> For the complete documentation index, see [llms.txt](https://i58215.gitbook.io/component-sdl2-en/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://i58215.gitbook.io/component-sdl2-en/getting-started-with-the-library/work-with-class-identifiers.md).

# Work with class identifiers

Class identifiers are a very important part of the components, since the whole appearance is built on their basis. 

There are several methods for interacting with them. The names of the methods speak for themselves.

```cpp
bool hasClass(const string& className) const;
Component* removeClass(const string& className);
Component* addClass(const string& className);
Component* toggleClass(const string& className);
```

As an example, when interactions with class identifiers may be needed, this is the creation of a component for `Checkbox` in which the change of picture may occur due to the change of the class identifier from `unchecked` to `checked` and vice versa.

### Getting components by class identifier

As already described earlier, an identifier is used to access a component. But the component can be obtained by the class identifier, but with some caveats. 

Since several components can have the same class identifier, a **collection** of components with the given class identifier will be returned. 

To get the collection, use the `getElementsByClassName` method:

```cpp
Components getElementsByClassName(const string& className);
```

The `Components` class is a wrapper over a collection of components. The following actions can be performed on the collection:

1. Set each element of the collection to the same listener using the `addEventListener` method, which completely repeats the method of the `Component` class;

2. Call for each collection item a callback function with the first parameter passing a pointer to the current collection item using the `each` method

   ```cpp
   void each(function< void(Component* sender)> callback);
   ```

3. Get the component by number using the `at` method or the `operator[]`:

   ```cpp
   Component* at(size_t index);
   Component* operator[](size_t index);
   ```

4. Get the size of the collection using the `size` method:

   ```cpp
   size_t size();
   ```