First-class citizen (programming)
(it is potentially confusing that the 'class' in first-class here refers not to OO classes but the sense of quality - first-rate, second-rate, etc.)
First-class citizen tends to refer to a language in which a particular thing is like any other - and without any trickery.
For example, in Javascript and Python, functions are first-class objects:
They are part of the language's typing system, you can construct them, you can return them, you can hand them around and assign them to variables.
Contrast this with C++, where functions themselves are not first class objects - after compilation, they do not exist as things you can hand around and inspect, only really as "if you put the right things on the stack and jump here, things will happen as expected". (...with footnotes, I know.)
Raphael Finkel splits this particular case out out a bit more:
- first-class - An entity you can pass as a parameter, return, and assign to a variable
- second-class - an entity you can passed as a parameter, cannot returned from a function, and cannot store into a variable
- third-class - an enitity you cannot pass as a parameter, cannot returned from a function, and cannot store into a variable
(TODO: examples)