delegates and temporary struct

[Steven Schveighoffer]

On Thu, 16 May 2013 18:53:55 -0400, Jack Applegame <japplegame at gmail.com>  
wrote:

> Look at this code:
>> import std.stdio;
>>
>> class Foo {
>>  int value = 123456;
>> }
>>
>> struct Bar {
>>  this(Foo f) { foo = f; }
>>  @property auto lambda() {
>>    return (){ writefln("value = %s", foo.value); };
>>  }
>>  Foo foo;
>> }
>> auto getLambda(Foo f) {
>>  return Bar(f).lambda; // lambda closure points to temporary Bar on the  
>> stack
>> }
>> void main() {
>>  Foo foo = new Foo;
>>  auto lambda = getLambda(foo);
>>  lambda(); // undefined behaviour? prints some number, but not 123456
>> }
> It compiles, but result is unpredictable.
>
> Platform: Windows 7, dmd 2.062
>
> For reproducing this behaviour it's necessary to omit -inline and -O  
> flags, try simple "rdmd test.d"

Using struct delegates is VERY frought with danger.  You must ensure the  
struct is alive when the delegate is called.  I don't think the compiler  
can detect the cases where it's not and allocate a closure.

I also have found this out the hard way...

Only recourse is to ensure the struct is on the heap.

-Steve