On Saturday, 5 August 2017 at 18:17:49 UTC, Simon Bürger wrote:
If a lambda function uses a local variable, that variable is
captured using a hidden this-pointer. But this capturing is
always by reference. Example:
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int i = 1;
auto dg = (){ writefln("%s", i); };
i = 2;
dg(); // prints '2'
Is there a way to make the delegate "capture by value" so that
the call prints '1'?
Note that in C++, both variants are available using
[&]() { printf("%d", i); }
and
[=]() { printf("%d", i); }
respectively.
I asked about this a couple of day ago:
http://forum.dlang.org/thread/ckkswkkvhfojbcczi...@forum.dlang.org
The problem is that the lambda captures the entire enclosing
stack frame. This is actually a bug because the lambda should
only capture the enclosing *scope*, not the entire stack frame of
the function. So even if you were to copy `i` into a temporary in
some nested scope where a lambda was declared (this works in C#
for example), that temporary would still reside in the same stack
frame as the outer `i`, which means there would still be only one
copy of it.
There is a workaround in Timon's post here:
http://forum.dlang.org/post/om2aqp$2e9t$1...@digitalmars.com
Basically, that workaround wraps the nested scope in another
lambda to force the creation of a separate stack frame.
