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

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.开发者_如何学运维 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?

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.

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

In contrast to @Matthieu's answer, the FIDS says the following under 2.14.8 [lex.ext] p1:

If a token matches both user-defined-literal and another literal kind, it is treated as the latter.
[ Example: 123_km is a user-defined-literal, but 12LL is an integer-literal. — end example ]

So even if you define those literals, the predefined ones are taken and there is no ambiguity.

No standard library components have staked a claim on a user-defined literal. @Bo mentioned complex as a possibility.

Another is bitset:

template<char... Bits>
  inline constexpr std::bitset<sizeof...(Bits)>
  operator"" bits()
  {
    return std::bitset<sizeof...(Bits)>((char []){Bits..., '\0'});
  }

I predict there will be proposals for literal operators for various library components in the upcoming library TR2 enhancements.

I anticipate some collisions over the suffix namespace. You can declare literal operators in a namespace and prevent multiple ambiguous definitions but you can't distinguish the actual suffixes by namespace in an actual literal. We'll see.