std::string::erase

darkness@linux:~$ cat eval.cpp
#include <cstdlib>
#include <iostream>
#include <string>


int main()
{
    const std::string str = "    foo   ";

    size_t spaces;
    for(spaces = 0; str[spaces] == ' '; ++spaces);

    str.erase(0, spaces - 1);

    std::cout << "Number of leading spaces: " << spaces << std::endl;
    std::cout << "New string: \"" << str << "\"" << std::endl;

    return EXIT_SUCCESS;
}

darkness@linux:~$ g++ -o eval eval.cpp
eval.cpp: In function âint main()â:
eval.cpp:13: error: passing âconst std::stringâ as âthisâ argument of âstd::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::erase(typename _Alloc::rebind<_CharT>::other::size_type, typename _Alloc::rebind<_CharT>::other::size_type) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]â discards qualifiers

Number of leading spaces: 4
New string: "foo   "

    const std::string str = "    foo   ";

    str.erase(0, spaces - 1);

error: passing const XXX as argument to YYY discards qualifies

void str_ltrim (string& str)
{
	string mstr;
	int i=0, j, len;

	if (str.empty()) return;

	mstr = str;

	while (mstr[i] == ' ') {
		j = i;
		i++;
	};

	if (j == 0) return;

	len = mstr.length() - j;
	str = mstr.substr(j+1, len);
}

void str_rtrim (string& str)
{
	string mstr;
	int i=0, j, len;

	if (str.empty()) return;

	mstr = str;
	len = mstr.length();
	i = len-1;
	
	while (mstr[i] == ' ') {
		j = i;
		i--;
	};

	if (j == len) return;

	str = mstr.substr(0, j);
}

void strip_lspace( string & line )
{
   string::size_type space;

   space = line.find_first_not_of( ' ' );
   if( space == string::npos )
      space = 0;
   line = line.substr( space, line.length(  ) );

   space = line.find_first_not_of( '\t' );
   if( space == string::npos )
      space = 0;
   line = line.substr( space, line.length(  ) );
}

void strip_tspace( string & line )
{
   string::size_type space;

   space = line.find_last_not_of( ' ' );
   if( space != string::npos )
      line = line.substr( 0, space + 1 );
}

void strip_spaces( string & line )
{
   strip_lspace( line );
   strip_tspace( line );
}

#define SPACES " \t\r\n"


inline string trim_right (const string & s, const string & t = SPACES)
  { 
  string d (s); 
  string::size_type i (d.find_last_not_of (t));
  if (i == string::npos)
    return "";
  else
   return d.erase (d.find_last_not_of (t) + 1) ; 
  }  // end of trim_right

inline string trim_left (const string & s, const string & t = SPACES) 
  { 
  string d (s); 
  return d.erase (0, s.find_first_not_of (t)) ; 
  }  // end of trim_left

inline string trim (const string & s, const string & t = SPACES)
  { 
  string d (s); 
  return trim_left (trim_right (d, t), t) ; 
  }  // end of trim