fn area(&self) -> f64 { 3.14159 * self.r * self.r }
}
impl Area for Square {
    fn area(&self) -> f64 { self.side * self.side }
}

// Static dispatch (zero-cost generics)
fn print_area_static<T: Area>(s: &T) {
    println!("{}", s.area());
}

// Dynamic dispatch (vtable)
fn print_area_dyn(s: &dyn Area) {
    println!("{}", s.area());
}

fn main() {
    let shapes: Vec<Box<dyn Area>> = vec![
        Box::new(Circle { r: 2.0 }),
        Box::new(Square { side: 3.0 }),
    ];
    for s in &shapes { print_area_dyn(s.as_ref()); }
}
```

### 매 visitor pattern (AST traversal)
```python
from dataclasses import dataclass
from typing import Union

# AST hierarchy
@dataclass
class Num: value: float
@dataclass
class Add: left: "Expr"; right: "Expr"
@dataclass
class Mul: left: "Expr"; right: "Expr"

Expr = Union[Num, Add, Mul]

# Polymorphic dispatch via match (Python 3.10+)
def evaluate(e: Expr) -> float:
    match e:
        case Num(v): return v
        case Add(l, r): return evaluate(l) + evaluate(r)
        case Mul(l, r): return evaluate(l) * evaluate(r)

# (3 + 4) * 5
print(evaluate(Mul(Add(Num(3), Num(4)), Num(5))))  # 35
```

### 매 LSP-violation 의 detector (mypy + tests)
```python
# Runtime LSP check — for each subclass override, parameter contravariant + return covariant
import inspect
from typing import get_type_hints

def check_lsp(parent: type, child: type) -> list[str]:
    issues = []
    for name in dir(parent):
        if name.startswith("_") or not callable(getattr(parent, name)):
            continue
        try:
            p_hints = get_type_hints(getattr(parent, name))
            c_hints = get_type_hints(getattr(child, name))
        except Exception:
            continue
        # Return type must be subtype of parent's return
        if "return" in p_hints and "return" in c_hints:
            if not issubclass(c_hints["return"], p_hints["return"]):
                issues.append(f"{child.__name__}.{name}: return type widens parent")
    return issues
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| 매 is-a 의 stable | Inheritance OK — keep depth ≤ 2 |
| 매 has-a / can-do | Composition + protocol/interface |
| 매 cross-cutting (logging, metrics) | Decorator / mixin / aspect |
| 매 algorithm variants | Strategy pattern (composition) |
| 매 type-safe collections | Parametric generics (`List<T>`) |
| 매 closed AST / variant data | Sum types + pattern match (Rust enum, Scala sealed) |

**기본값**: 매 composition + interface — inheritance 의 only when 명확 is-a + LSP-honoring.

## 🔗 Graph
- 부모: [[Object-Oriented-Programming]] · [[Type Theory]]
- 변형: [[Subtype Polymorphism]] · [[Parametric Polymorphism]]
- 응용: [[Visitor Pattern]]
- Adjacent: [[Composition over Inheritance]] · [[Duck Typing]]

## 🤖 LLM 활용
**언제**: 매 boilerplate 의 generation (interface + N impls) / 매 LSP audit / 매 inheritance-to-composition refactor.
**언제 X**: 매 deep inheritance design — 매 LLM 의 over-inherit 의 tendency. Manual review 필수.

## ❌ 안티패턴
- **매 deep inheritance**: 매 4+ levels — fragile base class problem.
- **매 LSP violation**: 매 subclass 의 throws on parent-supported method (Penguin.fly).
- **매 inheritance for code reuse only**: 매 not is-a — use composition.
- **매 god parent class**: 매 parent 의 every responsibility — SRP violation.
- **매 multiple inheritance 의 diamond ignore**: 매 MRO 의 surprise behavior.

## 🧪 검증 / 중복
- Verified (Gamma et al. *Design Patterns* 1994; Liskov & Wing *A Behavioral Notion of Subtyping* 1994; Effective Java Item 18 "Favor composition" 2018).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — types of inheritance + polymorphism + multi-language patterns |