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kandi X-RAY | boost Summary
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QUESTION
In C++20, we got the capability to sleep on atomic variables, waiting for their value to change.
We do so by using the std::atomic::wait method.
Unfortunately, while wait has been standardized, wait_for and wait_until are not. Meaning that we cannot sleep on an atomic variable with a timeout.
Sleeping on an atomic variable is anyway implemented behind the scenes with WaitOnAddress on Windows and the futex system call on Linux.
Working around the above problem (no way to sleep on an atomic variable with a timeout), I could pass the memory address of an std::atomic to WaitOnAddress on Windows and it will (kinda) work with no UB, as the function gets void* as a parameter, and it's valid to cast std::atomic to void*
On Linux, it is unclear whether it's ok to mix std::atomic with futex. futex gets either a uint32_t* or a int32_t* (depending which manual you read), and casting std::atomic to u/int* is UB. On the other hand, the manual says
The uaddr argument points to the futex word. On all platforms, futexes are four-byte integers that must be aligned on a four- byte boundary. The operation to perform on the futex is specified in the futex_op argument; val is a value whose meaning and purpose depends on futex_op.
Hinting that alignas(4) std::atomic should work, and it doesn't matter which integer type is it is as long as the type has the size of 4 bytes and the alignment of 4.
Also, I have seen many places where this trick of combining atomics and futexes is implemented, including boost and TBB.
So what is the best way to sleep on an atomic variable with a timeout in a non UB way? Do we have to implement our own atomic class with OS primitives to achieve it correctly?
(Solutions like mixing atomics and condition variables exist, but sub-optimal)
...ANSWER
Answered 2021-Jun-15 at 20:48You shouldn't necessarily have to implement a full custom atomic API, it should actually be safe to simply pull out a pointer to the underlying data from the atomic and pass it to the system.
Since std::atomic does not offer some equivalent of native_handle like other synchronization primitives offer, you're going to be stuck doing some implementation-specific hacks to try to get it to interface with the native API.
For the most part, it's reasonably safe to assume that first member of these types in implementations will be the same as the T type -- at least for integral values [1]. This is an assurance that will make it possible to extract out this value.
... and casting
std::atomictou/int*is UB
This isn't actually the case.
std::atomic is guaranteed by the standard to be Standard-Layout Type. One helpful but often esoteric properties of standard layout types is that it is safe to reinterpret_cast a T to a value or reference of the first sub-object (e.g. the first member of the std::atomic).
As long as we can guarantee that the std::atomic contains only the u/int as a member (or at least, as its first member), then it's completely safe to extract out the type in this manner:
QUESTION
I've been attempting to create a node class which mimics a node on a graph. Currently, storage of the predecessor and successor nodes are stored via a node pointer vector: std::vector previous. The vectors for the predecessor/successor nodes are private variables and are accessible via setters/getters.
Currently, I am dealing with updating the pointer values when adding a new node. My current method to update the predecessor/successor nodes is through this method (the method is the same for successor/previous nodes, just name changes):
...ANSWER
Answered 2021-Jun-15 at 20:20I think this should get you going (edge-cases left to you to figure out, if any):
QUESTION
I am using boost (version 1.70.0) property tree. Is there a way to convert a node to XML string including the node itself, not just node's children?
If I have this XML:
...ANSWER
Answered 2021-Jun-14 at 20:24You can create a helper property tree to hold nothing but the extracted one. This involves some additional copying, but should otherwise work just fine:
QUESTION
I've been tasked with porting a piece of legacy software and the client has decided they want to update Boost from 1.34 to 1.75 in the process.
Unfortunately, I'm having this issue show up when compiling:
...ANSWER
Answered 2021-Jun-09 at 10:17I think this is a conflict with third-party headers which we've seen before here:
"xtime: ambiguous symbol" error, when including
In that case reordering the includes worked out. If that doesn't work in your situation, you should work out which library is to blame (usually its the one that contaminates global namespace with (macro) definitions).
And then you can report the defect to the respective maintainers.
QUESTION
I'm following JetBrains's tutorial on an Apple Silicon computer. I installed boost with MacPorts (sudo port install boost), the version is 1.71 The build of tests.cpp file fails with the following error:
...ANSWER
Answered 2021-Jan-03 at 12:57Add
QUESTION
I have csv file: Lets call it product.csv
...ANSWER
Answered 2021-Jun-13 at 20:31I don't think you have O(n) complexity, but a O(n^2), which means that for 100k lines your code will run for 220 minutes, not 22. What makes it worse is that you are reading the file each time you call findPreviousProduct. I would suggest first loading csv into memory and then searching it:
QUESTION
I am using laravel framework to check if it mobile using helper.php, but i get sometimes errors in laravel.log with: Undefined index: HTTP_USER_AGENT
My Code helper.php code:
...ANSWER
Answered 2021-Jun-14 at 05:54It is because, there is no case if it is NULL for HTTP_USER_AGENT you can modify:
QUESTION
I have a complicated Elasticsearch query like the following example. This query has two sub queries: a weighted bool query and a decay function. I am trying to understand how Elasticsearch aggregrates the scores from each sub queries. If I run the first sub query alone (the weighted bool query), my top score is 20. If I run the second sub query alone (the decay function), my score is 1. However, if I run both sub queries together, my top score is 15. Can someone explain this?
My second related question is how to weight the scores from the two sub queries?
...ANSWER
Answered 2021-Jun-13 at 15:43I found the answer myself by reading the elasticsearch document on the usage of function_score. function_score has a parameter boost_mode that specifies how query score and function score are combined. By default, boost_mode is set to multiply.
Besides the default multiply method, we could also set boost_mode to avg, and add a parameter weight to the above decay function exp, then the combined score will be: ( the_bool_query_score + the_decay_function_score * weight ) / ( 1 + weight ).
QUESTION
Problem: I have a class (PortableFoo) designed to be very portable. It contains a scoped class PortableBar. The surrounding codebase (call it Client A) requires both Foo and Bar to have a function that cannot be implemented portably, and Foo's implementation must call Bar's implementation. The following is a solution that compiles and works in GCC, but I know invokes undefined behavior when it casts the reference from base to derived:
...ANSWER
Answered 2021-Jun-11 at 17:37Is it possible to convert Base& to Derived& without object copying or undefined behavior?
Yes, it is possible on a condition that the base reference refers to a base sub object of dynamic type Derived. A minimal example:
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.
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