> For the complete documentation index, see [llms.txt](https://hai-shi.gitbook.io/cpython-internals/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hai-shi.gitbook.io/cpython-internals/12-objects-types/12.3-object-and-variable-object-types.md).

# 12.3 对象和可变长度对象类型

C 语言不是像 [Python](https://realpython.com/python3-object-oriented-programming/) 这样的面向对象语言，C 语言中的对象不会继承其他对象。`PyObject` 是每个 Python 对象的初始数据段，`PyObject *` 表示指向 Python 对象的指针。

当定义 Python 类型时，`typedef` 使用如下两个宏中的一个：

* `PyObject_HEAD (PyObject)` 用于简单的类型
* `PyObject_VAR_HEAD (PyVarObject)` 用于容器类型。

`PyObject` 比较简单，它有如下字段：

| 字段          | 类型             | 用途       |
| ----------- | -------------- | -------- |
| `ob_refcnt` | `Py_ssize_t`   | 实例的引用计数器 |
| `ob_type`   | `_typeobject*` | 对象的类型    |

例如，`cellobject` 除了基础字段外，声明了一个额外的字段 `ob_ref`：

```c
typedef struct {
    PyObject_HEAD
    PyObject *ob_ref; /* Content of the cell or NULL when empty */
} PyCellObject;
```

可变长度类型 `PyVarObject` 扩展了 `PyObject` 类型，有如下字段：

| 字段        | 类型           | 用途      |
| --------- | ------------ | ------- |
| `ob_base` | `PyObject`   | 基类型     |
| `ob_size` | `Py_ssize_t` | 包含的项目数量 |

例如，`int` 类型（`PyLongObject`），定义如下：

```c
struct _longobject {
    PyObject_VAR_HEAD
    digit ob_digit[1];
}; /* PyLongObject */
```


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