Question

Possible Duplicate:
How can a class have a member of its own type, isnt this infinite recursion?

The Code:

public class Test2{
    private Test2 subject = new Test2(); //Create Test2 object in Test2
    private int num;
}

The Questions:

1. Why does Java permit the above code to be executed, but C++ doesn’t?

2. Does the code above create infinite number of objects? Since Test2 itself contains a Test2 object which again contains a Test2 object which itself has a Test2 object and so on.

Answer 1

The key difference between the two languages regarding your problem is that Java is a language with reference semantics (with the exception of primitive types), and C++ a language with value semantics that allows reference semantics through references and pointers.

Syntax that looks similar in both language has complete different meanings, when in Java you create a reference (Test2 x = new Test2();) the equivalent construct in C++ would be using a pointer (Test2 *x = new Test2();).

A key difference is that it is simple to provide reference semantics on top of value semantics through the use of pointers, but it is impossible to provide value semantics on top of (pure) reference semantics. Some of the implications of this statement include not being able to control the layout of objects in Java in memory or the locality of data (for anything other than primitive types and arrays of primitive types), while on the other direction the finer control of objects in C++ allows you to mimic Java objects.