Destructors and RAII

The destructor

A constructor runs when an object is born. A destructor runs when it dies – when it goes out of scope or is deleted. Its name is the class name with a ~ (tilde) in front:

#include <iostream>
#include <string>
using namespace std;

class Guest {
    string name;
public:
    Guest(string n) : name(n) {
        cout << name << " arrives\n";
    }
    ~Guest() {
        cout << name << " leaves\n";
    }
};

int main() {
    Guest a("Ana");
    {
        Guest b("Ben");
        cout << "Party inside the braces\n";
    }                                   // b goes out of scope here
    cout << "Back in main\n";
    return 0;
}                                       // a goes out of scope here

Output

Ana arrives
Ben arrives
Party inside the braces
Ben leaves
Back in main
Ana leaves

Notice the order: Ben leaves as soon as his { } block ends. Ana leaves at the end of main. Objects are destroyed in the reverse order they were created.

RAII: let objects clean up after themselves

RAII stands for Resource Acquisition Is Initialization – a long name for a simple, powerful idea: get a resource in the constructor, give it back in the destructor. Then you can never forget to clean up.

#include <iostream>
using namespace std;

class IntBuffer {
    int* data;
    int size;
public:
    IntBuffer(int n) : data(new int[n]), size(n) {
        cout << "Reserved " << size << " ints\n";
    }
    ~IntBuffer() {
        delete[] data;
        cout << "Freed " << size << " ints\n";
    }
    int& at(int i) { return data[i]; }
};

int main() {
    IntBuffer buf(3);
    buf.at(0) = 10;
    buf.at(1) = 20;
    cout << buf.at(0) + buf.at(1) << "\n";
    return 0;                           // no delete needed: ~IntBuffer() does it
}

Output

Reserved 3 ints
30
Freed 3 ints

Think of it like this: RAII is like a hotel room key card that is automatically cancelled when you check out. You don't have to remember to cancel it; leaving does it for you.

Remember: vector, string and the file streams all use RAII. That's why you never have to free them yourself.