base::Bind + uncapturing C++11 lambda?

[Taiju Tsuiki]

Hi, C++ experts.

Can we allow binding uncapturing C++11 lambda with base::Bind into base::Callback?

I heard mojo::Callback was able to hold C++11 lambda, while base::Bind doesn't accept it directly.

Though I'm not sure we should allow general capturing lambda as a base::Bind argument, I think it's safe and useful to allow uncapturing ones.

I.e.:

int i = 42;

base::Closure cb1 = base::Bind([i](){});  // Reject this.

base::Closure cb2 = base::Bind([](int j){}, i);  // Allow this.

On implementation side, it will accept an object if its class have `R T::operator()(Args...)` and the instance is convertible to `R(*)(Args...)`.

WDYT?

[Daniel Cheng]

This seems reasonable to me and is the approach I advocated for in https://codereview.chromium.org/2090013004/.

Non-blocking random thought: in the future, I wonder if we should require annotating all pointer lifetimes for base::Bind to make code like this impossible:

  auto cb = base::Bind([](MyClass* c) { c->MyMethod(); }, raw_my_class_ptr);

I'm guessing this would be a pretty large project, and of course, it's already possible today: it's just lambdas make it a little easier.

Daniel

[Jeremy Roman]

FWIW, I think using captureless lambda in this way is reasonable (I just haven't gotten around to finishing a CL to do it). But base/OWNERS would have to OK it.

It already works if you explicitly cast to a function pointer; it's just that the template magic doesn't currently handle doing it automagically.

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[dan...@chromium.org]

That is in line with the guidance expressed in the style guide currently, and I think it would be nice. 

[Nico Weber]

Sounds good to me too (unless the implementation for this is complicated, since bind is already hard to understand).

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