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The Foundation Kit, or just Foundation for short, is an Objective-C framework in the OpenStep specification. It provides basic classes such as wrapper classes and data structure classes. This framework uses the prefix NS (for NeXTSTEP, or NeXT/Sun_Microsystems).
NSValue and NSNumber
NSValue is a wrapper class for C data types, and NSNumber is a wrapper class for C number data types such as int, double, and float. The data structures in Foundation Kit can only hold objects, not primitive types, so wrappers such as NSValue and NSNumber are used in those data structures.
The Foundation Kit is implemented in GNUstep's Base Package. This implementation is mostly comparable (4 classes are missing) and aims to be comparable with both the OpenStep API and later macOS additions.
The Foundation Kit is implemented in Cocotron, an open-source implementation of Cocoa.
PureFoundation is an open-source implementation of Foundation that implements Foundation by wrapping Core Foundation, just like in Cocoa, rather than create a separate Foundation from scratch like GNUstep and Cocotron.