Name: jl125535 Date: 07/10/2003


DESCRIPTION OF THE REQUEST :
Here's a thought for a feature to add to the language. In
C++, it is often convenient to attach a clean-up action to
the destructor of a class so that when a stack-allocated
instance of that class goes out of scope, the clean-up
action is executed automatically.

Typical uses are for File open/close, Lock acquire/release,
Log enter/exit, Variable increment/decrement, and in C++,
malloc/free.

However in Java, objects never have so-called 'automatic'
lifetime, and nor do they have (synchronous) destructors;
but Java has the somewhat similar try { .. } finally
mechanism, and has the special case 'synchronized(e) {..}'
for locks.

Therefore I propose the following small, compatible (except
for new keyword 'auto') addition to the compiler and
library, which generalises the 'synchronized' statement for
arbitrary interfaces. The syntactic benefit is I hope,
obvious, by analogy with the synchronized statement.

// this goes in java.lang
interface Automatic {
    void start();
    void stop();
}

// example implementation class
class Trace implements Automatic {
    public void start() { System.out.println("start"); }
    public void stop() { System.out.println("stop"); }
}

// example use:
class s
{
    // you write:
    public static void main(String[] args) {
        Trace tr = new Trace();
        auto(tr) { // argument must implement Automatic
            if(args.length == 0) return;
            System.out.println("here");
        }
    }
    // the compiler desugars this to:
    public static void main(String[] args) {
        Trace tr = new Trace();
        Automatic auto$0 = tr;
        try {
            auto$0.start();
            if(args.length == 0) return;
            System.out.println("here");
        }
        finally {
            auto$0.stop();
        }
    }
}


So the additional syntax as shown here is a statement of the
form:

    auto(EXPR) BLOCK

In addition it would be nice to support the form:

    auto(EXPR);

also, which would implicitly use the remainder of the block
as the BLOCK argument. This permits even more localised
code changes. e.g.

void f() {
    auto(new Trace());
    /* this is the block */
}

(The most obvious objection I can see is the 'kitchen sink'
argument).

(Review ID: 181875)
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