Editing Class invariant (section)

====C++====
The [[Loki (C++)]] library provides a framework written by [[Richard Sposato]] for checking class invariants, static data invariants, and [[exception safety]] level.

This is an example of how a class can use Loki::Checker to verify that invariants remain true after an object changes. The example uses a geopoint object to store a location on Earth as a coordinate of latitude and longitude.

The geopoint invariants are:
* latitude may not be more than 90° north.
* latitude may not be less than -90° south.
* longitude may not be more than 180° east.
* longitude may not be less than -180° west.

<syntaxhighlight lang="C++">
#include <loki/Checker.h>  // Needed to check class invariants.

#include <Degrees.hpp>

class GeoPoint {
 public:
  GeoPoint(Degrees latitude, Degrees longitude);

  /// Move function will move location of GeoPoint.
  void Move(Degrees latitude_change, Degrees longitude_change) {
    // The checker object calls IsValid at function entry and exit to prove this
    // GeoPoint object is valid. The checker also guarantees GeoPoint::Move
    // function will never throw.
    CheckFor::CheckForNoThrow checker(this, &IsValid);

    latitude_ += latitude_change;
    if (latitude_ >= 90.0) latitude_ = 90.0;
    if (latitude_ <= -90.0) latitude_ = -90.0;

    longitude_ += longitude_change;
    while (longitude_ >= 180.0) longitude_ -= 360.0;
    while (longitude_ <= -180.0) longitude_ += 360.0;
  }

 private:
  /** @note CheckFor performs validity checking in many functions to determine
   if the code violated any invariants, if any content changed, or if the
   function threw an exception.
   */
  using CheckFor = ::Loki::CheckFor<const GeoPoint>;

  /// This function checks all object invariants.
  bool IsValid() const {
    assert(this != nullptr);
    assert(latitude_ >= -90.0);
    assert(latitude_ <= 90.0);
    assert(longitude_ >= -180.0);
    assert(longitude_ <= 180.0);
    return true;
  }

  Degrees latitude_;   ///< Degrees from equator. Positive is north, negative is
                       ///< south.
  Degrees longitude_;  ///< Degrees from Prime Meridian. Positive is east,
                       ///< negative is west.
}
</syntaxhighlight>