Question

I have noticed this strange thing

fstream obj(filename , ios::in);
obj.seekp(7);

is the same as

fstream obj(filename , ios::in);
obj.seekg(7);

seekg and seekp do the same this and cause the same outcome, although i specified ios::in flag only

Why are they both work with fstream? And what is the difference between seekp and seekg with fstream?

Answer 1

basic_fstream is derived from basic_iostream which is derived from basic_istream and basic_ostream. So, basic_fstream has function seekp from basic_ostream and function seekg from basic_ifstream.

In short, in your case, calls to seekp and seekg do same actions, since actions perfomed by basic_filebuf::seekpos depends only on open mode for basic_filebuf.

basic_ostream<charT,traits>& seekp(pos_type pos);

Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::out). In case of failure,
the function calls setstate(failbit) (which may throw ios_base::failure).

Where pubseekpos calls seekpos (which is virtual, so, calls basic_filebuf::seekpos)

pos_type seekpos(pos_type sp,
ios_base::openmode which = ios_base::in | ios_base::out);

Alters the file position, if possible, to correspond to the position stored in sp (as described below).
Altering the file position performs as follows:

1. if (om & ios_base::out) != 0, then update the output sequence and write any unshift sequence;

2. set the file position to sp;

3. if (om & ios_base::in) != 0, then update the input sequence;

where om is the open mode passed to the last call to open(). The operation fails if is_open() returns
false.

Since you open file with ios_base::in function does 2 and 3 puncts.

basic_istream<charT,traits>& seekg(pos_type pos);

Effects: Behaves as an unformatted input function (as described in 27.7.2.3, paragraph 1), except that
the function first clears eofbit, it does not count the number of characters extracted, and it does not
affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail()
!= true, executes rdbuf()->pubseekpos(pos, ios_base::in). In case of failure, the function calls
setstate(failbit) (which may throw ios_base::failure).