C++ std::ofstream - Move the put pointer

Question

I am writing some data to a file. Occasionally, I want to write a block of data from memory, and then move the put pointer along either 1, 2 or 3 bytes to maintain a 4 byte data boundary format.

I could make a new block of data containing zeros and write this, but this seems unnecessary and clumsy. How can I move the put pointer along 1, 2 or 3 bytes?

I am not sure how to do this, because if I call seekp() surely I will move the pointer outside of the current file size? Whereas I assume ofstream.write() deals with this correctly? ie: It resizes the file somehow while writing data?

I'm quite sure it's not possible to do it any other way than to just write something. That's the way the stream classes work. — MicroVirus, Nov 18 '15 at 00:31

score 1 · Answer 1 · answered Nov 18 '15 at 00:07

I am assuming you are doing something like, except instead of writing two bytes of data you want to write 4 bytes with some padding.

#include <fstream>

using namespace std;

struct data
{
    char first;
    char second;
};

int _tmain(int argc, _TCHAR* argv[])
{
    ofstream outFile;
    data data1;
    data data2;

    data1.first = 'a';
    data1.second = 'b';
    data2.first = 'c';
    data2.second = 'd';

    outFile.open("somefile.dat");

    outFile.write(reinterpret_cast<char*>(&data1), sizeof(data));
    outFile.write(reinterpret_cast<char*>(&data2), sizeof(data));

    outFile.close();

    return 0;
}

One option is to simply make the struct 4 bytes. This could have a disadvantage as it could increase memory footprint.

Using seekp probably is not a good option, I tried it and it sort of worked but not really.

    outFile.write(reinterpret_cast<char*>(&data1), sizeof(data));
    outFile.seekp(2, ios_base::cur);
    outFile.write(reinterpret_cast<char*>(&data2), sizeof(data));
    outFile.seekp(2, ios_base::cur);

This did succeed in adding padding after data1 but not data2. Moving the pointer past the just isn't a good idea as it doesn't change the file size. I tried writing 0 bytes after seekp but this didn't work either.

Honestly I would implement a helper function to provide this functionality. Seems much cleaner this way. Here is a simple example:

#include <fstream>

using namespace std;

struct data
{
    char first;
    char second;
};

void WriteWithPadding(ofstream* outFile, data d, int width);

int _tmain(int argc, _TCHAR* argv[])
{
    ofstream* outFile = new ofstream();
    data data1;
    data data2;

    data1.first = 'a';
    data1.second = 'b';
    data2.first = 'c';
    data2.second = 'd';

    outFile->open("somefile.dat");

    WriteWithPadding(outFile, data1, 4);
    WriteWithPadding(outFile, data1, 4);

    outFile->close();
    delete outFile;

    return 0;
}

void WriteWithPadding(ofstream* outFile, data d, int width)
{
    if (sizeof(d) > width)
        throw;

    width = width - sizeof(d); // width is now amount of padding required
    outFile->write(reinterpret_cast<char*>(&d), sizeof(data));

    // Add Padding
    for (int i = 0; i < width; i++)
    {
        outFile->put(0);
    }
}

Nice first post! Welcome to StackOverflow. – MicroVirus Nov 18 '15 at 00:42 — MicroVirus, Nov 18 '15 at 00:42

score 0 · Answer 2 · answered Nov 17 '15 at 23:39

Just to be pedantic, I assume you have opened your file with ios::binary, because you'll have issues if you haven't.

When writing a file, the file is only as large as the number of bytes you have written to your file. So if you write three bytes to the file, you will have a three-byte file.

To maintain a four-byte resolution, you must make sure to write four bytes at a time -- if you write a three-byte object, write an additional byte (zero?) to bring it up to four bytes.

Hope this helps.

C++ std::ofstream - Move the put pointer

2 Answers2