C++ Weekly - Ep 126 - Lambdas With Destructors

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[superscatboy]

C++ Weekly, 2029: "Concrete types deprecated, everything is a lambda now"

[ChnSailor]

You are one of the few people I will watch, going back slowly into all of your content! (I’ve been doing C/C++ for 20+ years)

[vertigo]

One can never have too many videos about lambdas. I'm really enjoying every one of them.

[devluz]

Might come in handy if we ever have a "most ugly C++" competition

[applicative_functor]

Lambda, which capture another lambda inline, which have structure declaration and usage. I was struck to the core of my soul by the fact that it is possible at all !

Is it possible to unsee this?! :D Why not add few levels of nesting templates, make them mutable, and mix with SFINAE, type-traits and move-semantics? :D

[goshisanniichi]

I could see it used for timing, something like function profiling. It's not that necessary anymore, but you have the lambda grab a mutex on creation and then release it on destruction, that way you don't have to worry about things like mutexes not being release in the even exceptions and things like that.

[NonTwinBrothers]

I had to find this video again just to convince myself it wasn't a dream

[rabbitdrink]

if i were to use something like that id most likely just put together a struct with operator() or use a finally object from some utility library

[ukaszdrozdz]

This looks like it could have a use in a SCOPE_EXIT{ } implementation.

[zackyezek]

Or you can just write a regular, old school functor struct directly and bypass both the unnecessary extra wrapping layers AND the write only lambda code.

You can still do things with those that no lambda can, like a recursive calls. And in this case it’d be faster too.

[YTonYahoo]

What's the difference between defining i in the capture clause and defining it as a local variable within the lambda function body?

[shushens]

I find the name misleading, because the destructor does not destroy the lambda.

[DusanJovanovicDBJ]

That lambda has no destructor. That struct S has a destructor.

[AxelStrem]

If I ever happen to have a container of mutexes, I'll make sure to use a "destructor lambda" with std::for_each to lock them all :D

[danielrhouck]

Wouldn't a simpler lambda with a destruction be [s=std::string("Hello")](){}?

[X_Baron]

I'm puzzled by that struct S being in scope after the outer lambda.

[foomoo]

Not sure this is really a “destructor” for the lambda … it gets called twice . Also not clear why you’d need it

[alexeiz]

Why do these kind of programming technics remind me Javascript?

[cmdlp]

I think it can be useful for debugging and optimizing. When I write functions taking a lambda function as a parameter, I always pass them by value, because in most cases the lambdas just capture a reference to all variables, this should be as fast as copying a pointer, but when other people use the functions, they might capture variables by value, this could help me to improve the code, so that lambdas are not copied all over the place.

But do you think it is better to pass lambdas by value, by l-value reference or universal reference when writing functions like in the <algorithm>-header?

[markusbock]

I usually use a class that is construct able with a std::function only to make a scope guard for when using a c api that requires cleanup. Or if possible I use boost scope exit