DOCUMENT NUMBER? Adding string-slicing to C++

1. Table of Contents

Changelog
Motivation and Scope
Impact on the Standard
Design Decisions
Technical Specifications
Acknowledgements

2. Changelog

R1: Removed default parameter from std::basic_string::slice(size_t start, size_t end, size_t step)

3. Motivation and Scope

Parsing and string manipulation in C++ used to be very cumbersome, with seemingly basic and trivial methods missing from std::basic_string. The introduction of C++20 and C++23 resolved some of these issues by adding the above listed utility functions. I believe we can make string manipulation in C++ even better by adding more of these utility functions to std::basic_string, and one option I always miss, that is present in other programming languages (such as Python), is string-slicing. Python’s string-slicing is very graceful and easy-to-use, but C++ does not support that syntax.
Instead, I propose to add several functions to std::basic_string to emulate string-slicing.
The functions I propose to add to std::basic_string are the following:

namespace std
{
  /* 1. */ basic_string_view  basic_string::slice(size_t start, size_t end);
  /* 2. */ basic_string       basic_string::slice(size_t start, size_t end, size_t step);
  /* 3. */ basic_string_view  basic_string::first(size_t count);
  /* 4. */ basic_string_view  basic_string::last(size_t count);
}

4. Impact on the Standard

Since these are only trivial functions requiring no major changes to the language or changes to existing API, the impact of this proposal on the standard is minimal.
These functions can already be implemented in the current version of C++23 without any extra changes.
Implementation will be left up to the vendor of course, but since these are trivial functions, we can provide a "template" implementation.

5. Design Decisions

There is a choice in whether a std::basic_string is returned, or a std::basic_string_view is returned by these new utility functions.
It is best for these functions to return std::basic_string_view (with the exception of 2 where it is required to return std::basic_string) since:

These functions will most often be used to find something in a string, often not requiring a new dynamic allocation to be made.
std::basic_string::contains(), std::basic_string::starts_with() and std::basic_string::ends_with() all take a std::basic_string_view as a parameter. Therefore, the return value of the proposed functions matching up with these is a benefit.
If the user wants a std::basic_string instead of a std::basic_string_view, they can always construct a std::basic_string.

6. Technical Specifications

std::basic_string::slice() 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.
- end is the ending index (exclusive) of where to end the slice. end is automatically capped t the size of the string.
std::basic_string::slice() takes 3 parameters: size_t start, size_t end and size_t step and returns a std::basic_string.
- start is the starting index (inclusive) of where to start the slice.
- end is the ending index (exclusive) of where to end the slice. end is automatically capped t the size of the string.
- step is the increment between each index in the string to be included in the slice.
std::basic_string::first() takes 1 parameter: size_t count and returns a std::basic_string_view.
- count is the amount of characters to be included (counting from index 0) in the slice.
- count is automatically capped at the size of the string and cannot throw an exception or crash.
std::basic_string::last() takes 1 parameter: size_t count and returns a std::basic_string_view.
- count is the amount of characters to be included (counting from the last index) in the slice.
- count is automatically capped at the size of the string and cannot throw an exception or crash.

These are easily implemented functions and depend on specific vendor-implementation of std::basic_string, but here are some sample implementations:

namespace std
{
  // functions should obviously be part of std::basic_string
  // these are now free functions to show off implementation
	
	
  // 1
  std::string_view slice(const std::string& source, size_t start, size_t end)
  {
    if (end >= source.size())
    {
      end = source.size();
    }
		
    return std::string_view(source.data() + start, end - start);
  }
	
  // 2
  std::string slice(const std::string& source, size_t start, size_t end, size_t step)
  {
    if (end >= source.size())
    {
      end = source.size();
    }
		
    if (step == 0)
    {
      step = 1;
    }
		
    std::string slicedString{};
		
    for (size_t i{ start }; i < end; i += step)
    {
      slicedString.push_back(source[i]);
    }
		
    return slicedString;
  }
	
  // 3
  std::string_view first(const std::string& source, size_t count)
  {
    if (count >= source.size())
    {
      count = source.size();
    }
		
    return std::string_view(source.data(), count);
  }
	
  // 4
  std::string_view last(const std::string& source, size_t count)
  {
    if (count >= source.size())
    {
      count = source.size();
    }
		
    return std::string_view(source.data() + (source.size() - count), count);
  }
}

7. Acknowledgements

Thanks to both Zhihao Yuan and Nathaniel Rupprecht for their suggestions to the API of these new functions.