markdown multiple dispatch（多重指派）与函数重载的关系

Posted 2021-05-05

## multiple dispatch与method overloading的关系

- multiple dispatch是在runtime根据参数类型决定要调用的方法的技术
- method overloading是在compile time根据参数类型决定要调用的方法.

对于静态语言而言, 可以通过模拟实现multiple dispatch. 对于动态语言而言, 实际上两者基本上是同义词. 但并不是说动态语言天生支持multiple dispatch, 静态语言就不支持multiple dispatch.

## Python中的multiple dispatch

Python原生是不支持multiple dispatch的, 可以通过第三方库来模拟:

```python
from multimethods import Dispatch
from game_objects import Asteroid, Spaceship
from game_behaviors import ASFunc, SSFunc, SAFunc
collide = Dispatch()
collide.add_rule((Asteroid,  Spaceship), ASFunc)
collide.add_rule((Spaceship, Spaceship), SSFunc)
collide.add_rule((Spaceship, Asteroid), SAFunc)
def AAFunc(a, b):
    """Behavior when asteroid hits asteroid"""
    # ...define new behavior...
collide.add_rule((Asteroid, Asteroid), AAFunc)

# ...later...
collide(thing1, thing2)
```

## Java中的multiple dispatch

```java
interface Collideable {
    void collideWith(final Collideable other);
    /* These methods would need different names in a language without method overloading. */
    void collideWith(final Asteroid asteroid);
    void collideWith(final Spaceship spaceship);
}

class Asteroid implements Collideable {
    public void collideWith(final Collideable other) {
        // Call collideWith(Asteroid) on the other object.
        other.collideWith(this);
    }
    public void collideWith(final Asteroid asteroid) {
        // Handle Asteroid-Asteroid collision.
    }
    public void collideWith(final Spaceship spaceship) {
        // Handle Asteroid-Spaceship collision.
    }
}

class Spaceship implements Collideable {
    public void collideWith(final Collideable other) {
        // Call collideWith(Spaceship) on the other object.
        other.collideWith(this);
    }
    public void collideWith(final Asteroid asteroid) {
        // Handle Spaceship-Asteroid collision.
    }
    public void collideWith(final Spaceship spaceship) {
        // Handle Spaceship-Spaceship collision.
    }
}
```

## 参考链接

- [Multiple Dispatch](https://en.wikipedia.org/wiki/Multiple_dispatch)
- [Multiple Dispatch vs. Function Overloading](https://softwareengineering.stackexchange.com/questions/125084/multiple-dispatch-vs-function-overloading)