File Positioning

C++ streams maintain internal markers to track the current read and write positions. Using seek and tell operations, you can jump to specific bytes inside a file to implement random access.

Clearing State Flags

If you read to the end of a file, the stream sets the eof and fail flags. If these flags are active, subsequent seek operations will fail silently. You must call stream.clear() to reset the flags before calling seekg() or seekp().

Code Walkthrough

Calculates file size in bytes and reads data from a specific offset.

#include <iostream>
#include <fstream>
std::streamsize getFileSize(const std::string& filepath) {
    std::ifstream file(filepath, std::ios::binary);
    if (!file) return 0;
// Seek to the end of file
    file.seekg(0, std::ios::end);
    // Tell position (returns bytes from beginning)
    std::streamsize size = file.tellg();
    return size;
}
void readFromOffset(const std::string& filepath, std::streamoff offset) {
    std::ifstream file(filepath, std::ios::binary);
    if (!file) return;
// Seek to specific offset from the beginning
    file.seekg(offset, std::ios::beg);
char buffer[10];
    if (file.read(buffer, sizeof(buffer))) {
        std::cout << "Read 10 bytes from offset " << offset << std::endl;
    }
}
int main() {
    std::cout << "File Size: " << getFileSize("data.bin") << " bytes\n";
    readFromOffset("data.bin", 4);
    return 0;
}

Interview-Relevant Information