constexpr

[PROGR4MM3R]

I've been learning about constants and I can't really get the key difference between const and constexpr. It said something about "evaluated at compile time" but that makes no sense to me.

[JLBorges]

const's primary function is to express the idea that an object is not modified through an interface (even though the object may very well be modified through other interfaces) - http://www.stroustrup.com/C++11FAQ.html#constexpr

struct A { /* ..... */ } ;

void foo( const A& c ) ; // foo can't modify the object through c

void bar( A& a ) // bar can modify the object through a
{
      // modify a
      foo(a) ; // foo can't modify the object 
}

In addition to be able to evaluate expressions at compile time, we want to be able to require expressions to be evaluated at compile time; constexpr in front of a variable definition does that (and implies const):
...
constexpr's primary function is to extend the range of what can be computed at compile time, making such computation type safe. - Stroustrup

Also see: http://en.wikipedia.org/wiki/C%2B%2B11#constexpr_.E2.80.93_Generalized_constant_expressions