Range Widgets
Gtk::Scale and Gtk::Scrollbar both inherit from Gtk::Range and share much functionality. They contain a "trough" and a "slider" (sometimes called a "thumbwheel" in other GUI environments). Dragging the slider with the pointer moves it within the trough, while clicking in the trough advances the slider towards the location of the click, either completely, or by a designated amount, depending on which mouse button is used. This should be familiar scrollbar behaviour.
As will be explained in the Adjustment section, all Range widgets are associated with a Adjustment object. To change the lower, upper, and current values used by the widget you need to use the methods of its Adjustment, which you can get with the get_adjustment() method. The Range widgets' default constructors create an Adjustment automatically, or you can specify an existing Adjustment, maybe to share it with another widget. See the Adjustments section for further details.
Reference

Scrollbar Widgets
These are standard scrollbars. They should be used only to scroll another widget, such as, a Gtk::Entry, or a Gtk::Viewport, though it's usually easier to use the Gtk::ScrolledWindow widget in most cases.
The orientation of a Gtk::Scrollbar can be either horizontal or vertical.
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Scale Widgets
Gtk::Scale widgets (or "sliders") allow the user to visually select and manipulate a value within a specific range. You might use one, for instance, to adjust the magnification level on a zoomed preview of a picture, or to control the brightness of a colour, or to specify the number of minutes of inactivity before a screensaver takes over the screen.
As with Scrollbars, the orientation can be either horizontal or vertical. The default constructor creates an Adjustment with all of its values set to 0.0. This isn't useful so you will need to set some Adjustment details to get meaningful behaviour.

Useful methods
Scale widgets can display their current value as a number next to the trough. By default they show the value, but you can change this with the set_draw_value() method.
The value displayed by a scale widget is rounded to one decimal point by default, as is the value field in its Gtk::Adjustment. You can change this with the set_digits() method.
Also, the value can be drawn in different positions relative to the trough, specified by the set_value_pos() method.
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Example
This example displays a window with three range widgets all connected to the same adjustment, along with a couple of controls for adjusting some of the parameters mentioned above and in the section on adjustments, so you can see how they affect the way these widgets work for the user.

Range Widgets
Source Code