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QUESTION
I want to create a toString function for Tuples with a variadic amount of a specific type (arithmetic types for now).
Something like this
...ANSWER
Answered 2021-Jun-16 at 03:17You can check all the types are identical or not with the help of fold expression (since C++17).
E.g.
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 was following along with this tutorial on creating a concurrent counter struct for a usize value: ConcurrentCounter. As I understand it, this wrapper struct allows us to mutate our usize value, with more concise syntax, for example:my_counter.increment(1) vs. my_counter.lock().unwrap().increment(1).
Now in this tutorial our value is of type usize, but what if we wanted to use a f32, i32, or u32 value instead?
I thought that I could do this with generic type arguments:
...ANSWER
Answered 2021-Jun-15 at 23:55I haven't come across such a ConcurrentCounter library, but crates.io is huge, maybe you find something. However, if you are mostly concerned with primitives such as i32, there is a better alternative call: Atomics, definitely worth checking out.
Nevertheless, your approach of generalizing the ConcurrentCounter is going in a good direction. In the context of operation overloading, std::ops is worth a look. Specifically, you need Add, Sub, and Mul, respectively. Also, you need a Copy bound (alternatively, a Clone would also do). So you were pretty close:
QUESTION
so I'm struggling with these things:
I have method that returns istream input and takes istream input as a parameter, sends values to vector and stores them in it. Now, when I've entered 1 value, I'm trying to make a check if vector already contains that value, here is my code to understand it better:
...ANSWER
Answered 2021-Jun-15 at 20:14first of all, you can check count of std::vector to see if given key exists
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'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 new to rust and I was reading up on using futures and async / await in rust, and built a simple tcp server using it. I then decided to write a quick benchmark, by sending requests to the server at a constant rate, but I am having some strange issues.
The below code should send a request every 0.001 seconds, and it does, except the program reports strange run times. This is the output:
...ANSWER
Answered 2021-Jun-15 at 20:06You are not measuring the elapsed time correctly:
total_send_timemeasures the duration of thespawn()call, but as the actual task is executed asynchronously,start_in.elapsed()does not give you any information about how much time the task actually takes.The
ran intime, as measured bystart.elapsed()is also not useful at all. As you are using blocking sleep operation, you are just measuring how much time your app has spent in thestd::thread::sleep()Last but not least, your
time_to_sleepcalculation is completely incorrect, because of the issue mentioned in point 1.
QUESTION
I am aware that floating-point numbers are tricky. But today I encountered a case that I cannot explain (and cannot reproduce using a standalone C++ code).
The code within a large project looks like this:
...ANSWER
Answered 2021-Jun-15 at 09:57Barring the undefined behavior which can be easily be fixed, you're seeing the effect of denormal numbers. They're extremely slow (see Why does changing 0.1f to 0 slow down performance by 10x?) so in modern FPUs there are usually denormals-are-zero (DAZ) and flush-to-zero (FTZ) flags to control the denormal behavior. When DAZ is set the denormals will compare equal to zero which is what you observed
Currently you'll need platform-specific code to disable it. Here's how it's done in x86:
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You can use std like any standard Python library. You will need to make sure that you have a development environment consisting of a Python distribution including header files, a compiler, pip, and git installed. Make sure that your pip, setuptools, and wheel are up to date. When using pip it is generally recommended to install packages in a virtual environment to avoid changes to the system.
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