Question: What does * and *+ mean for a lambda?

[ruiyuanlu]

Hi, I tried to figure out the meaning of *identity and *+identity used in ExceptionTest.cpp but had no idea. I've never seen sth like this brefore. Why are these operators used on the lambda function identity?

Many thx for your help.

auto identity = [](int i) { return i; };
EXPECT_EQ(3, folly::catch_exception<int>(returner(3), +identity));
EXPECT_EQ(3, folly::catch_exception<int>(returner(3), *+identity));

[Orvid]

@yfeldblum Any comment here?

[yfeldblum]

Every capture-free or static lambda has a built-in conversion to pointer-to-function. IIUC, the use of + triggers conversion because the compiler looks for any intermediate built-in conversion that would allow + on the converted-to type. Unary + of a pointer is a no-op. And *+ is just getting a reference-to-function from pointer-to-function the way * normally does.

https://en.cppreference.com/w/cpp/language/lambda
https://en.cppreference.com/w/cpp/language/implicit_conversion