Constructors & Destructors

A constructor initializes an object when it's created; a destructor runs automatically when it's destroyed — when a stack object goes out of scope, or a heap object is explicitly deleted.

class Dog {
public:
    std::string name;

    Dog(std::string n) : name(n) { // member initializer list
        std::cout << name << " created" << std::endl;
    }

    ~Dog() {
        std::cout << name << " destroyed" << std::endl;
    }
};

int main() {
    Dog rex("Rex");
    std::cout << "Using rex..." << std::endl;
    return 0;
    // rex's destructor runs automatically here
}

Rex created
Using rex...
Rex destroyed

This automatic, guaranteed-to-run destructor is the foundation of RAII (Resource Acquisition Is Initialization) — a class that acquires a resource in its constructor and releases it in its destructor can never leak that resource, covered fully in Level 2.

Common mistakes

• Preferring name = n; in the constructor body over the member initializer list (: name(n)) — the initializer list is more efficient and, for some member types, required.