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QUESTION
I was reading this code (source):
...ANSWER
Answered 2021-Jun-16 at 02:16The n2 - n1 in the case of a negative number as a result when converted to bool will yield true. So n1 turns out to be less than n2. That's why it is a bad practice to use ints in such Boolean context.
Yes, as stated in the documentation:
...comparison function object which returns true if the first argument is less than the second
But the implementation of the comparison here leads to failure. Try this and see for yourself:
QUESTION
Giving a bit of context. I'm using c++17. I'm using pointer T* data because this will interop with cuda code. I'm trying write a parallel version (on CPU) of a histogram creator. The sequential version:
ANSWER
Answered 2021-Jun-16 at 00:46The issue you are having has nothing to do with templates. You cannot invoke std::async() on a member function without binding it to an instance. Wrapping the call in a lambda does the trick.
Here's an example:
QUESTION
I have a simple code where I try to define a vector as one of two initializer lists using a ternary operator:
...ANSWER
Answered 2021-Jun-15 at 21:48Because you can't have braces in that context. If you look at cppreference on list initialization, you see that the case inside a ternary operator isn't defined. So it can't be parsed correctly and you get the error you have.
You'd have to use something like this :
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 am currently trying to build OpenPose. First, I will try to describe the environment and then the error emerging from it. Caffe, being built from source, resides in its entirety in [/Users...]/openpose/3rdparty instead of the usual location (I redact some parts of the filepaths in this post for privacy). All of its include files can be found in [/Users...]/openpose/3rdparty/caffe/include/caffe. After entering this command:
...ANSWER
Answered 2021-Jun-15 at 18:43You are using cmake. The makefiles generated by cmake don't conform to "standard" makefile conventions; in particular they don't use the CXXFLAGS variable.
When you're using cmake, you're not expected to modify the compiler options by changing the invocation of make. Instead, you're expected to modify the compiler options by either editing the CMakeLists.txt file, or else by providing an overridden value to the cmake command line that is used to generate your makefiles.
QUESTION
According to the OpenCV Docs, we can use cv::FileStorage to read/write custom data structure from/to config files (XML, YAML, JSON):
ANSWER
Answered 2021-Jun-15 at 15:05The issue is due to the intruduction of namespace, indeed you can get a similar issue with this code:
QUESTION
In my iOS app "Progression" there is rarely a crash (1 crash in ~1000+ Sessions) I am currently not able to fix. The message is
Progression: protocol witness for TrainingSetSessionManager.update(object:weight:reps:) in conformance TrainingSetSessionDataManager + 40
This crash points me to the following method:
...ANSWER
Answered 2021-Jun-15 at 13:26While editing my initial question to add more context as Jay proposed I think it found the issue.
What probably happens? The view where the crash is, contains a table view. Each cell will be configured before being presented. I use a flag which holds the information, if the amount of weight for this cell (it is a strength workout app) has been initially set or is a change. When prepareForReuse is being called, this flag has not been reset. And that now means scrolling through the table view triggers a DB write for each reused cell, that leads to unnecessary writes to the db. Unnecessary, because the exact same number is already saved in the db.
My speculation: Scrolling fast could maybe lead to a race condition (I have read something about that issue with realm) and that maybe causes this weird crash, because there are multiple single writes initiated in a short time.
Solution: I now reset the flag on prepareForReuse to its initial value to prevent this misbehaviour.
The crash only happens when the cell is set up and the described behaviour happens. Therefor I'm quite confident I fixed the issue finally. Let's see. -- I was not able to reproduce the issue, but it also only happens pretty rare.
QUESTION
I'd like to run a simple neural network model which uses Keras on a Rasperry microcontroller. I get a problem when I use a layer. The code is defined like this:
...ANSWER
Answered 2021-May-25 at 01:08I had the same problem, man. I want to transplant tflite to the development board of CEVA. There is no problem in compiling. In the process of running, there is also an error in AddBuiltin(full_connect). At present, the only possible situation I guess is that some devices can not support tflite.
QUESTION
This morning I created an MSTest project in C#, and for one of the JSON resources, Visual Studio is showing this warning:
...ANSWER
Answered 2021-Feb-02 at 17:38Apparently all you have to do is to close out the file. It seems to only show when the file is opened.
Perhaps a later version of Visual Studio can make this warning behave more consistently with a standard warning in VS. Really it behaves very much like a refactoring / code cleanup suggestion (which commonly has a grey, squiggly line), rather than an actual warning. It's like it's just been mislabeled in development or whatever. However the good thing is that as long as the file is closed, it doesn't pollute the build or the errors window with warning messages.
QUESTION
I am currently trying to write some ECS in C++. Inside my ECS (Entity component system), I have a set of entities which all have a set of components. Like a position, rotation, etc.. What I want to do is implement a function which returns an iterator to iterate over the entities which fullfill a few requirements. These requirements are grouped into the following categories:
- required
- requires_one
- excludes
Ideally, I would call the function like this:
...ANSWER
Answered 2021-Jun-15 at 13:09common solution:
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