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Re: [std-proposals] Lambda type not isolated within function

From: Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]>
Date: Thu, 20 Apr 2023 11:09:45 -0400
On Thu, Apr 20, 2023 at 10:54 AM Ville Voutilainen wrote:

> On Thu, 20 Apr 2023 at 17:35, Frederick Virchanza Gotham wrote:
> >
> > How about allowing the creation of new lambda object (with captures)
> > by using initialiser braces?

`{3}` resembles a sequence of integers, not a callable. If this were to
exist at all, it would be an explicit constructor:
    using LambdaType = decltype([arg=0]() { return arg + 3; });
    auto mylambda = LambdaType(3);
    assert(mylambda() == 6);

Why would we? If you go through all that trouble, just write a
> function object class. The syntax you suggest doesn't
> work - the lambda in Func is a lambda that captures the argument of
> Func, to create this new kind of lambda you
> would need something else,

@Ville, I agree 100% with the "Why would we?" part, but I don't see what
you mean about "new kind of lambda." AIUI, there is nothing *physically*
preventing us from giving a captureful lambda a non-explicit constructor
from the types of its captures, just like we have in C++20 given a
captureless lambda a non-explicit constructor from the types of its
captures (i.e., a zero-argument constructor, because it has zero
captures). What keeps WG21 from adding that (mis)feature is its lack of
*desirability*, not implementability.

The biggest usability issue is that either you'd have to permit
    int x = 1, y = 2;
    auto lam = [=]() { return x + y; };
    decltype(lam) copy = decltype(lam)(3, 4);
or disallow it.
If you disallow it, then `[=]` means something very different from `[x,y]`.
Again, that's not *physically* problematic, it's just kinda weird from the
naïve programmer's POV.
If you allow it, then either you have to specify the order of the
constructor arguments (does `3, 4` initialize `x, y` or `y, x`?), which is
ugly; or leave it up to the implementation, which is a portability trap.

That's enough problems that it's clear why we don't give captureful lambdas
any non-special-member-function constructors. But there's nothing
physically preventing such a feature. A bold enough compiler could even
make every lambda type an *aggregate*, if it wanted to. There's nothing
*physically* difficult about that.


Received on 2023-04-20 15:09:58