[comp.lang.c++] The meaning of this

levy@fine.Princeton.EDU (Silvio Levy) (03/01/89)

I created the |iline| (input line) class to automate the common (for me)
situation where a program must parse its input line by line,
and it doesn't know a priori how long the input lines are going to be.
An |iline| is used like an |istream| (of which it is a derived class),
with the following two additional functions: a constructor which takes
an existing |istream| as an argument, and binds the |iline| to it;
and |fill| which fills the |iline| with the next line from the |istream|.

Below is a simplified version of the code (it doesn't handle lines
of arbitrary length).  I can post the full thing if there is interest.

My questions are:
(1) How have other people handled this situation?  I started using C++
very recently, and may be taking a completely wrong, or at least
inefficient, approach.
(2) If I replace |this->istream| with |istream| in the body of
|iline::fill|, the program doesn't behave correctly.  The |istream|
structure within the |iline| simply doesn't get updated.  But how
can |this->istream| differ from |istream| if |istream| is a member
function of the object pointed to by this?  Where is that explained
in the C++ book?

Thanks.

*************************************************

#include <stream.h>

class iline: public istream
{
  istream* src_p;  // source istream
  int size;        // size of char array to hold a line of input
  char* beg;       // beginning of ditto
public:
  iline(istream& src_=cin, int size_=100);
  ~iline() { delete beg; };
  iline& fill(char term='\n');
};

// The implicit call to |istream()| sets the state to |bad|, so reading
// will fail from an |iline| which hasn't yet been |fill|ed.

iline::iline(istream& src_=cin, int size_=100)
{
  beg=new char[size=size_];
  src_p=&src_;
};

// In reality we should make sure a whole line has been read,
// and increase the size of the char array if not.

iline& iline::fill(char term='\n')
{
  clear();
  failif(!src_p->getline(beg,size-1,term));
  if (!fail())
    this->istream(size,beg); 
  return *this;
}

// An example of use

main()
{
  iline foo;  // bound to cin by default
  int i;
  while (foo.fill()) {
    while (foo >> i)
      cout << i << ";";
    cout << "\n";
  }
}