Question

I’m porting some C++ to Java, and encountered the following shorthand..but have no idea what this means or even how to go about Googling for it:

class mailbox {
public:
 mailbox(): sent(0),received(0),msg(0) { }

I don’t understand what the method calls(?) after mailbox() signify.

Thanks.

Answer 1

Assuming sent, received and msg are int, it is equivalent to:

mailbox() {
    sent = 0;
    received = 0;
    msg = 0;
}

This syntax exists because, unlike Java where all objects are pointers, in C++ you can have member variables that it’s type does not have a default constructor. For exemple:

class Foo {
public:
    Foo(int x);
};

class Bar {
public:
    Bar(int x) { // ERROR, Foo does not have a default constructor
        foo = Foo(x);
    }

    Bar() : Foo(0) {} // OK, using Foo(int) instead of Foo()

private:
    Foo foo;
};

Note that in the example, even if Foo had a default constructor, Bar(int) would construct Foo twice.