Hi, I've been playing around with having classes allocated on the stack. I am using the scope keyword for local variables. I assumed that the scope modifier for function parameters was designed to prevent these from escaping, but the code below doesn't show any errors. Is there some newer feature I should be using? Thanks.

import std.stdio;

class Foo
{
}

class Bar
{
	Foo foo;
}

void doSomething(scope Foo z, Bar b)
{
	b.foo = z;
}

void main()
{
	Bar b = new Bar;
	scope foo = new Foo;
	doSomething(foo, b);
}

On 2/22/20 10:01 PM, Russ wrote:
> Is there some newer feature I should be using?

scope escapes are only disallowed in @safe code.

Adding @safe: to the top of this file makes it not compile with a complaint:

Error: scope variable z assigned to non-scope b.foo

Which I think is what you expected.

As to why it happens without @safe, you may be managing your global variable carefully and ensuring it's unset by the time foo goes away. The compiler can't know.

Another reason to enable safe-by-default...

-Steve

On Sunday, 23 February 2020 at 03:26:17 UTC, Steven Schveighoffer wrote:
> On 2/22/20 10:01 PM, Russ wrote:
>> Is there some newer feature I should be using?
>
> scope escapes are only disallowed in @safe code.
>
> Adding @safe: to the top of this file makes it not compile with a complaint:
>
> Error: scope variable z assigned to non-scope b.foo
>
> Which I think is what you expected.
>
> As to why it happens without @safe, you may be managing your global variable carefully and ensuring it's unset by the time foo goes away. The compiler can't know.
>
> Another reason to enable safe-by-default...
>
> -Steve

Perfect, thank you! I agree that this sort of thing should be on by default. Better to opt-out than opt-in for memory safety.