What are the rules for calling the base class constructor?

What are the C++ rules for calling the base class constructor from a derived class?

For example, I know in Java, you must do it as the first line of the subclass constructor (and if you don’t, an implicit call to a no-arg super constructor is assumed – giving you a compile error if that’s missing).

Answer

Base class constructors are automatically called for you if they have no argument. If you want to call a superclass constructor with an argument, you must use the subclass’s constructor initialization list. Unlike Java, C++ supports multiple inheritance (for better or worse), so the base class must be referred to by name, rather than “super()”.

class SuperClass
{
    public:

        SuperClass(int foo)
        {
            // do something with foo
        }
};

class SubClass : public SuperClass
{
    public:

        SubClass(int foo, int bar)
        : SuperClass(foo)    // Call the superclass constructor in the subclass' initialization list.
        {
            // do something with bar
        }
};

More info on the constructor’s initialization list here and here.