std::strstreambuf::seekoff

C++

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Technical Specifications

Input/output library

I/O manipulators

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

strstreambuf (deprecated)

basic_syncbuf (C++20)

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ios_base

basic_ios

basic_istream

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basic_istringstream

basic_ostringstream

basic_stringstream

Array I/O

istrstream (deprecated)

ostrstream (deprecated)

strstream (deprecated)

Synchronized Output

basic_osyncstream (C++20)

Types

streamoff

streamsize

fpos

Error category interface

iostream_category (C++11)

io_errc (C++11)

std::strstreambuf

Public member functions

strstreambuf::strstreambuf

strstreambuf::~strstreambuf

strstreambuf::freeze

strstreambuf::str

strstreambuf::pcount

Protected member functions

strstreambuf::underflow

strstreambuf::pbackfail

strstreambuf::overflow

strstreambuf::setbuf

strstreambuf::seekoff

strstreambuf::seekpos

protected: virtual pos_type seekoff(off_type off, ios_base::seekdir way, ios_base::openmode which = ios_base::in | ios_base::out);

Repositions std::basic_streambuf::gptr and/or std::basic_streambuf::pptr, if possible, to the position that corresponds to exactly off characters from beginning, end, or current position of the get and/or put area of the buffer.

• If which includes ios_base::in and this buffer is open for reading, then repositions the read pointer std::basic_streambuf::gptr inside the get area as described below
• If which includes ios_base::out and this buffer is open for writing, then repositions the write pointer std::basic_streambuf::pptr inside the put area as described below
• If which includes both ios_base::in and ios_base::out and the buffer is open for both reading and writing, and when is either ios_base::beg or ios_base::end, then repositions both read and write pointers as described below.
• Otherwise, this function fails.

If the pointer (either gptr or pptr or both) is repositioned, it is done as follows:

1) If the pointer to be repositioned is a null pointer and the new offset newoff would be non-zero, this function fails.

2) The new pointer offset newoff of type off_type is determined

a) if way == ios_base::beg, then newoff is zero

b) if way == ios_base::cur, then newoff is the current position of the pointer (gptr()-eback() or pptr()-pbase())

c) if way == ios_base::end, then newoff is the length of the entire initialized part of the buffer (if overallocation is used, the high watermark pointer minus the beginning pointer)

3) If newoff + off is negative or out of bounds of the initialized part of the buffer, the function fails

4) Otherwise, the pointer is assigned as if by gptr() = eback() + newoff + off or pptr() = pbase() + newoff + off

Parameters

off	-	relative position to set the next pointer(s) 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
which	-	defines whether the input sequences, the output sequence, or both are affected. It can be one or a combination of the following constants: Constant Explanation in affect the input sequence out affect the output sequence

Return value

pos_type(newoff) on success, pos_type(off_type(-1)) on failure and if pos_type cannot represent the resulting stream position.

Example

Run this code

#include <iostream>
#include <strstream>

int main()
{
    char a[] = "123";
    std::strstream ss(a, sizeof a); // in/out
    std::cout << "put pos = " << ss.tellp()
              << " get pos = " << ss.tellg() << '\n';

    // absolute positioning both pointers
    ss.rdbuf()->pubseekoff(1, std::ios_base::beg); // move both forward
    std::cout << "put pos = " << ss.tellp()
              << " get pos = " << ss.tellg() << '\n';

    // try to move both pointers 1 forward from current position
    if(-1 == ss.rdbuf()->pubseekoff(1, std::ios_base::cur))
        std::cout << "moving both pointers from current position failed\n";
    std::cout << "put pos = " << ss.tellp()
              << " get pos = " << ss.tellg() << '\n';

    // move the write pointer 1 forward, but not the read pointer
    // can also be called as ss.seekp(1, std::ios_base::cur);
    ss.rdbuf()->pubseekoff(1, std::ios_base::cur, std::ios_base::out);
    std::cout << "put pos = " << ss.tellp()
              << " get pos = " << ss.tellg() << '\n';

    ss << 'a'; // write at put position
    std::cout << "Wrote 'a' at put position, the buffer is now: '";
    std::cout.write(a, sizeof a);
    std::cout << "'\n";
    char ch;
    ss >> ch;
    std::cout << "reading at get position gives '" << ch << "'\n";
}

Output:

put pos = 0 get pos = 0
put pos = 1 get pos = 1
moving both pointers from current position failed
put pos = 1 get pos = 1
put pos = 2 get pos = 1
Wrote 'a' at put position, the buffer is now: '12a'
reading at get position gives '2'

See also

seekpos [virtual]	repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function of std::basic_streambuf)
seekoff [virtual]	repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function of std::basic_streambuf)
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)
seekoff [virtual]	repositions the file position, using relative addressing (virtual protected member function of std::basic_filebuf)