When should literal classes be used in C++?

Question

Can someone tell me when are literal classes needed in C++?
I am getting a little confused from constexpr constructors, constexpr members, and I can't see what the point is. I'd like to see some practical use of it.

Also I'd want to know if a set member function needs to be constexpr, i.e.:

constexpr void set_num(int a) { num = a; }

Answer 1

constexpr fixes a problem in C++98 when using numeric limits. Before C++11 an expression such as

std::numeric_limits<short>::max()

can not be used as integral constant, although it is almost equal to macro INT_MAX. with C++11, such an expression is declared as constexpr so that, for example, you can use it to declare arrays or in compile-time computations (metaprogramming):

std::array<float,std::numeric_limits<short>::max()> a;