main.d(61): Error: temp_[i_] isn't mutable

[Ellery Newcomer]

On 06/19/2010 12:30 PM, Ben Hanson wrote:
> Hi there,
>
> I'm converting the following C++ function to D:
>
>      void negate ()
>      {
>          const std::size_t max_chars_ = sizeof (CharT) == 1 ?
>              num_chars : num_wchar_ts;
>          CharT curr_char_ = sizeof (CharT) == 1 ? -128 : 0;
>          string temp_;
>          const CharT *curr_ = _charset.c_str ();
>          const CharT *chars_end_ = curr_ + _charset.size ();
>
>          _negated = !_negated;
>          temp_.resize (max_chars_ - _charset.size ());
>
>          CharT *ptr_ = const_cast<CharT *>  (temp_.c_str ());
>          std::size_t i_ = 0;
>
>          while (curr_<  chars_end_)
>          {
>              while (*curr_>  curr_char_)
>              {
>                  *ptr_ = curr_char_;
>                  ++ptr_;
>                  ++curr_char_;
>                  ++i_;
>              }
>
>              ++curr_char_;
>              ++curr_;
>              ++i_;
>          }
>
>          for (; i_<  max_chars_; ++i_)
>          {
>              *ptr_ = curr_char_;
>              ++ptr_;
>              ++curr_char_;
>          }
>
>          _charset = temp_;
>      }
>
> Here's the complete source:
>
> module main;
>
> import std.algorithm;
> import std.string;
>
> template regex(StringT)
> {
> struct basic_string_token
> {
>      bool _negated = false;
> 	StringT _charset;
> 	typedef typeof(StringT.init[0]) CharT;
> 	enum size_t MAX_CHARS = CharT.max + 1;
>
> 	this(const bool negated_, ref StringT charset_)
> 	{
> 		_negated = negated_;
> 		_charset = charset_;
> 	}
>
> 	void remove_duplicates()
> 	{
> 		_charset.sort;
> 		_charset = squeeze(_charset);
> 	}
>
> 	void normalise()
> 	{
> 		if (_charset.length == MAX_CHARS)
> 		{
>              _negated = !_negated;
> 			_charset.clear();
> 		}
> 		else if (_charset.length>  MAX_CHARS / 2)
> 		{
> 			negate();
> 		}
> 	}
>
> 	void negate()
> 	{
> 		CharT curr_char_ = MAX_CHARS == 256 ? 0x80 : 0;
> 		StringT temp_;
> 		size_t curr_ = 0;
> 		size_t end_ = _charset.length;
> 		size_t i_ = 0;
>
>          _negated = !_negated;
> 		temp_.length = MAX_CHARS - end_;
>
> 		while (curr_<  end_)
> 		{
> 			while (_charset[curr_]>  curr_char_)
> 			{
> 				temp_[i_] = curr_char_;
> 				++curr_char_;
> 				++i_;
> 			}
>
>              ++curr_char_;
>              ++curr_;
>              ++i_;
> 		}
>
>          for (; i_<  MAX_CHARS; ++i_)
>          {
>              temp_ ~= curr_char_;
>              ++curr_char_;
>          }
>
> 		_charset = temp_;
> 	}
> };
> }
>
> int main(char[][]argv)
> {
> 	regex!(string).basic_string_token token_;
>
> 	token_._charset = "cccbba";
> 	token_.remove_duplicates();
> 	token_.negate();
> 	return 0;
> }
>
> Can anyone explain the error 'main.d(61): Error: temp_[i_] isn't
> mutable'? Can I use pointers instead like the C++ code? What's the
> best approach for maximum efficiency in D (pointers would make the
> conversion easier to, I guess).
>
> Thanks,
>
> Ben

because strings aren't mutable. char[]'s on the other hand are, so you 
could just change temp_'s type to

import std.traits;

Unqual!(typeof(StringT.init[0]))[] temp_;

or something like that.