C++将字符串转换成大写、小写的函数

#include <string>     std::string toLower( std::string str )  {      for ( int i = 0; i < str.length(); i++ )      {          /* Only convert upper-case letters */          if ( str[i] > 64 && str[i] < 91 )              str[i] += 32;      }      return str;  }     std::string toUpper( std::string str )  {      for ( int i = 0; i < str.length(); i++ )      {          /* Only convert lower-case letters */          if ( str[i] > 96 && str[i] < 123 )              str[i] -= 32;      }      return str;  }     bool isNpos( size_t iter )  {      return iter == std::string::npos;  }