Features:

  * //implementation-defined//: each implementation must provide a specific, well-defined behavior for a construct and that behavior must be documented
  * //unspecified//: a range of possible behaviors are acceptable, but the implementer is not obliged to specify which actually occur
  * //undefined//: no reasonable behavior is required by an implementation

Implementations:

  * //hosted//
  * //freestanding// (TC++PL4, p. 137)