2D Array seems to change values as c++ class member -

i have got strange behavior when using double pointer array in programm this:

 /* in a.h     */  class a{    private:      int _n;      int _m;      int _p;      int _dim;      int** _t1;      int** _t2;     public:      ~a();      a();      a(const a& a);      a& operator=(const a& a);      a(int n, int m, std::string filename);  };   int readfile(int* p, int* dim, int*** t1, std::string filename);   /* in a.cpp    */  a::a(int n, int m, std::string filename){    _n = n;    _m = m;    _t1 = null;    _t2 = null;     int ierr;    ierr = readfile(&_p, &_dim, &_t1, filename);    // print _t1[i][j] here : no problem     // fill _t2 _t1 : problem!!    _t2 = new int*[_p];     // check allocation     for(int k = 0; k < _p; k++){        _t2[k] = new int[_dim];     }      for(int k = 0; k < _p; k++){         for(int j = 0; j < _dim; j++){           _t2[k][j] = 0;         }      }       int in, ie, nv;       for(int k = 0; k < _p; k++){           nv = _t1[k][_dim];           if(nv == dim){              for(int in = 0; in < nv; in++){                  in = _t1[k][in];                  ie = _t2[in][_dim];                  _t2[in][ie] = k;                  _t2[in][_dim] += 1;              }           }        }   }   int readfile(int* p, int* dim, int*** t1, std::string filename){     std::ifstream fid;      std::string line;    std::stringstream pline;     fid.open(filename.c_str(), std::ifstream::in);    if( !fid.is_open() ){         fprintf(stderr, "error : can't open %s ...\n", filename.c_str() );         exit( -1 );     }      getline(fid, line);     pline.str(line);     pline >> (*p);      getline(fid, line);     pline.str(line);     pline >> (*dim);      *t1 = new int*[(*p)];     // check allocation     for(int k = 0; k < *p; k++){        (*t1)[k] = new int[(*dim)];     }      int j;     for(int k = 0; k < *p; k++){        getline(fid, line);        pline.str(line);        j = 0;        while( pline >> (*t1)[k][j] ){ j += 1};     }     fid.close();      return ( 0 );  } 

the fact is: double pointer '_t1' being allocated , filled function 'readfile(...)'. ok since simple 'print' shows values have been read file. now, array '_t2' allocated inside class constructor; when start filling _t2 _t1, appears values in array _t1 have changed, , don't figure out why! after calling function readfile(...), nothing else performed on array _t1, when print value during assignment of _t2, no more same first print. i've been thinking array _t1 has been passed correctly function readfile(...) reference, may i'm missing something! may find something!!!