A proposal to add a generalized callable negator (Revision 4)

Introduction

This proposal is to add function template not_fn that will allow to create negator of any callable object.

Revision history

Changes since N4022:

• Changed the paragraph numbers to match newest working draft.
• Changed the requires clause to apply to constructed functor instead of function argument.
• Updated wording for argument_type, first_argument_type and second_argument_type to match resolution of LWG issue #2387.
• Changed definition of fn to forwarding call wrapper.
• Replaced decay<F>::type with decay_t<F>.
• Corrected spelling mistakes in Motivation and Scope section.
• Removed nested typedefs and added note, based on LWG review.

Changes since N3800:

• Updated proposed wording per LWG guidance.

Changes since N3699:

• Changed the requirements of not_fn to allow wrapping of move-only types.
• Added missing Throws clause.
• Included changes to 20.9 [function.objects]/2 section.
• Corrected link to "Generic (Polymorphic) Lambda Expressions" proposal.
• Changed formating of the document.

Motivation and Scope

The standard negators not1 and not2 accept only unary and binary functors that define argument_type or first_argument_type and second_argument_type respectively, which make them unusable with results of standard library functions such as bind and mem_fn. Furthermore, with relation to N3421, they cannot be used with new operator functor specializations.

This proposal addresses the problem by introducing a template function not_fn that returns complement of arbitrary predicate.

Comparison with lambda

While the polymorphic lambda expressions (N3649) may be used to negate arbitrary functor, the proposed function offers more compact syntax:

  std::partition(v.begin(), v.end(), [f](auto& p) { return !f(p); });
  std::partition(v.begin(), v.end(), std::not_fn(f));

Furthermore, the use of lambda expression requires separate treatment of member pointers:

  std::partition(v.begin(), v.end(), [pm](auto& p) { return !std::mem_fn(pm)(p); });
  std::partition(v.begin(), v.end(), std::not_fn(pm));

Deprecation

With the incorporation of proposed functionality the old standard negators should be deprecated.

Design Decisions

New specializations of unary_negate, binary_negate

Problem addressed by this paper may be solved by introducing perfect forwarding specializations of unary_negate and binary_negate for types that do not define argument_type and first_argument_type, second_argument_type nested types respectively, without breaking existing code. Although this solution does not address functions with arbitrary number of arguments and requires additional implementation burden.

Return type

The perfect forwarding of return type was chosen instead of hard-coded bool. Similar argumentation to the one provided in N3421 applies.

In Rapperswil decided not to define this type.

Argument typedefs

The argument_type, first_argument_type and second_argument_type nested types are required by implementation to be consistent with most of library functors, that define these types where they are well defined for target callable type. Furthermore it is increasing the amount of existing code that will remain valid after changing not1, not2 to not_fn.

In Rapperswil decided not to define these types.

Impact On The Standard

This proposal has no dependencies beyond a C++11 compiler and Standard Library implementation. (It depends on perfect forwarding, decltype and trailing return types.)

Nothing depends on this proposal.

Proposed wording

After the declaration of not2 in the section 20.9 [function.objects]/2 (Header <functional> synopsis), add:

  // 20.9.9, negators
  template <class F> unspecified not_fn(F&& f);

After paragraph 20.9.8 Negators [negators], insert a new paragraph. (Chapter [bind] (Function template bind) becomes 20.9.?)

20.9.9 Function template not_fn [not_fn]

  template <class F>
    unspecified not_fn(F&& f);

In the text that follows:

• FD is the type decay_t<F>,
• fd is an lvalue of type FD constructed from std::forward<F>(f),
• fn is a forwarding call wrapper created as a result of not_fn(f),

Requires:

is_constructible<FD, F>::value shall be true. fd shall be a callable object ([func.def] 20.9.1).

Returns:

A forwarding call wrapper fn such that the expression fn(a1, a2, ..., aN) is equivalent to !INVOKE(fd, a1, a2, ..., aN) ([func.require] 20.9.2).

Throws:

Nothing unless the construction of fd throws an exception.

Remarks:

The return type shall satisfy the requirements of MoveConstructible. If FD satisfies the requirements of CopyConstructible, then the return type shall satisfy the requirements of CopyConstructible. [ Note: This implies that FD is MoveConstructible. — end note ]

[ Note: Function template not_fn can usually provide a better solution than using the negators not1 and not2. — end note ]

Acknowledgements

Anna Salwa originally proposed not_fn in discussion group ISO C++ Standard - Future Proposals.

Mateusz Kwiatkowski, Jonathan Wakely and Daniel Krügler offered many useful suggestions and corrections to the proposal.

References

1. Stephan T. Lavavej, "Making Operator Functors greater<>" (N3421, http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3421.htm)
2. Faisal Vali, Herb Sutter, Dave Abrahams, "Generic (Polymorphic) Lambda Expressions (Revision 3) " (N3649, http://isocpp.org/files/papers/N3649.html)