Replace char in string with some string inplace

Question

i want to replace a character in the string with a string. can i do it in-place? As the new string has length greater than original string.Question is that can i do with

Answer 1

Here is a code that minimises the number of assignments and allocations. It is based on the following answer to a similar question: https://stackoverflow.com/a/32322122/3903076

The cases where the replacement string has length 0 or 1 are handled separately. Else, the string has to grow.

If there is not enough capacity, then an external buffer will be necessary anyway, so we just do copy-replace and swap.

The interesting case is when the string already has enough capacity, so we can actually do a non-trivial in-place replacement. We do that with a reverse copy-replace, stopping when we do not need to replace anything else.

This can be seen in the last line of the function.

void replaceChar(std::string& input, const std::string& replacementString, char charToReplace)
{
  if (replacementString.empty()) {
    input.erase(std::remove(input.begin(), input.end(), charToReplace), input.end());
    return;
  }
  if (replacementString.size() == 1) {
    std::replace(input.begin(), input.end(), charToReplace, replacementString.front());
    return;
  }

  const auto first_instance = std::find(input.begin(), input.end(), charToReplace);
  auto count = std::count(first_instance, input.end(), charToReplace);
  const auto extra_size = count * (replacementString.size() - 1);
  const auto new_size = input.size() + extra_size;

  if (input.capacity() < new_size) {
    std::string aux;
    aux.reserve(new_size);
    replace_with_range_copy(input.cbegin(), input.cend(), std::back_inserter(aux), charToReplace, replacementString.cbegin(), replacementString.cend());
    input.swap(aux);
    return;
  }

  input.resize(new_size);

  const auto rlast = std::make_reverse_iterator(first_instance);
  const auto rfirst = input.rbegin();
  const auto old_rfirst = rfirst + extra_size;

  replace_with_range_copy(old_rfirst, rlast, rfirst, charToReplace, replacementString.crbegin(), replacementString.crend());
}

Here is an implementation of the replace_with_range_copy algorithm:

template 
OutputIt replace_with_range_copy(InputIt1 first, InputIt1 last, OutputIt d_first, const T& old_value, InputIt2 new_first, InputIt2 new_last)
{
  InputIt1 next;
  while (true) {
    if (first == last) return d_first;
    next = std::find(first, last, old_value);
    d_first = std::copy(first, next, d_first);
    if (next == last) return d_first;
    d_first = std::copy(new_first, new_last, d_first);
    first = std::next(next);
  }
}