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istream_iterator invokes reading from the keyboard by just defining it!!

JUANDENT (411)
Hi,

The following code opens for reading from the keyboard instead of waiting for the vector: (this does not occur if the istream is an ifstream - associated with a file)


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istream_iterator<string> in {cin }; // this reads the keyboard immediately why??
istream_iterator<string> eos;
vector<string> vec{ in, eos };     // this is where I expect reading the keyboard!!


JLBorges (13770)
The actual read operation is performed when the iterator is incremented, not when it is dereferenced.
The first object is read when the iterator is constructed.
Dereferencing only returns a copy of the most recently read object.
https://en.cppreference.com/w/cpp/iterator/istream_iterator
JUANDENT (411)
This activates reading but I can never close the input unless I write Ctrl-Z or Ctrl-C... why doesn't it end reading when I press ENTER?


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vector<string> vec{ istream_iterator<string> {cin }, istream_iterator<string> {}};
bool eof = cin.eof();  // this return true


must do cin.clear() in order to continue reading from cin... Why necessary?

JUANDENT (411)
this line of reading does not end when pressed ENTER, why?

 
 
vector<string> vec{ istream_iterator<string> {cin }, istream_iterator<string> {}}



Peter87 (11234)
Pressing enter does not end the stream.
JUANDENT (411)
Then how come ENTER terminates input for this instruction?

 
 
istream_iterator<string> in {cin };




Try it out please
Last edited on
Peter87 (11234)
Because of what JLBorges posted.
The first object is read when the iterator is constructed.

Once it has read that one first string token the code continues.
Last edited on
seeplus (6590)
Pressing enter does not end the stream.


Correct. This iterates the stream until eof is found (istream_iterator<string> {}). For Windows cin, eof is ctrl-z. So to terminate the input enter ctrl-z on a line by itself.

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#include <iostream>
#include <vector>
#include <string>

using namespace std;

int main() {
	vector<string> vec{ istream_iterator<string> {cin }, istream_iterator<string> {}};

	for (const auto& v : vec)
		cout << v << '\n';
}


This will generate vec from the input with a new element inserted for every white space in the input. The input is terminated with ctrl-z


foobar barfoo
^Z
foobar
barfoo


foorbar barfoo is entered. Then ctrl-Z to terminate. Then the contents of vec are displayed.
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