std::barrier is_nothrow_invocable_v<CompletionFunction&> shall be true error - c++

Question

I am trying to simulate a dice roll program, to be more specific get the number of rolls necessary to get n dices to be 6 at the same time.
So this is a textbook example of fork-join model in order to simulate rolls simultaneously and then check result after each iteration.
#include <iostream>
#include <vector>
#include <thread>
#include <random>
#include <array>
#include <barrier>
auto random_int(int min, int max)
{
static thread_local auto engine = std::default_random_engine{ std::random_device{}() };
auto dist = std::uniform_int_distribution<>(min, max);
return dist(engine);
}
int main()
{
constexpr auto n = 5;
auto done = false;
auto dice = std::array<int, n>{};
auto threads = std::vector<std::thread>{};
auto n_turns = 0;
auto check_result = [&]
{
++n_turns;
auto is_six = [](int i) {return 6 == i; };
done = std::all_of(std::begin(dice), std::end(dice), is_six);
};
auto bar = std::barrier{ n,check_result };
for (int i = 0; i < n; ++i)
{
threads.emplace_back([&, i]{
while (!done)
{
dice[i] = random_int(1, 6);
bar.arrive_and_wait();
}});
}
for (auto&& t : threads)
{
t.join();
}
std::cout << n_turns << std::endl;
}
And I am getting the following error:
error C2338: N4861 [thread.barrier.class]/5:
is_nothrow_invocable_v<CompletionFunction&> shall be true
1>C:\Users\eduar\source\repos\C++20\C++20\main.cpp(114): message : see
reference to class template instantiation
'std::barriermain::<lambda_1>' being compiled
Can someone please hint what am I doing wrong and how to fix this?

Answer 1

The issue is in the error message. Which is great, even cites exactly the part of the standard which has this requirement: [thread.barrier.class]/5:
CompletionFunction shall meet the Cpp17MoveConstructible (Table 28) and Cpp17Destructible (Table 32) requirements.
is_­nothrow_­invocable_­v<CompletionFunction&> shall be true.
You're currently missing the last part: your lambda isn't nothrow-invocable. That's an easy fix tho:
auto check_result = [&]() noexcept
// ^~~~~~~~
{
++n_turns;
auto is_six = [](int i) {return i == 6; };
done = std::all_of(std::begin(dice), std::end(dice), is_six);
};
I also took the opportunity to flip your Yoda conditional, because there is no reason to write Yoda conditionals.