C++: What is RAII in C++?

1#include <fstream>
2#include <string>
3
4class FileHandler {
5private:
6    std::ifstream file;
7
8public:
9    FileHandler(const std::string& path) {
10        file.open(path);  // Acquire
11        if (!file.is_open()) {
12            throw std::runtime_error("Cannot open file");
13        }
14    }
15
16    ~FileHandler() {
17        if (file.is_open()) {
18            file.close();  // Release
19        }
20    }
21
22    std::string readLine() {
23        std::string line;
24        std::getline(file, line);
25        return line;
26    }
27};
28
29void func() {
30    FileHandler file("data.txt");
31    // File automatically closes when file goes out of scope
32    std::string line = file.readLine();
33}

1#include <mutex>
2
3class Lock {
4private:
5    std::mutex& mtx;
6
7public:
8    Lock(std::mutex& m) : mtx(m) {
9        mtx.lock();  // Acquire
10    }
11
12    ~Lock() {
13        mtx.unlock();  // Release
14    }
15};
16
17std::mutex mtx;
18{
19    Lock lock(mtx);
20    // Critical section
21}  // Automatically unlocked!