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### 1308. Concerns about `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:

1. They require that `T` is `CopyConstructible`. IMO that is too much of a constraint for a generic, general purpose function like `std::min<T>`.
2. They potentially throw an exception, even if `T`'s less-than-operator throws nothing. (And of course, less-than typically throws nothing.)
3. They are inconsistent with C++03 `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)
4. They do not conveniently support use cases that need to have a reference to the minimum or maximum object itself, rather than just a copy.
5. They potentially perform badly: possibly O(n), when the arguments themselves have a size of n.

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:

1. It is likely that those variadic templates can be declared `constexpr`, now that CWG issue #991 is in drafting status.
2. They provide complete compile-time protection against accidentally passing zero arguments.

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 as NAD. ]