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match_results
can't satisfy the requirements of a containerSection: 32.9 [re.results] Status: C++17 Submitter: S. B. Tam Opened: 2016-01-24 Last modified: 2017-07-30
Priority: 3
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Discussion:
N4567 32.9 [re.results] p2 mentions
The class template
match_results
shall satisfy the requirements of an allocator-aware container and of a sequence container, as specifed in 23.2.3, except that only operations defined for const-qualified sequence containers are supported.
However, this is impossible because match_results
has a operator==
whose semantics differs from the one
required in Table 95 — "Container requirements".
a == b
is an equivalence relation and means equal(a.begin(), a.end(), b.begin(), b.end())
.
But for match_results
, a == b
and equal(a.begin(), a.end(), b.begin(), b.end())
can give different
results. For example:
#include <iostream> #include <regex> #include <string> #include <algorithm> int main() { std::regex re("a*"); std::string target("baaab"); std::smatch a; std::regex_search(target, a, re); std::string target2("raaau"); std::smatch b; std::regex_search(target2, b, re); std::cout << std::boolalpha; std::cout << (a == b) << '\n'; // false std::cout << std::equal(a.begin(), a.end(), b.begin(), b.end()) << '\n'; // true }
[2016-02, Issues Telecon]
Marshall: The submitter is absolutely right, but the proposed resolution is insufficient. We should avoid "shall", for once.2016-05: Marshall cleans up the wording around the change
[2016-08 - Chicago]
Thurs AM: Moved to Tentatively Ready
Previous resolution [SUPERSEDED]:
This wording is relative to N4567.
Change 32.9 [re.results] p2 as indicated:
-2- The class template
match_results
shall satisfy the requirements of an allocator-aware container and of a sequence container, as specified in 23.2.3, except that only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.
Proposed resolution:
This wording is relative to N4567.
Change 32.9 [re.results] p2 as indicated:
-2- The class template
match_results
shall satisfysatisfies the requirements of an allocator-aware container and of a sequence container,as specified in(23.2.3), except that only operations defined for const-qualified sequence containers are supported and the semantics of comparison functions are different from those required for a container.
[Drafting note: (Post-Issaquah) Due to the outdated N4567 wording the project editor accepted the following merge suggestion into N4606 wording: — end drafting note]
This wording is relative to N4606.
Change 32.9 [re.results] p2 as indicated:
-2- The class template
match_results
satisfies the requirements of an allocator-aware container and of a sequence container,as specified in(24.2.2 [container.requirements.general]and, 24.2.4 [sequence.reqmts])respectively, except that only operations defined for const-qualified sequence containers are supported and the semantics of comparison functions are different from those required for a container.