What kind of "Traits" are used/defined in the C++0x Standard

Question

A trait in C++ encapsulates a family of operations that allow an Algorithm or Data Structure to operator with that type with which it is instantiated. char_traits are an example for grouping string- and file-required functions.

But not all traits have "trait" in their name, right? numeric_limits comes to mind. Is this a "Trait", too? Even without the name "trait" in it?

So, are there other Templates that could/should be considered a "Trait"? Besides the examples I found:

• allocator_traits how to get memory
• pointer_traits how to access an object indirectly
• type_traits meta programming
• char_taits for sequence of symbols
• iterator_traits how to get forward, backward and to the element
• regex_traits for... regexes.

I guess, what I am asking, too, is there a pure definition for traits?

Some things I am especially unsure about are:

• numeric_limits mentioned above
• <chrono>s customization "traits", [20.11.4], i.e. duration_values
• what about Hashing? Can the functor hash<> be considered to be a trait?
• If thats the case, are not all requirements "traits", like "CopyAssignable", etc?
• And then, are the abandoned "Concepts" the ultimate "trait"-Definition?

Update: The question what exactly makes a trait a trait seems a bit controversy in the details. Maybe a another question could be answered: Is there a comprehensive list which of the trait-like classes are new to C++0x, and which ones have already been in C++03? Maybe someone knows of a link to somewhere?

Answer 1

Here is an attempted list of the traits divided by standard. I could quite easily be overlooking some.

new C++11 traits:

is_error_code_enum is_error_condition_enum pointer_traits allocator_traits Just about everything in <type_traits> treat_as_floating_point duration_values uses_allocator regex_traits 

C++98/03 traits:

numeric_limits char_traits iterator_traits 

Answer 2

• *numeric_limits* definitely represents a set of traits for the numeric types.
• all requirements like "CopyAssignable" etc. are indeed traits see this paper on traits

  For the others I cannot comment but when in doubt:

  Think of a trait as a small object whose main purpose is to carry information used by another object or algorithm to determine "policy" or "implementation details". - Bjarne Stroustrup

  Update: to just make my small contribution to the extensive list Howard provided:

• time-related traites
• regex traits

I was wrongly under the impression that the type traits and regex traits beeing part of the TR1 are technically not part of the new traits bunch in C++0x(even though the type traits have been greatly extended by the new upcoming standard). See Howard's comment and clarification about that.