istream_iterator behaviour - c++

Question

I have two pieces of code.They work properly when it is used alone in the main().
vector<int> v;
cout << "Enter sequance of integers "<< "(press q to quit) : ";
istream_iterator<int> start_cin(cin);
istream_iterator<int> end_of_cin;
copy(start_cin,end_of_cin,back_inserter(v));
for ( vector<int>::iterator It = v.begin();It != v.end(); It++ )
cout << *It << "\t";
cout << endl;
and
vector<string> vS;
cout << "Enter three strings : ";
for ( int i = 0; i < 3; i++ )
vS.push_back(*istream_iterator<string>(cin));
ostream_iterator<string> sIt(cout,", ");
copy(vS.begin(),vS.end(),sIt);
cout << endl;
When these two part use together,i.e
#include <iostream>
#include <algorithm>
#include <vector>
#include <iterator>
#include <string>
using namespace std;
int main ()
{
// first part
vector<int> v;
cout << "Enter sequance of integers "<< "(press q to quit) : ";
istream_iterator<int> start_cin(cin);
istream_iterator<int> end_of_cin;
copy(start_cin,end_of_cin,back_inserter(v));
for ( vector<int>::iterator It = v.begin();It != v.end(); It++ )
cout << *It << " \t";
cout << endl;
vector<string> vS;
cout << "Enter three strings : ";
for ( int i = 0; i < 3; i++ )
vS.push_back(*istream_iterator<string>(cin));
ostream_iterator<string> sIt(cout,", ");
copy(vS.begin(),vS.end(),sIt);
cout << endl;
return 0;
}
here first part worked but second part give output: Enter Three Strings : , , ,.
I want to know that what is the reason behind this behaviour?
Thanks.

Answer 1

After the copy() has completed cin will be in an unreadable state (!cin.good()), due to the failed read of the "integer" q. This means the subsequent for loop will fail to read anything.
Add:
cin.clear();
cin.ignore(); // To skip the unread "q"
before the for loop.
EDIT:
As commented by James Kanze, check to ensure "q" was the cause of the termination of the copy():
...
cin.clear();
string int_read_terminator;
cin >> int_read_terminator;
if ("q" != int_read_terminator)
{
cerr << "Integer copy() failure: " << int_read_terminator << "\n";
}
else
{
...

Answer 2

You've just encountered one of the problems with input_iterator: it
requires the entire file to be of one type. There are several ways of
working around this; the most general is to insert a filtering streambuf
between the actual source and the stream. Thus, for example, the first
part of your stream should terminate when you enter a single line with
just a 'q', something like:
class UntilQStreambuf : public std::streambuf
{
std::streambuf* mySource;
char myBuffer;
bool myIsAtStartOfLine;
protected:
int underflow()
{
int results = mySource->sbumpc();
if ( results == 'q'
&& myIsAtStartOfLine
&& mySource->sgetc() == '\n' ) {
mySource->sbumpc(); // remove terminator line.
results = traits_type::eof();
}
if ( results != traits_type::eof() ) {
myBuffer = results;
setg( &myBuffer, &myBuffer, &myBuffer + 1 );
}
return results;
}
public:
UntilQStreambuf( std::istream& source )
: mySource( source->rdbuf() )
, myIsAtStartOfLine( true )
{
}
};
(I think boost::iostream has some support which would make this
significantly simpler to write.) You then set up a separate stream for
reading the numbers, using the streambuf from std::cin (or
whereever):
std::vector<int>
getNumbers( std::istream& source )
{
UntilQStreambuf localSB( source );
std::istream src( &localSB );
std::vector<int> results( (std::istream_iterator<int>( src )),
(std::istream_iterator<int>()) );
if ( !src.eof() ) {
// Some other error occurred...
}
return results;
}
By using a separate stream, the end condition won't be set in the
original stream, and you can continue with it later (perhaps using more
of the same technique).