Comparators and projections

Most sorting algorithms in cpp-sort accept comparison and/or projection parameters. The library therefore considers these kinds of functions to be first-class citizens too and provides dedicated comparators, projections and tools to combine them and to solve common related problems.

All the functions and classes in cpp-sort that take comparison or projection functions as parameters expect Callable parameters, which correspond to anything that can be used as the first parameter of std::invoke. This allows to pass entities such as pointers to members or pointer to member functions to the sorting algorithms; it should work out-of-the-box without any wrapping needed on the user side.

Related utilities

Several of the miscellaneous utilities provided by the library are meant to interact with comparators and projections:

• as_comparison and as_projection are used to make it explicit whether an ambiguous function object should be used for comparison or for projection.
• as_function can be used to turn any Callable into an object invokable with regular parentheses.
• is_probably_branchless_comparison and is_probably_branchless_projection are type traits that can be used to mark whether functions are likely to be branchless when called with a specific type.
• identity is the default projection returning the argument it is passed without modifying it.

LWG3031

Sorters and adapters in the library accept comparators taking their parameters by non-const reference, and should work as expected as long as comparators do not actually modify their parameters in a way that affects the comparison result. This is mostly meant to support legacy comparators, but it also covers niche use cases such as when a comparator needs to "mark" the objects that have been compared in an intrusive fashion.

This additional guarantee is allowed by the resolution of LWG3031. However when a comparator can take its parameters by both const and non-const reference, it is required to return consistent results no matter which overload is used (see the LWG issue for an example of inconsistent results).

New in version 1.7.0