Which user-defined-literals are predefined by the standard?

Question

My question sounds like a contradiction, but I don't know how else to refer to the new literal syntax other than user-defined-literal.

std::string operator "" s ( const char* str, size_t len )
{
   return std::string( str, len );
}

assert( "foo"s == "bar"s );

I remember hearing that user defined literals should start with an _ prefix. That would imply that the library defines some non-prefixed literals for us.

Does the standard provide some UDLs in the the standard library?
If yes, what are they?

Answer 1

The standard library actually defines no user defined literals. We would perhaps have expected complex numbers, but no.

On the other hand, there is also a proposal to remove them again

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3250.html

so we don't yet know what happens.

Answer 2

The language already use regular literals suffixes, for example 1U.

It would become ambiguous if you were to use U as a user-defined-literal, thus the recommendation.

integer-suffix: u, U, l, L, ll, LL

floating-suffix: f, F, l, L