How to catch current stream position before istream_iterator

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Krzysztofelechowski-3963 asked Oct 3, '20 BarrnetZhou-MSFT edited Oct 22, '20

How to catch current stream position before istream_iterator

I am trying to create an extension of class istream_iterator that saves the input stream position before each read. How should I implement the constructor so that it can catch the stream position at the time the constructor was called? Because it is a subclass of istream_iterator, the constructor constructs the base class first; but after the base class has been constructed, the stream position will have changed (this is how istream_iterator works). Short of reimplementing istream_iterator (which is probably the best option because the istream_iterator does not let me ask its underlying stream for its current position, meaning that I need to keep another reference to the same input stream), what are my options here?

c++

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Answer 1 · 2020-10-05T16:39:21.367Z

Viorel-1 answered Oct 5, '20 BarrnetZhou-MSFT edited Oct 22, '20

Check the next approach too:

 class MyIterator : istream_iterator<char>
 {
 public:
    
     MyIterator( istream& stream )
         : MyIterator( stream, stream.tellg() )
     {
     }
    
 private:
    
     MyIterator( istream& stream, istream::pos_type pos )
         : istream_iterator( stream ), startPos( pos )
     {
     }
    
     istream::pos_type const startPos;
 };

· 2

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Krzysztofelechowski-3963 · Oct 05, 2020 at 04:44 PM

Thank you, I shall try that.

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BarrnetZhou-MSFT Krzysztofelechowski-3963 · Oct 14, 2020 at 08:35 AM

Is this problem solved? If it is solved, could you mark it as answer. This will be beneficial to other community. @Krzysztofelechowski-3963

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Answer 2 · 2020-10-03T16:15:11.597Z

IgorTandetnik-1300 answered Oct 3, '20 Krzysztofelechowski-3963 commented Oct 5, '20

Something like this should work:

 MyIterator::MyIterator(istream& stream)
     : istream_iterator((cur_pos = stream.tellg(), stream)) {}

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WayneAKing-0228 · Oct 03, 2020 at 06:23 PM

Something is nagging at the back of my mind. I recall some often-stated
advice that one should not derive from STL classes. Relates to the absence of
a virtual destructor as I recall. Or does this only apply to STL containers?

Wayne

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Krzysztofelechowski-3963 · Oct 03, 2020 at 06:57 PM

What is cur_pos?

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BarrySchwarz-8780 Krzysztofelechowski-3963 · Oct 03, 2020 at 07:29 PM

It is a variable of the type returned by tellg. It will probably work with any integer type at least as large as int.

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Krzysztofelechowski-3963 BarrySchwarz-8780 · Oct 03, 2020 at 07:34 PM

Where is cur_pos declared?

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