This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of Resolved status.

2757. std::string{}.insert(3, "ABCDE", 0, 1) is ambiguous

Section: 23.4.3.7.4 [string.insert] Status: Resolved Submitter: Marshall Clow Opened: 2016-07-30 Last modified: 2020-09-06

Priority: 1

View all other issues in [string.insert].

View all issues with Resolved status.

Discussion:

Before C++17, we had the following signature to std::basic_string:

basic_string&
  insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos);

Unlike most of the other member functions on std::basic_string, there were not corresponding versions that take a charT* or (charT *, size).

In p0254r2, we added:

basic_string&
  insert(size_type pos1, basic_string_view<charT, traits> sv, size_type pos2, size_type n = npos);

which made the code above ambiguous. There are two conversions from "const charT*", one to basic_string, and the other to basic_string_view, and they're both equally good (in the view of the compiler).

This ambiguity also occurs with the calls

assign(const basic_string& str,             size_type pos, size_type n = npos);
assign(basic_string_view<charT, traits> sv, size_type pos, size_type n = npos);

but I will file a separate issue (2758) for that.

A solution is to add even more overloads to insert, to make it match all the other member functions of basic_string, which come in fours (string, pointer, pointer + size, string_view).

[2016-08-03, Chicago, Robert Douglas provides wording]

Previous resolution [SUPERSEDED]:

This wording is relative to N4606.

1. In 23.4.3 [basic.string] modify the synopsis for basic_string as follows:

   namespace std {
     template<class charT, class traits = char_traits<charT>,
       class Allocator = allocator<charT>>
     class basic_string {
     public:
       […]
       template<class T>
       basic_string& insert(size_type pos1, basic_string_view<charT, traits>T sv,
                            size_type pos2, size_type n = npos);
       […]
     };
   }
   

2. In 23.4.3.7.4 [string.insert], modify basic_string_view overload as follows:

   template<class T>
   basic_string& insert(size_type pos1, basic_string_view<charT, traits>T sv,
                        size_type pos2, size_type n = npos);
   

   […]

   -?- Remarks: This function shall not participate in overload resolution unless is_same_v<T, basic_string_view<charT, traits>> is true.

[2016-08-04, Chicago, Robert Douglas comments]

For the sake of simplicity, the previous wording suggestion has been merged into the proposed wording of LWG 2758.

[08-2016, Chicago]

Fri PM: Move to Tentatively Ready (along with 2758).

[2016-09-09 Issues Resolution Telecon]

Since 2758 has been moved back to Open, move this one, too

[2016-10 Telecon]

Ville's wording for 2758 has been implemented in libstdc++ and libc++. Move 2758 to Tentatively Ready and this one to Tentatively Resolved

Proposed resolution:

This issue is resolved by the proposed wording for LWG 2758.