Lambda capture by value

[Steven Schveighoffer]

On 2/24/20 3:32 PM, H. S. Teoh wrote:

> To fix this, copy the value of 'i' to a local variable inside the loop
> body, then the lambda will correctly capture a unique per-iteration
> instance of the variable.  Like this:
> 
>      foreach (i; iota(5))
>      {
> 	auto _i = i;
>          printers[i] = () { write(_i); };
>      }

Nope. It doesn't ;) Because actually, _i is reused for the loop iteration.

You are thinking that the capture happens on the delegate creation. In 
essence, it just sets a flag to the compiler that the enclosing function 
must be placed on the heap.

Adam's method works because the nested call's stack frame is captured 
when you return that delegate.

In other words, it looks like this:

void main()
{
    static struct __mainStackFrame
    {
        int i;
        int _i;
    }

    __mainStackFrame *frame = new __mainStackFrame; // here's where the 
capture happens
    with(*frame)
    {
       // main's function body, with all variables already being declared
    }
}

-Steve