C++

Reference

<ostream>

ostream

public member function
<ostream> <iostream>

std::ostream::seekp

(1)
ostream& seekp (streampos pos);
(2)
ostream& seekp (streamoff off, ios_base::seekdir way);
Set position in output sequence
Sets the position where the next character is to be inserted into the output stream.

Internally, if member fail returns true, the function returns.
Otherwise, it calls either pubseekpos (1) or pubseekoff (2) on its associated stream buffer object (if any).
Internally, the function accesses the output sequence by first constructing a sentry object without evaluating it. Then, if member fail returns true, the function returns.
Otherwise, it calls either pubseekpos (1) or pubseekoff (2) on its associated stream buffer object (if any).
Finally, it destroys the sentry object before returning.

Notice that the function will work even if the eofbit flag is set before the call, but it will not modify it.

Parameters

pos
New absolute position within the stream (relative to the beginning).
streampos is an fpos type (it can be converted to/from integral types).
off
Offset value, relative to the way parameter.
streamoff is an offset type (generally, a signed integral type).
way
Object of type ios_base::seekdir. It may take any of the following constant values:
valueoffset is relative to...
ios_base::begbeginning of the stream
ios_base::curcurrent position in the stream
ios_base::endend of the stream

Return Value

The ostream object (*this).

Errors are signaled by modifying the internal state flags:
flagerror
eofbit-
failbitThe internal call to pubseekpos in (1) failed (it may also be set if the construction of sentry failed).
badbitAnother error occurred on the stream (such as when the function catches an exception thrown by an internal operation).
When set, the integrity of the stream may have been affected.
flagerror
eofbit-
failbitThe internal call to either pubseekpos or pubseekoff failed (it may also be set if the construction of sentry failed).
badbitAnother error occurred on the stream (such as when the function catches an exception thrown by an internal operation).
When set, the integrity of the stream may have been affected.
Multiple flags may be set by a single operation.

If the operation sets an internal state flag that was registered with member exceptions, the function throws an exception of member type failure.

Example

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// position in output stream
#include <fstream>      // std::ofstream

int main () {

  std::ofstream outfile;
  outfile.open ("test.txt");

  outfile.write ("This is an apple",16);
  long pos = outfile.tellp();
  outfile.seekp (pos-7);
  outfile.write (" sam",4);

  outfile.close();

  return 0;
}

In this example, tellp is used to get the position in the stream after the writing operation. The pointer is then moved back 7 characters to modify the file at that position, so the final content of the file is:
This is a sample


Data races

Modifies the stream object.
Concurrent access to the same stream object may cause data races.

Exception safety

Basic guarantee: if an exception is thrown, the object is in a valid state.
It throws an exception of member type failure if the resulting error state flag is not goodbit and member exceptions was set to throw for that state.
Any exception thrown by an internal operation is caught and handled by the function, setting badbit. If badbit was set on the last call to exceptions, the function rethrows the caught exception.

See also

ostream::tellp
Get position in output sequence (public member function)
istream::seekg
Set position in input sequence (public member function)