CODEWITHSUNDEEP
c++parametersfstreamseekg
con-f-use
con-f-use

Reputation: 4028

After reading from an fstream how do I set the curor back to pre-read position?

I'm modifying a reading routine for binary data. Unfortunately I'm not that firm in C++ anymore, which is the language the routine is written in. The routine starts reading some data. After that I want it to look at a buffered value which I also read from the file. Depending on the value the code should either do something and continue normally afterwards or undo reading the buffer and continue normally.

My problem is the undoing or reverting the cursor position, if you will. The stripped code looks something like this:

int buffer;

std::fstream inputFile;
inputFile.open( "Filename", std::ios::in | std::ios::binary );

... // read some data from inputFile

// read buffer value
inputFile.read( reinterpret_cast<char *>(&buffer), sizeof(buffer) );

if( buffer == 256 ) {
    ... // do something here
} else
    // make it so nothing (including reading the buffer earlier) happened
    inputFile.seekg( -1*sizeof(buffer), std::ios::cur ); // <---- is this right?
    // or do I need to do it this way?
    inputFile.seekg( -1*sizeof(buffer)/sizeof(char), std::ios:cur );
}

I assume I can use negative values in seekg() as I find int only logicall and haven't read anything to the contrary. Which way above is correct? Or basically I'm asking what does seekg() actually expect as first argument?

The C++ Reference only says this:

 istream& seekg (streamoff off, ios_base::seekdir way);
 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:
    value   offset is relative to...
    ios_base::beg   beginning of the stream
    ios_base::cur   current position in the stream
    ios_base::end   end of the stream

Which doesn't tell me the unit off is measured in (bytes, chars, ints?).

Upvotes: 1

Views: 1054

Answers (1)

Dietmar K&#252;hl
Dietmar K&#252;hl

Reputation: 153895

The version of seekg() taking a whence argument takes an std::streamoff as argument. It can be negative. There is no need to divide by sizeof(char) as sizeof(char) is defined to be 1. Since stream aleays operate on characters, the units used by streams are characters, i.e., the type of the first template argument of the stream.

Upvotes: 1

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