android布局文件中android:icon="?attr/menuIconCamera"找不到对应图标路径

如

<item android:id="@+id/camera"
    android:title="Camera"
    android:icon="?attr/menuIconCamera"
    android:showAsAction="ifRoom" />

其中图标不是通常用的@drawable/ic_menu_camera_holo_light,而用?attr/menuIconCamera代替，而找到menuIconCamera，却没有图标对应

<resources>
<declare-styleable name="AppTheme">
    <attr name="menuIconCamera" format="reference" />
    <attr name="menuIconToggle" format="reference" />
</declare-styleable>

SDK中对此一段描述：

For example, here's how you can reference an attribute to set the text color to match the "primary" text color of the system theme:

<EditText id="text"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:textColor="?android:textColorSecondary"
    android:text="@string/hello_world" />

Here, the android:textColor attribute specifies the name of a style attribute in the current theme. Android now uses the value applied to the android:textColorSecondary style attribute as the value for android:textColor in this widget. Because the system resource tool knows that an attribute resource is expected in this context, you do not need to explicitly state the type (which would be ?android:attr/textColorSecondary)—you can exclude the attr type.（在这里,android:textColor属性指定一个样式的名称属性在当前主题。Android现在使用的值应用于Android:textColorSecondary样式属性的值为Android:textColor在这个小部件。因为系统资源工具知道一个属性资源预计将在这种情况下,您不需要显式地声明类型(这将是? android:attr / textColorSecondary)-你可以排除attr类型。）

其实要找到menuIconCamera对应图标可以在styles.xml中找到

<item name="menuIconCamera">@drawable/ic_menu_camera_holo_light</item>