How can I use a floating-point value as a non-type template parameter?

Question

Is there any reason for not being able to have double as a parameter type with templates?
For example:

template<int N>//-< Legal
struct
{
};  

template<double N>//-< Illegal
struct
{
};  

Is there any update on this in C++11?

Answer 1

This is to do with precision, floating point numbers cannot be precisely represented, and the likelyhood of you referring to the same type can depend on how the number is represented. Consider instead using an integer mantissa and exponent as the template parameters...

Answer 2

Given that floating point arithmetics only give fuzzy results (sqrt(2.0)*sqrt(2.0) might not be equal 2.0), how do you propose using double as template arguments could be useful? If you had a template

template<double D>  // not allowed in C++
class X {};

and specialize that

template<>
class X<2.0> {};

then

X<1.0+1.0> x;

might not refer to the specialization.

I'd say that limits its usefulness so severely that I'd call it crippled.