.unget

[masterinex]

can someone explain to me what is.unget() mean

/*
Practice Coding Exercises
Using command line arguments read in the file called Horoscope.txt
*/

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

using namespace std;

//A function which gets a word from the stream and returns the stream to the call
istream& getword(istream& is, string& word)
{
	char ch;
	while (is.get(ch))
	{
		if (isalpha(ch))
			break;
	}


	//'’'


	if (!is)
		return is;

	string buffer;
	buffer += ch;
	while (is.get(ch))
	{
		if (isalpha(ch))
			buffer += ch;
		else
			break;
	}

	if (is)
		is.unget();

	if (is.eof())
		is.clear(ios::eofbit);

	word = std::move(buffer);

	return is;
}

int main(int argc, char* argv[])
{
	//1. Read in the filename and open an input filestream
	ifstream inFile(argv[1]);
	//Test for success
	if (!inFile)
	{
		cerr << "Input file opening failed.\n";
		return EXIT_FAILURE;
	}

	//2. Get the words from the file and store in a vector
	vector<string> v;
	string word;
	while (getword(inFile, word))
		v.push_back(word);
	//Finished reading in file


	cout << "Finished reading in file!\n";


}

[SamuelAdams]

http://www.cplusplus.com/reference/istream/istream/unget/

[masterinex]

i found this example confusing , because it is using cin, what do i get if i combine is with unget ?

and what is difference btw unget and putback ?

another expression that i am not familiar with is word = std::move(buffer);

[masterinex]

what do i get if i use unget with an infile stream object like is instead of using it with cin ?

also what does this line mean word = std::move(buffer); ?

[masterinex]

aa

[masterinex]

aa

[masterinex]

aa

[coder777]

also what does this line mean word = std::move(buffer); ?

See:

http://en.cppreference.com/w/cpp/utility/move

It is actually like copy but might be a bit faster.

what is difference btw unget and putback ?

See:

http://www.cplusplus.com/reference/istream/istream/unget/?kw=unget
http://www.cplusplus.com/reference/istream/istream/putback/

unget() has no parameter. It restores the last char.
putback() has a parameter. It actually writes a char without moving the write index.

what do i get if i use unget with an infile stream object like is instead of using it with cin ?

There shouldn't be a difference. But I have never used it so just test it.

[masterinex]

i understand that if u use cin , unget will put the character back into the stream, but i still don't understand if i combine unget with infile like infile.unget() . will it still put the last character back into the stream ?

i can comment out this section of code and i tried running without it and it has no effect:


if (is)
is.unget();

[coder777]

It has no effect because it is guaranteed that is (line 40) will return false and hence is.unget(); will not be called. But I would think it has no effect anyway since the stream is in an error state. What is the effect you want?

[masterinex]

if unget puts the char back into the stream again for read, would not i read the same char twice
at the line while (is.get(ch)) ?

[coder777]

if unget puts the char back into the stream again for read, would not i read the same char twice
at the line while (is.get(ch)) ?

No, because it is not inside the loop. If it where inside you would read the same character over and over again which would lead to a infinite loop if the character where actually alpha.