The storage and carrying of glasses (normal or sun) on/in a rucksack can be a bit of a pain. Where do you put them, in your pocket, in the hip belt pocket (if you have one) in the side pockets, the top lid?
There are two simple 25mm split rings on the back of the pouch, which will slide through most shoulder harnesses and straps.
The pouches are not waterproof so for safety's sake it would be worth putting the phone/GPS or similar in a waterproof bag during inclement weather.
Photo shows hard glass case as an example. Not supplied.