Any Summary

Discriminated types can contain values of different types and are quite different from indiscriminate (read void*) types. We always depend heavily on type safety in C++, and there are few situations in which we are willing to do without it.

This is for good reasons: Type safety keeps us from making mistakes and improves the performance of our code. So, we avoid indiscriminate types. Still, it is not uncommon to find oneself in need of heterogeneous storage, or to insulate clients from the details of types, or to gain the utmost flexibility at lower levels of a hierarchy. any provides this functionality while maintaining full type safety, and that makes it an excellent addition to our toolbox!

Use the Any library when

• You need to store values of heterogeneous types in containers

• Storage for unknown types is required

• Types are being passed through layers that need not know anything about the types

The design of Any also serves as a valuable lesson on how to encapsulate a type without effect on the type of the enclosing class. This design can be used to create generic function objects, generic iterators, and much more. It is an example of the power of encapsulation and polymorphism in conjunction with templates.

In the Standard Library, there are excellent tools for storing collections of elements. When the need for storage of heterogeneous types arises, we want to avoid having to use new collection types. any offers a solution that works in many cases with existing containers. In a way, the template class any extends the capabilities of the Standard Library containers by packaging disparate types in a homogeneous wrapper that allows them to be made elements of those aforementioned containers.

Adding Boost.Any to an existing code base is straightforward. It doesn't require changes to the design, and immediately increases flexibility where it's applied. The interface is small, making it a tool that is easily understood.

The Any library was created by Kevlin Henney, and like all Boost libraries, has been reviewed, influenced, and refined by the Boost community.