# 下标

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`[]` 不同于普通的方法

```python
a = [!1, !2]
a[0].inc!()
assert a == [2, 2]
```

回想一下，子例程的返回值不能是引用
这里的 `a[0]` 的类型显然应该是 `Ref!(Int!)`(`a[0]` 的类型取决于上下文)
所以 `[]` 实际上是特殊语法的一部分，就像 `.` 一样。与 Python 不同，它不能被重载
也无法在方法中重现 `[]` 的行为

```python
C = Class {i = Int!}
C.steal(self) =
    self::i
```

```python,compile_fail
C. get(ref self) =
    self::i # 类型错误:`self::i`是`Int!`(需要所有权)但`get`不拥有`self`
```

```python
# OK (分配)
c = C.new({i = 1})
i = c.steal()
i.inc!()
assert i == 2
# or (own_do!)
own_do! C.new({i = 1}).steal(), i => i.inc!()
```

```python
# NG
C.new({i = 1}).steal().inc!() # OwnershipWarning: `C.new({i = 1}).steal()` is not owned by anyone
# hint: assign to a variable or use `uwn_do!`
```

此外，`[]` 可以不承认，但元素不会移动

```python
a = [!1, !2]
i = a[0]
i.inc!()
assert a[1] == 2
a[0] # 所有权错误:`a[0]`被移动到`i`
```

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