Compile-time constant id

Question

Given the following:

template
class A
{
public:
    static const unsigned int ID = ?;
};

I want ID to generate a unique c

Answer 1

Another alternative is to consider the following class Data with the unique, static member field type:

template 
class Data
{
public:
    static const std::type_index type;
};
// do [static data member initialization](http://stackoverflow.com/q/11300652/3041008)
// by [generating unique type id](http://stackoverflow.com/q/26794944/3041008)
template 
std::type_index const Data::type = std::type_index(typeid(T));

produces the output (MinGWx64-gcc4.8.4 -std=c++11 -O2)

printf("%s %s\n", Data::type.name(), Data::type.name())
//prints "i f"

It's not exactly an integer id or pretty-printable string, nor a constexpr, but can be used as an index in (un)ordered associative containers.
It also appears to work if the Data.h header is included in multiple files (same hashCode() values).