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QUESTION
I have a function that returns boost::asio::awaitable
. What is the idiomatic way to convert this awaitable to std::future
?
ANSWER
Answered 2021-Jun-12 at 18:00Before we get into the answer, be warned:
You should not, under any circumstance, get()
or wait()
a future to a boost::asio::awaitable
from the same thread as the executor that is running the coroutine.
That being said.
That third parameter to co_spawn()
, the one almost every example blindly sets to the magic detached
constant? Its role is to tell boost::asio
what to do once the coroutine has finished. detached
simply means "do nothing". So the canonical way to fulfil a future from an awaitable<>
should be via that mechanism.
Thankfully, asio already provides the use_future
completion token. Pass that as the third parameter to co_spawn()
and it will return a std::future<>
of the matching return type.
QUESTION
I used to have mongodb (the unofficial package) installed on Ubuntu 20.04. I decided to change to the official one.(version 4.4) First, both packages got confilcted, but after wiping the old one, the install succeeded.
But, when I try to run the mongod service,it fails, and shows this:
...ANSWER
Answered 2021-Jun-04 at 06:00I've found a related question where the same author found the solution, please click here to see:
As a side note, if you have the same error, and provided solutions does not work, please, follow the link and try this answer,but paying special attention to remove both lib and log files created by mongodb,
QUESTION
I have the following unit test making a rest call, but everything after my axios.get isn't running.
restCalls.spec.js:
...ANSWER
Answered 2021-Jun-03 at 20:36The problem is your getData
function returns undefined. Though starts an axios request in 'backround'. And your test case calls it and immediatelly proceeds to the next line (expect(login == true)
).
To fix it you should make the getData
function return created by axios.get
promise. And add await
before getData
call in test case to wait for it to finish.
QUESTION
I am receiving data from a source (Twitch IRC) that does not specify the length of the data beforehand, and it never sends a consistent amount of data. This source uses "\r\n" as its delimiter, and I would like to receive data until this delimiter is found, stop receiving to process the received data and then continue receiving. I have tried a few solutions I came up with:
...ANSWER
Answered 2021-Jun-03 at 18:10You can buffer the data and extract whole messages when the separator is found. Example:
server.py
QUESTION
I have a problem with handling asio::error_code
values when they are received from another dll or executable. For instance, I may run an asynchronous operation with a handler:
ANSWER
Answered 2021-Jun-03 at 14:30This is what I meant with this comment
though boost::system::system_error could invite issues back
The trouble is, error categories are global singleton instances, with object identity (i.e. compared for equality by their address).
You'r ending up with multiple instances in multiple modules. The usual solution is to
- dynamically link to Boost System, so all libraries use the same copy (this does however sometimes run into initialization order issues)
- if that doesn't solve it, make sure all modules actually load the same library (version?)
- In recent boosts I think there's the option to build Boost System completely header-only. This may or may not involve new-fangled C++14 inline, I haven't checked.
If all else fails, do your own translation. There's related issues that might give you ideas:
- Is it possible to convert a boost::system::error_code to a std:error_code?
- Do note that exceptions may run into very similar issues due to RTTI not necessarily matching; exceptions need to be exported types and linked dynamically from the same module for the exception handlers to match the thrown runtime type id.
Is it normal or expected to compare only errorCode.value() against enums?
No it is not. According to some sources Boost as well as standard library promise to map generic error category to errc
which is standard - so you could do that, but you still have to figure out whether that is the category, so doesn't help your scenario.
QUESTION
I wonder why the following programme crashes. How to use awaitable
not with boost::asio::async_write
/async_read
functions.
Let's see:
...ANSWER
Answered 2021-Jun-01 at 15:54digging into the sources of boost::asio::awaitable
, I figured out that I just should make use of co_return
keyword. Surprisingly, it is not shipped with boost
. It is enabled either by -fcoroutines
flag or -std=c++20
. Unexpectedly. Having said that, it is solved.
QUESTION
I am trying to send HTTPS request to a server and receive the page contents by only using Boost.Asio(not Network.Ts or Beast or others) by these code :
...ANSWER
Answered 2021-May-30 at 18:53QUESTION
Recently I wrote a Discord-Bot in C++ with the sleepy-discord bot library. Now, the problem here is that when I run the bot it shows me the following errors:
...ANSWER
Answered 2021-May-29 at 21:34The error triggers when you so s.remote_endpoint
on a socket that is not connected/no longer connected.
It would happen e.g. when you try to print the endpoint with the socket after an IO error. The usual way to work around that is to store a copy of the remote endpoint as soon as a connection is established, so you don't have to retrieve it when it's too late.
On the question why it's happening on the particular VM, you have to shift focus to the root cause. It might be that accept
is failing (possibly due to limits like number of filedescriptors, available memory, etc.)
QUESTION
I've been trying to find out how to stream images with zeromq (i'm using the cppzmq wrapper, but raw API answers are fine). Naively, I set up
...ANSWER
Answered 2021-May-28 at 16:40Use zmq_msg_init_data
http://api.zeromq.org/master:zmq-msg-init-data
You can provide the memory pointer/size of your already allocated memory and zeromq will take ownership (skipping the extra allocation). Once its been processed and is no longer needed it will call the associated free function where your own code can clean up.
I have used this approach in the past with a memory pool/ circular buffer and it worked well.
QUESTION
Is it possible to determine the type of a class field based on the value assigned to it within the function of the class itself?
I am writing a wrapper for the ozo library which is based on the boost.Asio library in order to use it with c ++ 20 coroutines. Such a strange need arose in the inability to set the type of the conn field, which must be returned.
The type I need is deduced when the operator () is called on the object ozo :: request_op reqOp;
.A callback is passed to operator (), where the second argument is a variable of the type I need. The type defined for the second argument of the callback must be defined for the class field Awaiter conn.
In this case, I determined the conn type ConnectionPtr, but this is wrong, because it is suitable only for one specific case. I need to make this functor more versatile. There is also an option how to pass conn from await_suspend to await_resume, but as far as I know, this can only be done through the awaiter class.
...ANSWER
Answered 2021-May-28 at 03:38In general, you can’t do this because the type of the parameter conn
is determined elsewhere, by code that might in some cases be declared after this class is defined. If the function accepting the callback returns something whose type is based on the type the callback returns, you can write something like decltype(reqOp.reqOp(…,[](auto x) {return x;}))
, but otherwise the information about what type was passed is lost. (There are stateful metaprogramming tricks that might be used to exfiltrate the information with a similar trick, but they’re best avoided.)
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