在C ++中使用auto声明lambda变量的首选方法是什么？

Question

Does anyone have any insight into the pros and cons of the various ways to use auto to declare a lambda variable?
For eg:

 auto Val = [] { };           // #1
 const auto &LVRef = [] { };  // #2
 auto &&RVRef = [] { };       // #3

Can anyone think of a compelling reason to not always prefer #1?

I imagine for a template function, the preferred way to declare a callable parameter is to use a universal reference?

template<class F> void foo(F&& f) { f(); }

Or do most guidelines prefer to declare it as a non-reference parameter?

template<class F> void foo(F f) { f(); }

Thank you!

Answer 1

Can anyone think of a compelling reason to not always prefer #1?

No. #1 is simpler and there's no performance penalty vs. the other options.

Algorithms taking functors just take them by value normally, but using universal references is reasonable if you want to guarantee no copying the functor for some reason. If you're writing something that simply forwards the functor then use universal references.

Answer 2

I'd actually say #2, because I prefer differenting non-mutable and non-reference variables/functor, then declare every other variable const auto& . The reason I prefer to also use a reference is that you dont have to care if a function returns a generated value or a reference.