Variant Summary

The fact that discriminated unions are useful in everyday programming should come as no surprise, and the Boost.Variant library does an excellent job of providing efficient and easy-to-use variant types based upon discriminated unions. Because C++ unions aren't terribly useful for many types (they support only built-in types and POD types), the need for something else has been prevalent for a long time. Many attempts at creating discriminated unions have suffered from significant drawbacks. For example, previous attempts usually come with a fixed set of supported types, which seriously impedes maintainability and flexibility. Boost.Variant avoids this limitation through templates, which theoretically allows creating any variant type. Type-switching code has always been a problem when dealing with discriminated unions; it was necessary to test for the type of the current value before acting, creating maintenance headaches. Boost.Variant offers straightforward value extraction and typesafe visitation, which is a novel approach that elegantly solves that problem. Finally, efficiency has often been a concern with previous attempts, but this library addresses that too, by using stack-based storage rather than the heap.

Boost.Variant is a mature library, with a rich set of features that makes it easy and efficient to work with variant types. It nicely complements the Boost.Any library, and it should definitely be part of your professional C++ toolbox.

The authors of Boost.Variant are Eric Friedman and Itay Maman.