std::basic_istream<CharT,Traits>::seekg

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basic_istream& seekg( pos_type pos);	(1)
basic_istream& seekg( off_type off,std::ios_base::seekdir dir);	(2)

Sets input position indicator of the current associatedstreambuf object.

Before doing anything else,seekg clearseofbit. (since C++11)

seekg behaves asUnformattedInputFunction, except thatgcount() is not affected. After constructing and checking the sentry object,

1) iffail()!=true, sets the input position indicator to absolute (relative to the beginning of the file) valuepos. Specifically, executesrdbuf()->pubseekpos(pos,std::ios_base::in) (pubseekpos, in turn, calls seekpos of the specific buffer, such asbasic_filebuf::seekpos,basic_stringbuf::seekpos, orstrstreambuf::seekpos). In case of failure, callssetstate(std::ios_base::failbit).

2) iffail()!=true, sets the input position indicator to positionoff, relative to position, defined bydir. Specifically, executesrdbuf()->pubseekoff(off, dir,std::ios_base::in). In case of failure, callssetstate(std::ios_base::failbit).

[edit]Parameters

pos - absolute position to set the input position indicator to

off - relative position (positive or negative) to set the input position indicator to

dir

defines base position to apply the relative offset to. It can be one of the following constants:

Constant	Explanation
`beg`	the beginning of a stream
`end`	the ending of a stream
`cur`	the current position of stream position indicator

[edit]Return value

*this

[edit]Exceptions

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failure if an error occurred (the error state flag is notgoodbit) andexceptions() is set to throw for that state.

If an internal operation throws an exception, it is caught andbadbit is set. Ifexceptions() is set forbadbit, the exception is rethrown.

[edit]Notes

seekg(n) is not necessarily equivalent toseekg(n, ios::beg).std::basic_ifstream, for example, requires the absolute positionn to come fromtellg().

[edit]Example

Run this code

#include <iostream>#include <sstream>#include <string> int main(){std::string str="Hello, world";std::istringstream in(str);std::string word1, word2;     in>> word1;    in.seekg(0);// rewind    in>> word2; std::cout<<"word1 = "<< word1<<'\n'<<"word2 = "<< word2<<'\n';}

Output:

word1 = Hello,word2 = Hello,

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 129	C++98	there was no way to indicate a failure	sets`failbit` on failure
LWG 136	C++98	`seekg` could set the output stream	only sets the input stream
LWG 537	C++98	the type ofoff was`off_type&`	corrected to`off_type`

[edit]See also

tellg	returns the input position indicator (public member function)[edit]
tellp	returns the output position indicator (public member function of`std::basic_ostream<CharT,Traits>`)[edit]
seekp	sets the output position indicator (public member function of`std::basic_ostream<CharT,Traits>`)[edit]
pubseekpos	invokesseekpos() (public member function of`std::basic_streambuf<CharT,Traits>`)[edit]
seekpos [virtual]	repositions the file position, using absolute addressing (virtual protected member function of`std::basic_filebuf<CharT,Traits>`)[edit]
seekpos [virtual]	repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function of`std::basic_stringbuf<CharT,Traits,Allocator>`)[edit]
seekpos [virtual]	repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function of`std::strstreambuf`)[edit]
pubseekoff	invokesseekoff() (public member function of`std::basic_streambuf<CharT,Traits>`)[edit]
seekoff [virtual]	repositions the file position, using relative addressing (virtual protected member function of`std::basic_filebuf<CharT,Traits>`)[edit]
seekoff [virtual]	repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function of`std::basic_stringbuf<CharT,Traits,Allocator>`)[edit]
seekoff [virtual]	repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function of`std::strstreambuf`)[edit]