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### 2771. Broken Effects of some `basic_string::compare` functions in terms of `basic_string_view`

Section: 21.3.3.8.4 [string.compare] Status: C++17 Submitter: Daniel Krügler Opened: 2016-09-05 Last modified: 2017-07-30

Priority: 1

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Discussion:

Some `basic_string::compare` functions are specified in terms of a non-existing `basic_string_view` constructor, namely 21.3.3.8.4 [string.compare] p3,

>     return basic_string_view<charT, traits>(this.data(), pos1, n1).compare(sv);

and 21.3.3.8.4 [string.compare] p4:

>     return basic_string_view<charT, traits>(this.data(), pos1, n1).compare(sv, pos2, n2);

because there doesn't exist a `basic_string_view` constructor with three arguments.

Albeit this can be easily fixed by a proper combination of the existing constructor

>     constexpr basic_string_view(const charT* str, size_type len);

with the additional member function

>     constexpr basic_string_view substr(size_type pos = 0, size_type n = npos) const;

it should be decided whether adding the seemingly natural constructor

>     constexpr basic_string_view(const charT* str, size_type pos, size_type n);

could simplify matters. A counter argument for this addition might be, that `basic_string` doesn't provide this constructor either.

Another problem is related to the specification of 21.3.3.8.4 [string.compare] p4, which attempts to call a non-existing `basic_string_view::compare` overload that would match the signature:

>     constexpr int compare(basic_string_view str, size_type pos1, size_type n1) const;

[2016-09-09 Issues Resolution Telecon]

Marshall to investigate using P/R vs. adding the missing constructor.

[2016-10 Issues Resolution Telecon]

Marshall reports that P/R is better. Status to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

> [Drafting note: The wording changes below are for the same part of the standard as 2758. However, they do not conflict. This one changes the "Effects" of the routine, while the other changes the definition. — end drafting note]

1. Change 21.3.3.8.4 [string.compare] as indicated:

   >     int compare(size_type pos1, size_type n1,
   >                 basic_string_view<charT, traits> sv) const;

   > > -3- Effects: Equivalent to:

   > > >     return basic_string_view<charT, traits>(this.data(), size()).substr(pos1, n1).compare(sv);

   >     int compare(size_type pos1, size_type n1,
   >                 basic_string_view<charT, traits> sv,
   >                 size_type pos2, size_type n2 = npos) const;

   > > -4- Effects: Equivalent to:

   > > >     return basic_string_view<charT, traits>(this.data(), size()).substr(pos1, n1).compare(sv,.substr(pos2, n2));