A Class for Status and Optional Value

Committee: ISO/IEC JTC1 SC22 WG21 EWG Evolution
Document Number: P0262r0
Date: 2016-02-14
Authors: Lawrence Crowl, Chris Mysen
Reply To: Lawrence Crowl, Lawrence@Crowl.org

Introduction
Discussion
Solution
Examples
Revision History

Introduction

The current proposal for concurrent queues, P0260r0 C++ Concurrent Queues, has two separate waiting pop operations, one with a return value one with an output reference parameter. These functions provide essentially the same semantics, but differ how they signal disappointment. See P0157r0 Handling Disappointment in C++. 

Value queue::value_pop();
queue_op_status queue::wait_pop(Value&);

This design has two consequences.

Chandler Carruth suggested having a return value that comprised both status and value. 

something<queue_op_status, Value> queue::pop();

The essential point of the design is that something may or may not contain a value, and that accessing a non-existent value results in an exception.

This design would normalize the form of the functions, move exception generation to accessing a non-existent value, and enable use of types with no default constructor as elements.

This paper proposes a mechanism to provide this combined status and value. While such a proposal could be folded into the queue proposal alone, the true value of such a mechanism would be its widespread use throughout the library, particularly in concurrent data structures where it is not technically possible to provide separate access functions for status and value. Therefore, we should either commit to or abandon such a mechanism.

Discussion

The value is clearly related to the class template optional, as defined by N3793 A proposal to add a utility class to represent optional objects (Revision 5). The problem could be solved with a tuple of the status and an optional. That approach would require more verbose code at each use, which we prefer to avoid.

However, the need is almost directly met by the class template expected, as defined by N4109 A proposal to add a utility class to represent expected monad - Revision 1.

In our view, the weakness in the expected proposal is that you get either a value or an error, but not both. In the queue proposal, operations return a status, not an error. The distinction is that a queue's inability to deliver a value is often not an error, but simply a normal part of interacting with concurrent objects. For example, try_pop can return queue_op_status::empty, which clearly does not indicate an error.

The distinction has a design effect when more than one status may be associated with a value. For example, a concurrent queue could provide both a status indicating success with low contention and a status indicating success with high contention. In both cases, the value alone provides less information.

So, we prefer a design in which a status is always provided, but the value is optional. Users of the design will need to define which statuses have values.

Solution

The design is a simpler version of that which appears in N4109 A proposal to add a utility class to represent expected monad - Revision 1.

The working name for the proposed class is status_value. 

template<typename Status, typename Value> class status_value;

Construction of a status_value can be done with or without a value. 

status_value::status_value(Status s);
status_value::status_value(Status s, Value&& v);
status_value::status_value(Status s, const Value& v);

Construction of status_value must include a status. 

status_value::status_value() = delete;

A status_value may be moved. A copy operation would make the type unusable for non-copyable contained objects, so we do not provide a copy operation. 

status_value::status_value(status_value&& sv);

They may be queried for status. The design assumes that inlining will remove the cost of returning a reference for cheap copyable types. 

const Status& status_value::status() const;

They may be queried for whether or not they have a value. 

bool status_value::has_value() const;
status_value::operator bool() const;

They may provide access to their value. If they have no value, an exception of type Status, with the status value passed to the constructor, is thrown. 

const Value& status_value::value() const;
Value& status_value::value();
const Value& status_value::operator *() const;
Value& status_value::operator *();

This design enables moving out of the class by calling std::move on the result of the non-const functions.

Examples

The outlined solution changes typical code for the proposed concurrent queue from 

Value e = q.value_pop();

into 

Value e = q.value_pop().value();

and from 

Value e;
queue_op_status s = q.wait_pop(e);
if ( s == queue_op_status::success )
   do_something_with(e);
else
   do_something_else_with(s);

into 

auto sv = q.wait_pop();
if ( sv.status() == queue_op_status::success )
  do_something_with(sv.value());
else
  do_something_else_with(sv.status());

or for handling all statuses that have values 

if ( auto sv = q.wait_pop() )
  // Via the implicit conversion to bool, a value is known to be present.
  do_something_with(*sv);
else
  // Via the implicit conversion to bool, a value is known to be absent.
  do_something_else_with(sv.status());

With both sets of changes, the handling of disappointment is decided by the caller, not by the set of operations provided by the queue.

Revision History

This paper revises N4233 - 2014-10-10 as follows.