Want speed? Use constexpr meta-programming!

It's official: C++11 has two meta-programming languages embedded in it! One is based on templates and other one using constexpr. Templates have been extensively used for meta-programming in C++03. C++11 now gives you one more option of writing compile-time meta-programs using constexpr. The capabilities differ, however.

The meta-programming language that uses templates was discovered accidently and since then countless techniques have been developed. It is a pure functional language which allows you to manipulate compile-time integral literals and types but not floating point literals. Most people find the syntax of template meta-programming quite abominable because meta-functions must be implemented as structures and nested typedefs. Compile-time performance is also a pain point for this language feature.

The generalized constant expressions (constexpr for short) feature allows C++11 compiler to peek into the implementation of a function (even classes) and perform optimizations if the function uses constants (literals) only. Constants can be integral, floating point, as well as string literals. The signature of constexpr functions is just like regular C++ functions but the body has several restrictions, such as only one return statement is allowed. Nevertheless, the syntax of constexpr functions is significantly friendlier than that of template-based meta-functions. Contrary to the design of templates, designers of generalized constant expressions are well-aware of its meta-programming capabilities.

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