C++ - lesson 5 | HackIndia Skills

C++ - lesson 5 | HackIndia Skills | HackIndia

Example

Small file wrapper

This class owns a C FILE pointer. It opens the file in the constructor and closes it in the destructor.

Copying is disabled because two objects closing the same file would be unsafe. Move semantics comes next lesson, so keep this simple for now.

#include <cstdio>
#include <stdexcept>
#include <string>

class FileReader {
    FILE* file;

public:
    explicit FileReader(const std::string& path) : file(std::fopen(path.c_str(), "r")) {
        if (!file) {
            throw std::runtime_error("Could not open file: " + path);
        }
    }

    ~FileReader() {
        if (file) {
            std::fclose(file);
        }
    }

    FileReader(const FileReader&) = delete;
    FileReader& operator=(const FileReader&) = delete;

    int getChar() {
        return std::fgetc(file);
    }
};

int main() {
    FileReader input("sample.txt");
    int ch = input.getChar();
    return 0;
}

Common mistake

Common bad pattern

Do not make users remember to call cleanup manually. Someone will return early during a contest fix and forget it.

If your class has open(), close(), and no destructor doing cleanup, it is not protecting the rest of your code.

void process() {
    FILE* f = std::fopen("data.txt", "r");
    if (!f) return;

    // Bug risk: any early return before fclose leaks the file handle.
    if (someCondition()) return;

    std::fclose(f);
}

Design classes with RAII

One ownership rule

Why it matters

Small file wrapper

Design the class

Common bad pattern

Prefer existing RAII

README proof

Do this now

Quick check