c++ - In what case can this if-statement throw an exception? -

apparently, when code reaches if statement, string subscript out of range exception occurs.

// int // str std::string  while ( true ) {     // other stuff if( == str.size() ) // line throws exception     break; } 

in case can such simple if-statement throw exception? don't see it. shouldn't return 0 if reason comparison fails?

edit: full function in occurs. reads through file , takes in value of tokens of its. , if of relevancy, i'm using visual studio 2010 express , error says "debug assertion failed".

void function(string &str, int start) {     int outline;      // read attributes     int pos, pos2 = start;     while( true )     {                pos = skipwhitespace(str, pos2);         pos2 = findendoftoken(str, pos);          string token = str.substr(pos, pos2-pos);          pos = skipwhitespace(str, pos2);         if( pos == str.size() || str[pos] != '=' ) break;          pos = skipwhitespace(str, pos+1);         pos2 = findendoftoken(str, pos);         file<<"...part 3";          string value = str.substr(pos, pos2-pos);          if( token == "outline" )             outline = (short)strtol(value.c_str(), 0, 10);          if( pos == str.size() ) // <--- error here (at least, seems so)             break;           }      setoutline(outline); } 

and skipwhitespace() , findendoftoken() functions these two.

int skipwhitespace(string &str, int start) {     uint n = start;     while( n < str.size() )     {         char ch = str[n];         if( ch != ' ' &&              ch != '\t' &&              ch != '\r' &&              ch != '\n' )             break;          ++n;     }      return n; }  int findendoftoken(string &str, int start) {     uint n = start;     if( str[n] == '"' )     {         n++;         while( n < str.size() )         {             char ch = str[n];             if( ch == '"' )             {                 // include last quote char in token                 ++n;                 break;             }             ++n;         }     }     else     {         while( n < str.size() )         {             char ch = str[n];             if( ch == ' ' ||                 ch == '\t' ||                 ch == '\r' ||                 ch == '\n' ||                 ch == '=' )                 break;              ++n;         }     }      return n; } 

this line can never throw exception. sure real c++ exception , not crash?