[GIS] Difference between geometry and $geometry QGIS

Question

This may be trivial question, but I'm confused, what is the difference between geometry and $geometry in QGIS? I didn't find any examples of using this function.

Particularly I'm interested in geometry, in what cases it is helpful?

Answer 1

Right next to the list of functions your screenshot is of, should be the currently selected function's documentation. For me, with 2.18.12 those read:

$geometry Returns the geometry of the current feature. Can be used for processing with other functions.

geometry Returns a feature's geometry.

This is like the different of a method versus a function in object-oriented programming. $geometry is this/self's geometry. You can think of it more like an attribute or property maybe.

geometry on the other hand requires/allows you to pass a feature to it and it will return the corresponding geometry. As the documentation shows, you can use this to get the geometry from a attribute-based feature selection:

geometry(
  get_feature(layer, attributeField, value)
)

What else you can do with it is just limited by your creativity and the sources for features to pass to it. :)