Tagging of arguments ref/out, or just out

[bearophile]

This is a recently opened (not by me) enhancement request thread:
http://d.puremagic.com/issues/show_bug.cgi?id=6442

It proposes something that I remember was discussed and refused two times in past: to require (but only optionally!) "ref" and "out" at the calling point, as C#4 instead always requires (optionally for COM):

    void foo(ref int bar) { ... }
    int i = 0;
    foo(ref i);   // <------- here

    void foo(out int bar) { ... }
    int i = 0;
    foo(out i);   // <------- here


Jonathan M Davis has then argued that they clutter the code, and that making them optional makes them kind of useless. See the thread for more details.

-----------------

After thinking some about it, I have suggested a related but alternative proposal: to ask only for the "out" at the calling point, make it obligatory if you compile with -warning and optional otherwise (for a long time "override" was like this). I think having "out" at the calling point is more useful than "ref".

Currently D 2.054 gives no warnings/errors on a program like this (I think the C# compiler gives something here):


void foo(out int x) {
    x = 5;
}
void main() {
    int y = 10;
    foo(y);
}


The problem here is the initialization of y to 10 always gets ignored. Assigning something to y, *not using y in any way*, and then using it in a "out" function argument call, is in my opinion a code smell. It's wasted code at best, and sometimes it's related to possible semantic bugs.

Using "out" at the calling point doesn't fix that code, but helps the programmer to see that the assign of 10 to y is useless, and it's better to remove it:


void foo(out int x) {
    x = 5;
}
void main() {
    int y = 10;
    foo(out y);
}


In my opinion "ref" arguments don't have the same need of being tagged at the calling point because a function that uses "ref" often reads and writes the argument (otherwise you use "in" or "out"), so in a ref argument assigning something to y before the call is more often meaningful:


void foo(ref int x) {
    x++;
}
int main() {
    int y = 10;
    return foo(y);
}

Bye,
bearophile