19.7 Afternotes

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The ability for a program to inspect its own high-level properties (such as its type structures) is sometimes called reflection . Our framework therefore implements a form of compile-time reflection , which turns out to be a powerful ally to metaprogramming (see Chapter 17).

The idea of storing properties of types as members of template specializations dates back to at least the mid-1990s. Among the earlier serious applications of type classification templates was the __type_traits utility in the STL implementation distributed by SGI (then known as Silicon Graphics ). The SGI template was meant to represent some properties of its template argument (for example, whether it was a POD type or whether its destructor was trivial). This information was then used to optimize certain STL algorithms for the given type. An interesting feature of the SGI solution was that some SGI compilers recognized the __type_traits specializations and provided information about the arguments that could not be derived using standard techniques. (The generic implementation of the __type_traits template was safe to use, albeit suboptimal.)

The use of the SFINAE principle for type classification purposes had been noted when the SFINAE principle was clarified during the standardization effort. However, it was never formally documented, and as a result much effort was later spent trying to recreate some of the techniques described in this chapter. One of the notable early contributions was by Andrei Alexandrescu who made popular the use of the sizeof operator to determine the outcome of overload resolution.

Finally, we should note that a rather complete type classification template has been incorporated in the Boost library (see [BoostTypeTraits]). In turn , this implementation is the basis of an effort to add such a facility to the standard library. See also Section 13.10 on page 218 for a related language extension.

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