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initializer_list
overloads of min
,
max
, and minmax
Section: 27.8.9 [alg.min.max] Status: NAD Submitter: Niels Dekker Opened: 2010-02-02 Last modified: 2017-09-07
Priority: Not Prioritized
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Discussion:
In San Francisco, June 2008,
N2722
was adopted, replacing the variadic templates min
, max
, and
minmax
by overloads that have an initializer_list<T>
parameter. The paper showed benchmark results wherein initializer_list
versions of min
appeared to outperform the corresponding variadic
template. Unfortunately, in October 2009 a very serious error was detected in
the benchmark. (c++std-lib-25210).
In fact, an initializer_list<T>
version of min
often
appears to perform worse than the corresponding variadic template,
especially when T
has an expensive copy constructor
(c++std-lib-25253,
http://www.xs4all.nl/~nd/dekkerware/issues/n2772_fix).
IMO, the biggest problem of the initializer_list
overloads is that they
pass and return T
objects by value. Which has the following
consequences:
T
is CopyConstructible
. IMO that is too much of a
constraint for a generic, general purpose function like
std::min<T>
.
T
's less-than-operator
throws nothing. (And of course, less-than typically throws nothing.)
std::min
and std::max
.
Consider the subtle difference between const T& c1 = min(a,b);
and
const T& c2 = min({a,b});
(c++std-lib-25265)
In the future, this problem might be solvable by using an
initializer_list
of const references, instead:
const T& min(initializer_list<const T&>); const T& max(initializer_list<const T&>); pair<const T&, const T&> minmax(initializer_list<const T&>);
It is unlikely that C++0x will support initializer_list<const T&>
,
but technically it seems possible to add such a language
feature after C++0x
(c++std-core-15428).
Variadic templates of min
, max
, and minmax
, as
proposed by
N2551
(Sylvain Pion), do have some other advantages over initializer_list
overloads:
constexpr
,
now that
CWG issue #991 is in drafting status.
Unfortunately, the variadic templates of min
, max
, and
minmax
may still need further improvement, before having them in the
Standard Library. Especially the optional Compare
parameter appears to
be a concern. So for this moment I recommend to keep both versions out of C++0x,
and postpone further discussion until after C++0x.
[ 2010 Pittsburgh: Discussed and the LWG still prefers the initializer list solution of N2772. ]
Rationale:
We prefer the solution of N2772 which will be reapplied.
Proposed resolution:
Remove both variadic templates and initializer_list
overloads of
min
, max
, and minmax
from the synopsis in
27.1 [algorithms.general] and from 27.8.9 [alg.min.max].
[ Note: This proposed resolution will resolve LWG 915(i) as NAD. ]