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### 2059. C++0x ambiguity problem with `map::erase`

Section: 22.4.4 [map] Status: C++17 Submitter: Christopher Jefferson Opened: 2011-05-18 Last modified: 2017-07-30

Priority: 3

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Discussion:

`map::erase` (and several related methods) took an iterator in C++03, but take a `const_iterator` in C++0x. This breaks code where the map's `key_type` has a constructor which accepts an iterator (for example a template constructor), as the compiler cannot choose between `erase(const key_type&)` and `erase(const_iterator)`.

>     #include <map>

>     struct X
>     {
>       template<typename T>
>       X(T&) {}
>     };

>     bool operator<(const X&, const X&) { return false; }

>     void erasor(std::map<X,int>& s, X x)
>     {
>       std::map<X,int>::iterator it = s.find(x);
>       if (it != s.end())
>         s.erase(it);
>     }

[ 2011 Bloomington ]

This issue affects only associative container `erase` calls, and is not more general, as these are the only functions that are also overloaded on another single arguement that might cause confusion - the `erase` by key method. The complete resolution should simply restore the `iterator` overload in addition to the `const_iterator` overload for all eight associative containers.

Proposed wording supplied by Alan Talbot, and moved to Review.

[2012, Kona]

Moved back to Open by post-meeting issues processing group.

Pablo very unhappy about case of breaking code with ambiguous conversion between both iterator types.

Alisdair strongly in favor of proposed resolution, this change from C++11 bit Chris in real code, and it took a while to track down the cause.

Move to open, bring in front of a larger group

Proposed wording from Jeremiah: `erase(key)` shall not participate in overload resolution if `iterator` is convertible to `key`. Note that this means making `erase(key)` a template-method

Poll Chris to find out if he already fixed his code, or fixed his library

Jeremiah - allow both overloads, but `enable_if` the `const_iterator` form as a template, requiring `is_same` to match only `const_iterator`.

Poll PJ to see if he has already applied this fix?

[2015-02 Cologne]

AM: To summarize, we changed a signature and code broke. At what point do we stop and accept breakage in increasingly obscure code? VV: libc++ is still broken, but libstdc++ works, so they've fixed this — perhaps using this PR? [Checks] Yes, libstdc++ uses this solution, and has a comment pointing to LWG 2059. AM: This issue hasn't been looked at since Kona. In any case, we already have implementation experience now.

AM: I'd say let's ship it. We already have implementation experience (libstdc++ and MSVS). MC: And "tentatively ready" lets me try to implement this and see how it works.

Proposed resolution:

Editorial note: The following things are different between 22.2.6 [associative.reqmts] p.8 and 22.2.7 [unord.req] p.10. These should probably be reconciled.

> 1. First uses the convention "denotes"; second uses the convention "is".
> 2. First redundantly says: "If no such element exists, returns a.end()." in erase table entry, second does not.

22.2.6 [associative.reqmts] Associative containers

8 In Table 102, `X` denotes an associative container class, `a` denotes a value of `X`, `a_uniq` denotes a value of `X` when `X` supports unique keys, `a_eq` denotes a value of `X` when `X` supports multiple keys, `u` denotes an identifier, `i` and `j` satisfy input iterator requirements and refer to elements implicitly convertible to `value_type`, `[i,j)` denotes a valid range, `p` denotes a valid const iterator to `a`, `q` denotes a valid dereferenceable const iterator to `a`, `r` denotes a valid dereferenceable iterator to a, `[q1, q2)` denotes a valid range of const iterators in `a`, `il` designates an object of type `initializer_list<value_type>`, `t` denotes a value of `X::value_type`, `k` denotes a value of `X::key_type` and `c` denotes a value of type `X::key_compare`. `A` denotes the storage allocator used by `X`, if any, or `std::allocator<X::value_type>` otherwise, and `m` denotes an allocator of a type convertible to `A`.

22.2.6 [associative.reqmts] Associative containers Table 102

Add row:

`a.erase(r)` | `iterator` | erases the element pointed to by `r`. Returns an iterator pointing to the element immediately following `r` prior to the element being erased. If no such element exists, returns `a.end()`. | amortized constant

22.2.7 [unord.req] Unordered associative containers

10 In table 103: `X` is an unordered associative container class, `a` is an object of type `X`, `b` is a possibly const object of type `X`, `a_uniq` is an object of type `X` when `X` supports unique keys, `a_eq` is an object of type `X` when `X` supports equivalent keys, `i` and `j` are input iterators that refer to `value_type`, `[i, j)` is a valid range, `p` and `q2` are valid const iterators to `a`, `q` and `q1` are valid dereferenceable const iterators to `a`, `r` is a valid dereferenceable iterator to a, `[q1,q2)` is a valid range in `a`, `il` designates an object of type `initializer_list<value_type>`, `t` is a value of type `X::value_type`, `k` is a value of type `key_type`, `hf` is a possibly const value of type `hasher`, `eq` is a possibly const value of type `key_equal`, `n` is a value of type `size_type`, and `z` is a value of type `float`.

22.2.7 [unord.req] Unordered associative containers Table 103

Add row:

`a.erase(r)` | `iterator` | Erases the element pointed to by `r`. Returns the iterator immediately following `r` prior to the erasure. | Average case O(1), worst case O(`a.size()`).

22.4.4.1 [map.overview] Class template map overview p. 2

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

22.4.5.1 [multimap.overview] Class template multimap overview p. 2

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

22.4.6.1 [set.overview] Class template set overview p. 2

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

22.4.7.1 [multiset.overview] Class template multiset overview

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

22.5.4.1 [unord.map.overview] Class template unordered_map overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

22.5.5.1 [unord.multimap.overview] Class template unordered_multimap overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

22.5.6.1 [unord.set.overview] Class template unordered_set overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

22.5.7.1 [unord.multiset.overview] Class template unordered_multiset overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

C.5.12 [diff.cpp03.containers] C.2.12 Clause 23: containers library

23.2.3, 23.2.4

Change: Signature changes: from iterator to const_iterator parameters

Rationale: Overspecification. Effects: The signatures of the following member functions changed from taking an iterator to taking a const_iterator:

- insert(iter, val) for vector, deque, list, set, multiset, map, multimap
- insert(pos, beg, end) for vector, deque, list, forward_list
- erase(iter) for set, multiset, map, multimap
- erase(begin, end) for set, multiset, map, multimap
- all forms of list::splice
- all forms of list::merge

Valid C++ 2003 code that uses these functions may fail to compile with this International Standard.