The following user defined literal emits an error:
constexpr double operator "" _kg(double q)
{
return q*1000;
}
but if long is added the error will disappear and the code will work as follows:
constexpr double operator "" _kg(long double q)
{
return q*1000;
}
the error is:
‘constexpr double operator""_kg(double)’ has invalid argument list
The problem is only caused by the argument and the return type can be double without long.
Why is long needed?
C++11 draft n3290 has this to say about the parameters that user-defined literals can take (§13.5.8):
The declaration of a literal operator shall have a parameter-declaration-clause equivalent to one of the following:
const char* unsigned long long int long double char wchar_t char16_t char32_t const char*, std::size_t const wchar_t*, std::size_t const char16_t*, std::size_t const char32_t*, std::size_t
As you can see, double is not in that list, only long double is. So you have to use that for user-defined literals that expect a floating point number as an argument.