buffered std::ifstream to read from disk only once (C++)

Question

Is there a way to add buffering to a std::ifstream in the sense that seeking (seekg) and reading multiple times wouldn't cause any more reads than necessary.

I'd basically like to read a chunk of file using stream multiple times but I'd want to have the chunk read from disk only once.

The question is probably a bit off cuz I want to mix buffered reads and streams ...

For example:

char filename[] = "C:\\test.txt";
fstream inputfile;
char buffer[20];

inputfile.open(filename, ios::binary);

inputfile.seekg(2, ios::beg);
inputfile.read(buffer, 3);
cout << buffer << std::endl;

inputfile.seekg(2, ios::beg);
inputfile.read(buffer, 3);
cout << buffer3 << std::endl;

I'd want to have to read from disk only once.

Answer 1

Personally, I wouldn't worry about reading from the file multiple times: the system will keep the used buffers hot anyway. However, depending on the location of the file and swap space, different disks may be used.

The file stream itself does support a setbuf() function which could theoretically set the internally used buffer to a size chosen by the user. However, the only arguments which have to be supported and need to have an effect are setbuf(0, 0) which is quite the opposite effect, ie, the stream becomes unbuffered.

I guess, the easiest way to guarantee that the data isn't read from the stream again is to use a std::stringstream and use that instead of the file stream after initial reading, eg:

std::stringstream inputfile;
inputfile << std::ifstream(filename).rdbuf();
inputfile.seekg(0, std::ios_base::beg);

If it is undesirable to read the entire file stream first, a filtering stream could be used which reads the file whenever it reaches a section it hasn't read, yet. However, creating a corresponding stream buffer isn't that trivial and since I consider the original objective already questionable I would doubt that it has much of a benefit. Of course, you could create a simple stream which just does the initialization in the constructor and use that instead.