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2051. Explicit tuple constructors for more than one parameter

Section: 19.5.3 [tuple.tuple], 19.5.3.1 [tuple.cnstr] Status: Resolved Submitter: Ville Voutilainen Opened: 2011-05-01 Last modified: 2016-02-10

Priority: 2

View all other issues in [tuple.tuple].

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Discussion:

One of my constituents wrote the following:

-------snip------------

So far the only use I've found for std::tuple is as an ad-hoc type to emulate multiple return values. If the tuple ctor was made non-explicit one could almost think C++ supported multiple return values especially when combined with std::tie().

// assume types line_segment and point
// assume function double distance(point const&, point const&)

std::tuple<point, point>
closest_points(line_segment const& a, line_segment const& b) {
 point ax;
 point bx;
 /* some math */

 return {ax, bx};
}


double
distance(line_segment const& a, line_segment const& b) {
 point ax;
 point bx;
 std::tie(ax, bx) = closest_points(a, b);

 return distance(ax, bx);
}

-------snap----------

See also the messages starting from lib-29330.

Some notes:

  1. pair allows such a return
  2. a lambda with a deduced return type doesn't allow it for any type
  3. decltype refuses {1, 2}

I would recommend making non-unary tuple constructors non-explicit.

[Bloomington, 2011]

Move to NAD Future, this would be an extension to existing functionality.

[Portland, 2012]

Move to Open at the request of the Evolution Working Group.

[Lenexa 2015-05-05]

VV: While in the area of tuples, LWG 2051 should have status of WP, it is resolved by Daniel's "improving pair and tuple" paper.

MC: status Resolved, by N4387

Proposed resolution:

Resolved by the adoption of N4387.