DOCUMENT NUMBER? Adding .first() and .last() to strings

1. Table of Contents

• Changelog

  • R3

  • R2

  • R1

  • R0

• Motivation And Scope

• Impact on the Standard

• Technical Specifications

• Proposed Wording

  • Addition to string

  • std::basic_string::first

  • std::basic_string::last

  • Addition to string_view

  • std::basic_string_view::first

  • std::basic_string_view::last

2. Changelog

2.1. R3

• Remove noexcept from std::basic_string::first() and std::basic_string::last()

• Remove template parameters from proposed wording

• Added a const & and && overload for std::basic_string::first() and std::basic_string::last()

• Adjust Abstract

2.2. R2

• Add similarities to std::span in Motivation and Scope

2.3. R1

• Add more to Motivation and Scope

• Add Technical Specifications

• Add Proposed Wording

2.4. R0

• Initial revision

3. Motivation and Scope

To facilitate these simple operations, I suggest adding std::string::first() and std::string::last() (and similar counterparts to std::string_view). Having these 2 utility functions brings a couple of benefits:

1. It is more obvious and clear what the programmer intent is, .first(N) conveys better meaning as opposed to .substr(0, N), same for .last(N) and .substr(size() - N, N)

2. Since there are less arguments, and the argument is only a count, there is less potential for programmer error

3. It is generally simpler and less typing.

There is the argument of the std::string API already being far too large, and this being only a minor addition. Although this is true, the std::string API has some weird missing features, and the richness of the API is important, currently, programmers are often forced to make small utility functions themselves to do trivial operations on a string that should be part of the STL.

There is also already existing similar functionality in std::span. std::span::first and std::span::last already exist and implement the feature proposed in this paper, even though there is also the std::span::subspan function, suggesting that std::span::first() and std::span::last() had enough interest to be added besides std::span::subspan(). I believe it should be the same treatment for std::string and std::string_view, which only have .substr().

4. Impact on the Standard

5. Technical Specifications

1. std::basic_string::first() takes 1 parameter: size_t count, returns a std::basic_string and has a noexcept guarantee.

   • count is the amount of characters to included, starting from the start of the original string.

     • Determines the effective length of count as std::min(count, size())

2. std::basic_string::last() takes 1 parameter: size_t count, returns a std::basic_string and has a noexcept guarantee.

   • count is the amount of characters to be included, starting from the end of the original string.

     • Determines the effective length of count as std::min(count, size())

3. std::basic_string_view::first() takes 1 parameter: size_t count, returns a std::basic_string_view and has a noexcept guarantee.

   • count is the amount of characters to included, starting from the start of the original string_view.

     • Determines the effective length of count as std::min(count, size())

4. std::basic_string_view::last() takes 1 parameter: size_t count, returns a std::basic_string_view and has a noexcept guarantee.

   • count is the amount of characters to be included, starting from the end of the original string_view.

     • Determines the effective length of count as std::min(count, size())

6. Proposed Wording

6.1. Addition to <string>

// [...]
namespace std {
  // [...]
	
  // [string.ops], string operations
  // [...]
	
  constexpr basic_string first(size_t count) const &;
  constexpr basic_string first(size_t count) &&;

  constexpr basic_string last(size_t count) const &;
  constexpr basic_string last(size_t count) &&;
}

6.2. std::basic_string::first

• 23.4.3.?: basic_string::first [string.first]

  • constexpr basic_string first(size_t count) const;

    1. Effects: Determines the effective length xlen of the string to be returned as std::min(count, size()).
       Returns the characters in the range [data(), data() + xlen);

    2. Returns: basic_string(data(), data() + xlen)

6.3. std::basic_string::last

• 23.4.3.?: basic_string::last [string.last]

  • constexpr basic_string last(size_t count) const;

    1. Effects: Determines the effective length xlen of the string to be returned as std::min(count, size()).
       Returns the characters in the range [data() + size() - xlen, xlen);

    2. Returns: basic_string(data() + size() - xlen, data() + size())

6.4. Addition to <string_view>

// [...]
namespace std {
  // [...]
	
  // [string.ops], string operations
  // [...]
	
  constexpr basic_string_view first(size_t count) const noexcept;
  constexpr basic_string_view last(size_t count) const noexcept;
}

6.5. std::basic_string_view::first

• 23.3.3.?: basic_string_view::first [string.first]

  • constexpr basic_string_view first(size_t count) const noexcept;

    1. Effects: Determines the effective length xlen of the string to be returned as std::min(count, size()).
       Returns the characters in the range [data(), data() + xlen);

    2. Returns: basic_string_view(data(), data() + xlen)

6.6. std::basic_string_view::last

• 23.3.3.?: basic_string_view::last [string.last]

  • constexpr basic_string_view last(size_t count) const noexcept;

    1. Effects: Determines the effective length xlen of the string to be returned as std::min(count, size()).
       Returns the characters in the range [data() + size() - xlen, xlen);

    2. Returns: basic_string_view(data() + size() - xlen, data() + size())