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

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std::basic_istream
 
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 associated streambuf object.

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

seekg behaves as UnformattedInputFunction, except that gcount() is not affected. After constructing and checking the sentry object,

1) if fail() != true, sets the input position indicator to absolute (relative to the beginning of the file) value pos. Specifically, executes rdbuf()->pubseekpos(pos, std::ios_base::in) (pubseekpos, in turn, calls seekpos of the specific buffer, such as basic_filebuf::seekpos, basic_stringbuf::seekpos, or strstreambuf::seekpos). In case of failure, calls setstate(std::ios_base::failbit).
2) if fail() != true, sets the input position indicator to position off, relative to position, defined by dir. Specifically, executes rdbuf()->pubseekoff(off, dir, std::ios_base::in). In case of failure, calls setstate(std::ios_base::failbit).

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

Return value

*this

Exceptions

failure if an error occurred (the error state flag is not goodbit) and exceptions() is set to throw for that state.

If an internal operation throws an exception, it is caught and badbit is set. If exceptions() is set for badbit, the exception is rethrown.

Notes

seekg(n) is not necessarily equivalent to seekg(n, ios::beg). std::basic_ifstream, for example, requires the absolute position n to come from tellg().

Example

Run this code
#include <iostream>
#include <string>
#include <sstream>
 
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,

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 of off was off_type& corrected to off_type

See also

 returns the input position indicator
(public member function)
returns the output position indicator
(public member function of std::basic_ostream<CharT,Traits>)
sets the output position indicator
(public member function of std::basic_ostream<CharT,Traits>)
invokes seekpos()
(public member function of std::basic_streambuf<CharT,Traits>)
[virtual]
 repositions the file position, using absolute addressing
(virtual protected member function of std::basic_filebuf<CharT,Traits>)
[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>)
[virtual]
 repositions the next pointer in the input sequence, output sequence, or both using absolute addressing
(virtual protected member function of std::strstreambuf)
invokes seekoff()
(public member function of std::basic_streambuf<CharT,Traits>)
[virtual]
 repositions the file position, using relative addressing
(virtual protected member function of std::basic_filebuf<CharT,Traits>)
[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>)
[virtual]
 repositions the next pointer in the input sequence, output sequence, or both, using relative addressing
(virtual protected member function of std::strstreambuf)
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