How to keep the buffer of std::cin after using std::cin.getline()

I am trying to re-read the cin buffer after using cin.getline() 5 times, by calling cin.seekg(), but after calling cin.getline() again, instead of re-reading the data from the top, str becomes an empty string. Does cin.getline() flush the buffer? If so how do I stop that from happening?

#define PATH_MAX 512

using std::cin;



int main()

{

    char* str = new char[PATH_MAX + 1];



    for(int i = 0; i < 5; i++)

        cin.getline(str, PATH_MAX);



    cin.seekg(cin.beg);



    while(true)

        cin.getline(str, PATH_MAX);



    return 0;

}

edited Nov 25 '18 at 15:26

asked Nov 25 '18 at 15:23

John Lannister

325

1

seekg ends up calling streambuf::seekpos: "The base class version of this function has no effect. The derived classes may override this function to allow absolute positioning of the position indicator." Classes like filebuf and stringbuf override it and do something useful, but cin isn't using one of those. Bottom line: you cannot seek on cin. If you want to look at the data a second time, read it and store it somewhere.

– Igor Tandetnik
Nov 25 '18 at 15:29

1

cin.seekg(cin.beg); is incorrect, it should just be cin.seekg(0);, but most likely is that std::cin simply isn't seekable. There's no requirement that it should be.

– john
Nov 25 '18 at 15:29

add a comment |

#define PATH_MAX 512

using std::cin;



int main()

{

    char* str = new char[PATH_MAX + 1];



    for(int i = 0; i < 5; i++)

        cin.getline(str, PATH_MAX);



    cin.seekg(cin.beg);



    while(true)

        cin.getline(str, PATH_MAX);



    return 0;

}

edited Nov 25 '18 at 15:26

asked Nov 25 '18 at 15:23

John Lannister

325

1

seekg ends up calling streambuf::seekpos: "The base class version of this function has no effect. The derived classes may override this function to allow absolute positioning of the position indicator." Classes like filebuf and stringbuf override it and do something useful, but cin isn't using one of those. Bottom line: you cannot seek on cin. If you want to look at the data a second time, read it and store it somewhere.

– Igor Tandetnik
Nov 25 '18 at 15:29

1

cin.seekg(cin.beg); is incorrect, it should just be cin.seekg(0);, but most likely is that std::cin simply isn't seekable. There's no requirement that it should be.

– john
Nov 25 '18 at 15:29

add a comment |

#define PATH_MAX 512

using std::cin;



int main()

{

    char* str = new char[PATH_MAX + 1];



    for(int i = 0; i < 5; i++)

        cin.getline(str, PATH_MAX);



    cin.seekg(cin.beg);



    while(true)

        cin.getline(str, PATH_MAX);



    return 0;

}

edited Nov 25 '18 at 15:26

asked Nov 25 '18 at 15:23

John Lannister

325

#define PATH_MAX 512

using std::cin;



int main()

{

    char* str = new char[PATH_MAX + 1];



    for(int i = 0; i < 5; i++)

        cin.getline(str, PATH_MAX);



    cin.seekg(cin.beg);



    while(true)

        cin.getline(str, PATH_MAX);



    return 0;

}

c++ std cin istream

edited Nov 25 '18 at 15:26

asked Nov 25 '18 at 15:23

John Lannister

325

edited Nov 25 '18 at 15:26

asked Nov 25 '18 at 15:23

John Lannister

325

edited Nov 25 '18 at 15:26

asked Nov 25 '18 at 15:23

John Lannister

325

asked Nov 25 '18 at 15:23

John Lannister

325

asked Nov 25 '18 at 15:23

John Lannister

325

1

seekg ends up calling streambuf::seekpos: "The base class version of this function has no effect. The derived classes may override this function to allow absolute positioning of the position indicator." Classes like filebuf and stringbuf override it and do something useful, but cin isn't using one of those. Bottom line: you cannot seek on cin. If you want to look at the data a second time, read it and store it somewhere.

– Igor Tandetnik
Nov 25 '18 at 15:29

1

cin.seekg(cin.beg); is incorrect, it should just be cin.seekg(0);, but most likely is that std::cin simply isn't seekable. There's no requirement that it should be.

– john
Nov 25 '18 at 15:29

add a comment |

1

seekg ends up calling streambuf::seekpos: "The base class version of this function has no effect. The derived classes may override this function to allow absolute positioning of the position indicator." Classes like filebuf and stringbuf override it and do something useful, but cin isn't using one of those. Bottom line: you cannot seek on cin. If you want to look at the data a second time, read it and store it somewhere.

– Igor Tandetnik
Nov 25 '18 at 15:29

1

cin.seekg(cin.beg); is incorrect, it should just be cin.seekg(0);, but most likely is that std::cin simply isn't seekable. There's no requirement that it should be.

– john
Nov 25 '18 at 15:29

seekg ends up calling streambuf::seekpos: "The base class version of this function has no effect. The derived classes may override this function to allow absolute positioning of the position indicator." Classes like filebuf and stringbuf override it and do something useful, but cin isn't using one of those. Bottom line: you cannot seek on cin. If you want to look at the data a second time, read it and store it somewhere.

– Igor Tandetnik
Nov 25 '18 at 15:29

cin.seekg(cin.beg); is incorrect, it should just be cin.seekg(0);, but most likely is that std::cin simply isn't seekable. There's no requirement that it should be.

– john
Nov 25 '18 at 15:29

add a comment |

1 Answer
1

active

oldest

votes

I am trying to re-read the cin buffer after using cin.getline() 5 times

That's not possible with cin, terminal based input respectively.

What you can do is keeping track of the read input yourself, using a std::vector<std::string> keeping those lines read in 1st place. Here's a rough sketch:

#include <iostream>

#include <string>

#include <vector>    

using std::cin;

using std::string;



int main()

{

    std::vector<string> lines;

    string line;

    for(int i = 0; i < 5; i++) {

        std::getline(cin,line);

        lines.push_back(line);

    }



    auto linepos = lines.begin();

    while(linepos != lines.end()) {

        // cin.getline(str, PATH_MAX); instead do:

        // process *linepos

        ++linepos;

    }

}

answered Nov 25 '18 at 15:51

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73.6k1077144

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1 Answer
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1 Answer
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I am trying to re-read the cin buffer after using cin.getline() 5 times

That's not possible with cin, terminal based input respectively.

What you can do is keeping track of the read input yourself, using a std::vector<std::string> keeping those lines read in 1st place. Here's a rough sketch:

#include <iostream>

#include <string>

#include <vector>    

using std::cin;

using std::string;



int main()

{

    std::vector<string> lines;

    string line;

    for(int i = 0; i < 5; i++) {

        std::getline(cin,line);

        lines.push_back(line);

    }



    auto linepos = lines.begin();

    while(linepos != lines.end()) {

        // cin.getline(str, PATH_MAX); instead do:

        // process *linepos

        ++linepos;

    }

}

answered Nov 25 '18 at 15:51

πάντα ῥεῖ

73.6k1077144

add a comment |

I am trying to re-read the cin buffer after using cin.getline() 5 times

That's not possible with cin, terminal based input respectively.

What you can do is keeping track of the read input yourself, using a std::vector<std::string> keeping those lines read in 1st place. Here's a rough sketch:

#include <iostream>

#include <string>

#include <vector>    

using std::cin;

using std::string;



int main()

{

    std::vector<string> lines;

    string line;

    for(int i = 0; i < 5; i++) {

        std::getline(cin,line);

        lines.push_back(line);

    }



    auto linepos = lines.begin();

    while(linepos != lines.end()) {

        // cin.getline(str, PATH_MAX); instead do:

        // process *linepos

        ++linepos;

    }

}

answered Nov 25 '18 at 15:51

πάντα ῥεῖ

73.6k1077144

add a comment |

I am trying to re-read the cin buffer after using cin.getline() 5 times

That's not possible with cin, terminal based input respectively.

What you can do is keeping track of the read input yourself, using a std::vector<std::string> keeping those lines read in 1st place. Here's a rough sketch:

#include <iostream>

#include <string>

#include <vector>    

using std::cin;

using std::string;



int main()

{

    std::vector<string> lines;

    string line;

    for(int i = 0; i < 5; i++) {

        std::getline(cin,line);

        lines.push_back(line);

    }



    auto linepos = lines.begin();

    while(linepos != lines.end()) {

        // cin.getline(str, PATH_MAX); instead do:

        // process *linepos

        ++linepos;

    }

}

answered Nov 25 '18 at 15:51

πάντα ῥεῖ

73.6k1077144

I am trying to re-read the cin buffer after using cin.getline() 5 times

That's not possible with cin, terminal based input respectively.

What you can do is keeping track of the read input yourself, using a std::vector<std::string> keeping those lines read in 1st place. Here's a rough sketch:

#include <iostream>

#include <string>

#include <vector>    

using std::cin;

using std::string;



int main()

{

    std::vector<string> lines;

    string line;

    for(int i = 0; i < 5; i++) {

        std::getline(cin,line);

        lines.push_back(line);

    }



    auto linepos = lines.begin();

    while(linepos != lines.end()) {

        // cin.getline(str, PATH_MAX); instead do:

        // process *linepos

        ++linepos;

    }

}

answered Nov 25 '18 at 15:51

πάντα ῥεῖ

73.6k1077144

answered Nov 25 '18 at 15:51

πάντα ῥεῖ

73.6k1077144

answered Nov 25 '18 at 15:51

πάντα ῥεῖ

73.6k1077144

answered Nov 25 '18 at 15:51

πάντα ῥεῖ

73.6k1077144

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