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### 2306. `match_results::reference` should be `value_type&`, not `const value_type&`

Section: 32.9 [re.results] Status: C++14 Submitter: Matt Austern Opened: 2013-09-25 Last modified: 2017-07-05

Priority: 4

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Discussion:

The match_results class synopsis has

>     typedef const value_type& const_reference;
>     typedef const_reference reference;

We're getting too enthusiastic about types here by insisting that `reference` is a const reference, even though `match_results` is a read-only container. In the container requirements table (Table 96, in section 24.2.2.1 [container.requirements.general] we say that `Container::reference` is "lvalue of `T`" and `Container::const_reference` is "const lvalue of `T`".

That phrasing in the container requirements table is admittedly a little fuzzy and ought to be clarified (as discussed in lwg issue 2182), but in context it's clear that `Container::reference` ought to be a `T&` even for constant containers. In the rest of Clause 23 we see that `Container::reference` is `T&`, not `const T&`, even for const-qualified containers and that it's `T&`, not `const T&`, even for containers like `set` and `unordered_set` that provide const iterators only.

The way we handle const containers is just that in the case of a const-qualified container (including `match_results`) there are no operations that return `Container::reference`. That's already the case, so this issue is complaining about an unused typedef.

[2013-10-17: Daniel comments]

The `std::initializer_list` synopsis, 17.11 [support.initlist] shows a similar problem:

>     template<class E> class initializer_list {
>     public:
>       typedef E value_type;
>       typedef const E& reference;
>       typedef const E& const_reference;
>       […]
>     }

Given the fact that `std::initializer_list` doesn't meet the container requirements anyway (and is such a core-language related type) I recommend to stick with the current state.

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change the class template `match_results` header synopsis, 32.9 [re.results] p4 as indicated:

   >     typedef const value_type& const_reference;
   >     typedef const_referencevalue_type& reference;