Using operator>> with a pointer to ifstream object

Question

Why in the following code is the instruction os->operator>> input wrong? Is not the return value of operator>> the object *os?

#include <iostream>
#include <string>
#include <fstream>

using namespace std;

int main()
{
  double input;
  ifstream * os = new ifstream("prova.dat");
  os->operator>> input;
  return 0;
}

Don't use pointers to ifstreams! They're not intended to be used that way. They can be passed by reference where necessary. — Rook
– Rook, Commented May 12, 2014 at 16:30
@RoyIacob The same as for any other object shouldn't be used via raw pointer references whenever avoidable. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented May 12, 2014 at 16:35
Sorry, I thought input was a string. Either os->operator>>(input) or *os >> input works. — David G
– David G, Commented May 12, 2014 at 16:44
@RoyIacob: the standard classes have all been quite carefully written to handle their own resource creation and cleanup. By creating them on the heap, you lose all of the nice RAII facilities they provide you, you have to take your own responsibility for cleanup and various other things become harder to do. I can't think of any upsides to doing it at all. — Rook
– Rook, Commented May 12, 2014 at 17:16

masoud · Accepted Answer · 2014-05-12 16:50:01Z

5

When you want to use >> as a method, then you need to pass arguments by () like normal functions. To dereference it you should use these two ways:

os->operator >> (input);

or

*os >> input;

Note: Why pointers, when you can use automatic objects or references. Moreover, you need to manage the allocated object and free it.

answered May 12, 2014 at 16:50

masoud

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Comments

Rook · Accepted Answer · 2014-05-12 16:50:18Z

1

If you're creating pointers to streams like that, you're probably doing something wrong.

This should work fine:

int main()
{
  double input;
  std::ifstream os("prova.dat");
  os >> input;
  return 0;
}

Because the ifstream was allocated on the stack, it will be automatically cleaned up when the function finishes, which will close the underlying filehandle. You missed the explicit delete you'd need to do the same in your code.

If you need to pass your stream around, you can do so by reference:

double read_double(std::ifstream& stream)
{
  double d;
  stream >> d;
  return d;
}

int main()
{
  std::ifstream os("prova.dat");
  double input = read_double(os);
  return 0;
}

edited May 12, 2014 at 16:50

answered May 12, 2014 at 16:33

Rook

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6 Comments

Mooing Duck Over a year ago

Well, that's half an answer

Rook Over a year ago

@MooingDuck which half?

David G Over a year ago

@Rook The half that doesn't really answer the question.

David G Over a year ago

Right, but I think he was looking for an explanation for why his code didn't work. Telling him the recommended way is still good though.

Rook Over a year ago

Fair enough. MM at least has that bit covered, so the OP won't go away empty handed.

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