encoded-strings-proposal

C++ Proposal for encoded strings

• Document number: PxxxxRy
• Maks Mazurov <[email protected]>
• Target audience: LEWG, LWG

Table of contents

• Introduction
• Motivation
• Proposal
  • Standard Library Encodings
  • Encoding Tag

Introduction

The goal is to add compile-time information to string classes about used encoding along with conversion functions.

Motivation

• Provide more flexible replacement for deprecated std::wstring_convert.
• You already have std::string and/or std::string_view all around your code. Why copy to 3rd-party's encoding-aware string, when you can operate on standard string?
• ...

Proposal

Note: Declarations written as if they are inside std namespace (std:: is omitted). ... is all other template arguments.

• Add Encoding template argument to std::basic_string and std::basic_string_view.

template<
  class CharT,
  class Traits = char_traits<CharT>,
  class Allocator = allocator<CharT>,
  class Encoding = string::default_encoding
> class basic_string;

template<
  class CharT,
  class Traits = char_traits<CharT>,
  class Encoding = string_view::default_encoding
> class basic_string_view;

• Add template member function to_encoding to std::basic_string and std::basic_string_view.

template<class TargetEncoding>
basic_string<..., TargetEncoding> to_encoding() const;

Note: to_encoding may return std::basic_string with different CharT. This is required to support UTF-16 and similar encodings.

Standard Library Encodings

• std::ascii 7-bit ASCII encoding.

• std::native System native encoding.

• std::wide System native encoding for wide characters.

• std::utf8 UTF-8 (RFC 3629) encoding tag.

• std::utf16 UTF-16 (RFC 2781) encoding tag.

• std::utf32 UTF-32 encoding tag.

• std::string::default_encoding Implementation-defined encoding, can be any of specified above or another unrelated encoding. Not required to be same between program runs.

• std::string_view::default_encoding Must be same as std::string::default_encoding.

Implementation can provide additional encodings.

Encoding type

The implementation is allowed to store some information about encoding in static fields.

Example: We have encoding conversion library that identifies various encoding using string names. It is allowed to use encoding tag static field (preferably constexpr) named encoding_name.

struct win1251 {
  static constexpr const char* library_encoding_name = "cp1251";
  
  template<class TargetEncoding>
  static basic_string<..., TargetEncoding> to_encoding(const char* sptr, size_t length) {
    library_convert(TargetEncoding::library_encoding_name, win1251::library_encoding_name);
  }
};