On Fri, Jun 11, 2010 at 6:03 PM, Eric Niebler <eric@boostpro.com> wrote:
On 6/11/2010 12:36 PM, Eric Niebler wrote:
On 6/11/2010 12:27 PM, David Abrahams wrote:
At Fri, 11 Jun 2010 12:12:27 -0400, Eric Niebler wrote
makes it by-val. Under the hood, Phoenix would actually be storing every captured variable both by value and by reference.
!! Sounds expensive.
The plan has always been to make capture-by-value the default. Saving off a reference as well is free.
Well, there's a hitch.
ref[ bigobj += _1 ]
Users might reasonably expect this to not make a copy of bigobj, and the way to avoid it is not obvious:
ref[ ref(bigobj) += _1 ]
Clearly the user has already stated their intention to capture bigobj by reference and shouldn't have to say it again.
I think default capture modes may be unworkable if we want the default mode to be by-value.
unless you forbid 'naked' lambdas: int x = 0; auto f0 = lambda [ x += _1 ]; // capture x by val auto f1 = lambda_r [ x += _1 ]; // capture x by ref auto f2 = x += _1 ; // just a proto expression tree, not a lambda f0(1); //ok f2(1); //ok f3(1); // no operator() The lambda syntax becomes more heavy weight, but lambdas do stand up more in code. You would have to protect nested lambdas anyway ... -- gpd