A Class for Status and Optional Value

ISO/IEC JTC1 SC22 WG21 N4233 - 2014-10-10

Lawrence Crowl, Lawrence@Crowl.org
Chris Mysen, mysen@google.com

Introduction
Discussion
Solution
Examples

Introduction

The current proposal for concurrent queues, N3533 C++ Concurrent Queues, has separate return value and output parameter for its pop operations.

Value queue::value_pop();
queue_op_status queue::try_pop(Value&);
queue_op_status queue::nonblocking_pop(Value&);

This design has two consequences.

A bad status for value_pop will result in an exception.
Use of the try_pop and nonblocking_pop operations will require an Value that is essentially default constructible.

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

something<queue_op_status, Value> queue::value_pop();
something<queue_op_status, Value> queue::try_pop();
something<queue_op_status, Value> queue::nonblocking_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).

However, the need is almost direclty 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 status's 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. At present, we see no need for copying, but we could be convinced otherwise.

status_value::status_value(status_value&& sv);

They may be queried for status. The design assumes that the status is cheap and copyable, which enables more efficient return-by-value.

Status status_value::status() const;

They may be queried for wither 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 the constructor, is thrown. This design enables moving out of the class.

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

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.try_pop(e);
if ( s == queue_op_status::success )
   do_something_with(e);
else
   do_something_else_with(s);