Boost Users :
Subject: Re: [Boost-users] [asio] wrap lambda async_receive call
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2015-03-22 23:06:19
On 19/03/2015 19:16, Cheng Mo wrote:
> Hi Gavin:
> >Having said this, using lambdas can get tricky in real-world code, as
> >it's common to want to start the same read operation when the first
> >completes, and so on.
> Just like you said, will the code build a lambda expression when
> async_read() invoked ?
> So that "[=](const error_code& ec,size_t count" will be built servial
> times, it is kind of lavish for contructing the lambda expression, isn't ?
The compiler will build the code for a lambda only once regardless of
how many times it's called. But it also won't let you create a
recursive lambda, which is what you would need if you wanted to have a
read operation repeat itself when it completes (which is the most common
Well, actually that's not quite true -- you can type-erase a lambda via
std::function or similar and then call it recursively. But AFAIK you
can only do this when capturing by reference, and you can't safely do
that with a locally declared lambda when making async calls. So it's
not really useful in this context.
> It's better that build a function object in static. What do you think ?
Function objects have a similar lifetime issue, and making one static is
potentially dangerous if you could have multiple operations or
connections running concurrently. It's usually simplest to use a
private method of the containing class, which is what most of the
example code does.
(If you really want to hide it from clients of your class, then use the
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