Month: July 2020

C++ Channel: A thread-safe container for sharing data between threads

Threads synchronization is a common task in multithreading applications. You cannot get away without some form of protecting the data that is accessed from multiple threads. Some of the concepts of protecting data are mutexes and atomic variables, and they are common for programming languages that support multithreading.

There is another concept that offers the same features in a different way. Instead of requiring you to explicitly protecting data, it forces you to think about how data “flows” through your application and implicitly on the threads you’re using. This is what a channel is for and one of the languages that offer it is Go. A channel feels very natural to use, hiding a lower-level implementation of using data across threads.

And this is something I wanted to implement in C++ when I found out what the standard library offers for multi-threading. Why? Just to practice thread-safe C++. A channel has a far more complex and different implementation than you’ll find here. What I have done is a synchronized queue with a channel feeling.

I wanted to have a container that is very easy to use. Data should get in on some threads and come out on some other threads, and the operations must be thread-safe:

int in = 1;
in >> channel;

int out = 0;
out << channel; // out is 2

This is the most common and simple use case.

Another common situation is to continuously read data from a channel:

while (true) {
    out << channel;
}

Or, for a better C++ approach, using a range-based for loop:

for (auto out : channel) {
    // do something with "out"
}

Continue reading C++ Channel: A thread-safe container for sharing data between threads

GCC bug in noexcept operator

When something goes wrong, the first thing is to ask myself what did I do wrong. I don’t like just throwing the blame. And when I saw the not declared in this scope error related to the noexcept operator,  I was sure I was missing something. Maybe I had accidentally used a compiler extension, maybe I used something that’s undefined behavior. But I didn’t think it could be a GCC noexcept compiler bug.

I have a container that encapsulates a vector, for which I wanted to have a noexcept swap method. Ignore the implementation itself, the idea is I wanted to declare the method noexcept if the swap operations on the items I wanted to swap are also noexcept. It can be any other case that implies noexcept.

#include <vector>
#include <utility>
#include <cassert>

struct Container {
    std::vector<int> elements{};
    std::size_t count{};

    void swap(Container &c) noexcept(noexcept(elements.swap(c.elements)) && noexcept(std::swap(count, c.count))) {
        elements.swap(c.elements);
        std::swap(count, c.count);
    }
};

int main() {
    Container a{{1, 2}, 2};
    Container b{{3, 4, 5}, 3};

    a.swap(b);

    assert(a.elements == (std::vector<int>{3, 4, 5}));
    assert(a.count == 3);

    assert(b.elements == (std::vector<int>{1, 2}));
    assert(b.count == 2);
}

 

This tells that Container’s swap is noexcept if both swaps of elements and count are noexcept:

void swap(Container &c) noexcept(noexcept(elements.swap(c.elements)) && noexcept(std::swap(count, c.count)));

 

MSVC and Clang compile this, but GCC needs a newer version because on older ones it has the bug [DR 1207] “this” not being allowed in noexcept clauses, which I’ve found in this discussion.

If you see one of the following errors, try to update your compiler:

    • ‘elements’ was not declared in this scope
    • invalid use of incomplete type ‘struct Container’
    • invalid use of ‘this’ at top level