Issue 2989: path's stream insertion operator lets you insert everything under the sun (original) (raw)

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2989. path's stream insertion operator lets you insert everything under the sun

Section: 31.12.6.7 [fs.path.io] Status: C++20 Submitter: Billy O'Neal III Opened: 2017-06-27 Last modified: 2021-02-25

Priority: 2

View all issues with C++20 status.

Discussion:

The rules for converting a path to a narrow character sequence aren't necessarily the same as that iostreams should use. Note that this program creates a temporary path and stream inserts that, which likely destroys information.

#include #include #include

void foo() { using namespace std; using namespace std::experimental::filesystem::v1; wstring val(L"abc"); std::cout << val; }

Using the godbolt online compiler we get:

foo PROC sub rsp, 104 ; 00000068H lea rdx, OFFSET FLAT:$SG44895 lea rcx, QWORD PTR val$[rsp] call std::basic_string<wchar_t,std::char_traits,std::allocator >::basic_string<wchar_t,std::char_traits,std::allocator > lea rdx, QWORD PTR val$[rsp] lea rcx, QWORD PTR $T1[rsp] call ??$?0_WU?$char_traits@_W@std@@V?$allocator@_W@1@@path@v1@filesystem@experimental@std@@QEAA@AEBV?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@4@@Z lea rdx, QWORD PTR $T1[rsp] lea rcx, OFFSET FLAT:std::cout call std::experimental::filesystem::v1::operator<<<char,std::char_traits > lea rcx, QWORD PTR $T1[rsp] call std::experimental::filesystem::v1::path::path lea rcx, QWORD PTR val$[rsp] call std::basic_string<wchar_t,std::char_traits,std::allocator >::basic_string<wchar_t,std::char_traits,std::allocator > add rsp, 104 ; 00000068H ret 0 foo ENDP

This should either be disabled with a SFINAE constraint, use the auto_ptr user-defined conversion trick, or the stream insertion operators should be made "hidden friends" to prevent conversions to pathfrom being considered here.

[2017-07 Toronto Monday issue prioritization]

Priority 2

[2017-07 Toronto Saturday afternoon]

LWG confirmed they want the hidden friend solution, Billy O'Neal to provide wording.

[2018-1-26 issues processing telecon]

Status to 'Tentatively Ready'

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This resolution is relative to N4659.

1. Edit 31.12.4 [fs.filesystem.syn], header <filesystem> synopsis, as indicated:

   // 31.12.6.7 [fs.path.io], path inserter and extractor
   template <class charT, class traits>
   basic_ostream<charT, traits>&
   operator<<(basic_ostream<charT, traits>& os, const path& p);
   template <class charT, class traits>
   basic_istream<charT, traits>&
   operator>>(basic_istream<charT, traits>& is, path& p);

2. Edit 31.12.6.7 [fs.path.io] as indicated:

   [Drafting note: The project editor is kindly asked to consider to move sub-clause 31.12.6.7 [fs.path.io] before sub-clause 31.12.6.8 [fs.path.nonmember] (as a peer of it) — _end drafting note_]
   template <class charT, class traits>
   friend basic_ostream<charT, traits>&
   operator<<(basic_ostream<charT, traits>& os, const path& p);

   […]

   template <class charT, class traits>
   friend basic_istream<charT, traits>&
   operator>>(basic_istream<charT, traits>& is, path& p);

3. Edit 31.12.6 [fs.class.path] p2, class path synopsis, as indicated:

   namespace std::filesystem {
   class path {
   public:
   […]
   iterator begin() const;
   iterator end() const;

   // 31.12.6.7 [fs.path.io] path inserter and extractor
   template <class charT, class traits>
   friend basic_ostream<charT, traits>&

   operator<<(basic_ostream<charT, traits>& os, const path& p);  

template <class charT, class traits>
friend basic_istream<charT, traits>&
operator>>(basic_istream<charT, traits>& is, path& p);
};
}