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;
}
c++ std cin istream
asked Nov 25 '18 at 15:23
John LannisterJohn Lannister
325
add a comment |
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;
}
c++ std cin istream
asked Nov 25 '18 at 15:23
John LannisterJohn Lannister
325
1
seekgends up callingstreambuf::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 likefilebufandstringbufoverride it and do something useful, butcinisn't using one of those. Bottom line: you cannot seek oncin. 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 becin.seekg(0);, but most likely is thatstd::cinsimply isn't seekable. There's no requirement that it should be.
– john
Nov 25 '18 at 15:29
add a comment |
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;
}
c++ std cin istream
asked Nov 25 '18 at 15:23
John LannisterJohn Lannister
325
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;
}
c++ std cin istream
c++ std cin istream
asked Nov 25 '18 at 15:23
John LannisterJohn Lannister
325
asked Nov 25 '18 at 15:23
John LannisterJohn Lannister
325
edited Nov 25 '18 at 15:26
John Lannister
asked Nov 25 '18 at 15:23
John LannisterJohn Lannister
325
asked Nov 25 '18 at 15:23
John LannisterJohn Lannister
325
asked Nov 25 '18 at 15:23
John LannisterJohn Lannister
325
325
1
seekgends up callingstreambuf::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 likefilebufandstringbufoverride it and do something useful, butcinisn't using one of those. Bottom line: you cannot seek oncin. 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 becin.seekg(0);, but most likely is thatstd::cinsimply isn't seekable. There's no requirement that it should be.
– john
Nov 25 '18 at 15:29
add a comment |
1
seekgends up callingstreambuf::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 likefilebufandstringbufoverride it and do something useful, butcinisn't using one of those. Bottom line: you cannot seek oncin. 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 becin.seekg(0);, but most likely is thatstd::cinsimply isn't seekable. There's no requirement that it should be.
– john
Nov 25 '18 at 15:29
1
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
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
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
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
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I am trying to re-read the
cinbuffer after usingcin.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 ῥεῖπάντα ῥεῖ
73.6k1077144
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
I am trying to re-read the
cinbuffer after usingcin.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 ῥεῖπάντα ῥεῖ
73.6k1077144
add a comment |
I am trying to re-read the
cinbuffer after usingcin.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 ῥεῖπάντα ῥεῖ
73.6k1077144
add a comment |
I am trying to re-read the
cinbuffer after usingcin.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 ῥεῖπάντα ῥεῖ
73.6k1077144
I am trying to re-read the
cinbuffer after usingcin.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 ῥεῖπάντα ῥεῖ
73.6k1077144
answered Nov 25 '18 at 15:51
πάντα ῥεῖπάντα ῥεῖ
73.6k1077144
answered Nov 25 '18 at 15:51
πάντα ῥεῖπάντα ῥεῖ
73.6k1077144
answered Nov 25 '18 at 15:51
πάντα ῥεῖπάντα ῥεῖ
73.6k1077144
73.6k1077144
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1
seekgends up callingstreambuf::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 likefilebufandstringbufoverride it and do something useful, butcinisn't using one of those. Bottom line: you cannot seek oncin. 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 becin.seekg(0);, but most likely is thatstd::cinsimply isn't seekable. There's no requirement that it should be.– john
Nov 25 '18 at 15:29