How to access an element attribute value in C++ plugin?

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I have this kind of structure inside my world plugin:

<sdf version='1.6'>
  <world name='default>

    <plugin name="MyPlugin" filename="libMyPlugin.so">
      <my_box name="box1">
        <pose>0 0 2 0 0 0</pose>
        <geometry>
            <box>
                <size>5 5 4</size>
            </box>
        </geometry>
      </my_box>

      <my_box name="box2">
        <pose>10 5 2 0 0 0</pose>
        <geometry>
            <box>
                <size>5 5 4</size>
            </box>
        </geometry>
      </my_box>
    </plugin>

  </world>
</sdf>

In my world plugin I want to access both <my_box> elements and extract the positions and sizes.

I can get the first <my_box> element by

auto my_box_sdf = _sdf->GetElement("my_box");

I can check if it has a attribute 'name' by

my_box_sdf.GetAttributeSet("name");

But I don't know how to get the value of this attribute. I tried all the functions from here that seemed as if they could return the value, but nothing worked. How do I access the attribute value?

How do I get the other <my_box> element?