Danny Kalev wrote this post about constexpr:
constexpr is a new C++11 keyword that rids you of the need to create macros and hardcoded literals. It also guarantees, under certain conditions, that objects undergo static initialization. Danny Kalev shows how to embed constexpr in C++ applications to define constant expressions that might not be so constant otherwise.
The new C++11 keyword constexpr controls the evaluation time of an expression. By enforcing compile-time evaluation of its expression, constexpr lets you define true constant expressions that are crucial for time-critical applications, system programming, templates, and generally speaking, in any code that relies on compile-time constants.
constexpr is not available in Visual Studio 2010 or Visual Studio 2012 (not even with the November CTP).