c++ - Access violation reading location 0x00000006 -

i have following code finds strings contain no alphabets. cases mynumber123 shall not recognized , numberfinder() should return false , case 123 shall recognized , numberfinder() shall return true begin index of number.

the constructor:

caddressparser::caddressparser(string filename)     //constructor {     m_filename=filename;     int length=getlength(m_filename.c_str());     m_text =filereader(m_filename.c_str());     m_length=length; } 

which initializes string m_text contains contents of text file

somewhere along implementation come across following code:

for (i;i<m_length;i++) {     bool uppercasebeforenofound=false;      if(this->numberfinder (i).second)     {         //do calculations.     } } 

the numberfinder function implemented follows:

pair <int,bool> caddressparser::numberfinder(int counter) {     bool nofound=isdigit(m_text[counter]);      //number found? -> true     if(nofound)     {         int end=housenodigits(counter);         if(((counter-1)>=0) && ((counter +end-1) <m_length))         {             if((!(isalpha(m_text[counter-1]))) && (!isalpha(m_text[counter+end-1])))             {                 return make_pair(counter,nofound);      //return index if true             }         }      }      else return make_pair(0,nofound);  } 

now problem text file containing following text "he23 market street london q12 h13". error mentioned in headline , debugger takes me line in contains :

if(this->numberfinder (i).second)

i can't figure out why happening. please me figure out.

access violation error due null reference. 1 of function calling trying access null pointer. make sure isdigit function returns true or false, m_text points exiting memory location. if not need allocate memory. should check if filename null.