DOCUMENT NUMBER? Adding string-slicing to C++

1. Table of Contents

• Changelog

  • R6

  • R5

  • R4

  • R3

  • R2

  • R1

• Motivation and Scope

• Impact on the Standard

• Technical Specifications

• Proposed Wording

  • Addition to string

  • std::basic_string::subview

  • Addition to string_view

  • std:basic_string_view::subview

• Acknowledgements

2. Changelog

2.1. R6

• Remove multi-indexed operator[] for conformity reasons with matrices and other data structures

• Adjust proposed wording

• Adjust Motivation and Scope

• Adjust Impact on the Standard

• Remove first(n) and last(n). These will be added in a different paper.

• Removed Design Decisions

• Added .subview() also to std::basic_string_view

• Adjust function name from slice() to subview()

2.2. R5

• Added multi-indexed operator[] for unsafe variant of std::string::slice(size_t start, size_t end)

• Adjust proposed wording

2.3. R4

• Removed std::basic_string::slice(size_t start, size_t end, size_t step) since with the addition of std::stride_view it will no longer be necessary.
  

• Added proposed wording

• Made functions constexpr, const and noexcept (where applicable)

2.4. R3

• Added unit tests to a GitHub project.
  

2.5. R2

• std::basic_string::slice() now throws std::out_of_range when start >= size().
  

2.6. R1

• Removed default parameter from std::basic_string::slice(size_t start, size_t end, size_t step).
  

3. Motivation and Scope

const std::string google{ "www.google.com" };

// Subview
std::cout << google.subview(google.find('.') + 1, google.find_last_of('.')) << "\n";

// Substr
size_t start = google.find('.') + 1;
std::cout << google.substr(start, google.find_last_of('.') - start) << "\n";

Both do functionally the same thing, but .substr() requires a calculation to get the length and a need to store start since we need it to calculate the length of the substring (although this is not technically necessary, not doing so leads to code duplication), while .subview() can simply pass the same start and just .find_last_of() instead of having to calculate a length.

At the end of the day, this is more about adding to the richness of the API and making the API nicer for some programmers than it is about replacing .substr() or creating entirely new functionality.

4. Impact on the Standard

Implementation will be left up to the vendor of course, but since these are trivial functions, we can provide a "template" implementation.

5. Technical Specifications

1. std::basic_string::subview() takes 2 parameters: size_t start and size_t end and returns a std::basic_string.

   • start is the starting index (inclusive) of where to start the slice.

     • std::out_of_range is thrown when start >= size().

   • end is the ending index (exclusive) of where to end the slice.

     • if end > size() then end will be set to size()

     • if end < start then end will be set to start

2. std::basic_string_view::subview() takes 2 parameters: size_t start and size_t end and returns a std::basic_string_view.

   • start is the starting index (inclusive) of where to start the slice.

     • std::out_of_range is thrown when start >= size().

   • end is the ending index (exclusive) of where to end the slice.

     • if end > size() then end will be set to size()

     • if end < start then end will be set to start

These are easily implemented functions and depend on specific vendor-implementation of std::basic_string and std::basic_string_view, but I have provided unit tests and sample implementations here.

6. Proposed Wording

6.1. Addition to <string>

// [...]
namespace std {
  // [...]
	
  // [string.ops], string operations
  // [...]
  constexpr bool contains(const charT* x) const;
	
  constexpr basic_string subview(size_t start, size_t end) const;
}

6.2. std::basic_string::subview

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

  • constexpr basic_string<charT, traits> subview(size_t start, size_t end) const;

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

    2. Returns: basic_string<charT, traits>(data() + start, end)

    3. Throws: out_of_range if start >= size()

6.3. Addition to <string_view>

namespace std {
  // [...]
	
  // [string.ops], string operations
  // [...]
  constexpr basic_string_view substr(size_type pos = 0,
                    size_type n = npos) const;       // freestanding-deleted  
									   
  constexpr basic_string_view subview(size_t start, size_t end) const;
}

6.4. std::basic_string_view::subview

• 23.4.3.?: basic_string_view::subview [string.view.subview]

  • constexpr basic_string_view<charT, traits> subview(size_t start, size_t end) const;

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

    2. Returns: basic_string_view<charT, traits>(data() + start, end)

    3. Throws: out_of_range if start >= size()

7. Acknowledgements