Constructor/destructor order; parameterized base constructor

Single inheritance

Base class constructor runs first, then the derived class constructor. Destructors run in reverse: derived first, then base.

Multiple inheritance

Base class constructors are called in the order in which the bases are listed in the derived class declaration (not the order in the initializer list), then the derived constructor. Destructors run in the exact reverse order.

class D : public A, public B { ... };   // order: A(), B(), D()
                                        // destructors: ~D(), ~B(), ~A()

Passing arguments to a parameterized base constructor

The derived constructor passes values to the base constructor using a member initializer list after a colon. If the base class has no default constructor, this is compulsory.

#include <iostream>
using namespace std;

class A {
    int a;
public:
    A(int x) { a = x; cout << "A(" << a << ")" << endl; }
    ~A()     { cout << "~A" << endl; }
};

class B {
    int b;
public:
    B(int y) { b = y; cout << "B(" << b << ")" << endl; }
    ~B()     { cout << "~B" << endl; }
};

class D : public A, public B {
    int d;
public:
    // arguments for bases are passed in the initializer list
    D(int x, int y, int z) : B(y), A(x) {
        d = z;
        cout << "D(" << d << ")" << endl;
    }
    ~D() { cout << "~D" << endl; }
};

int main() {
    D obj(1, 2, 3);
    return 0;
}

Output:

A(1)
B(2)
D(3)
~D
~B
~A

Although B(y) is written before A(x) in the initializer list, A is constructed first because A is listed first in class D : public A, public B.