Thursday, June 13, 2013

Construction destruction

Take a look at this little program skeleton:

class Foo 
    private int x;
    private int y;
    public Foo(int x, int y) 
        this.x = x;
        this.y = y;
        SideEffects.Alpha(); // Notice: does not use "this"
static class SideEffects
    public static void Alpha() { ... }
    public static void Bravo() { ... }
    public static void Charlie() { ... }
    public static void M()
        Foo foo = new Foo(1, 2); 

Let's suppose we have three side effects: Alpha, Bravo and Charlie. What precisely they are is not important.

The question is: what do we know about the order in which Alpha, Bravo and Charlie execute when a call to M() occurs?

First off, clearly Alpha must happen before Bravo. The C# compiler and the jit compiler are not permitted to make any optimization that would cause the side effects of a single-threaded program to appear out of order. The construction of foo must be complete before control is passed to Bravo, which means that Alpha has already run.

QR: Inline image 1