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

Section: 24.4.4 [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 24.2.7 [associative.reqmts] p.8 and 24.2.8 [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.

24.2.7 [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.

24.2.7 [associative.reqmts] Associative containers Table 102

Add row:

24.2.8 [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.

24.2.8 [unord.req] Unordered associative containers Table 103

Add row:

24.4.4.1 [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);

24.4.5.1 [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);

24.4.6.1 [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);

24.4.7.1 [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);

24.5.4.1 [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);

24.5.5.1 [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);

24.5.6.1 [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);

24.5.7.1 [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.