Why does this simple thread code fail?

Question

I'm trying to make it do that I can't call threads from a loop. But when I run it I get a runtime error:

terminate called after throwing an instance of 'std::system_error'
what(): Invalid argument Thread #1

#include <iostream>
#include <vector>
#include <memory>
#include <thread>
#include <mutex>

std::mutex m;
static int thread_count;

auto foo = [&] {
    std::lock_guard<std::mutex> lock(m);
    std::cout << "Thread #" << ++thread_count << std::endl;
};

int main()
{
    std::vector<std::shared_ptr<std::thread>>
             threads(20, std::make_shared<std::thread>(foo));

    for (const auto& th : threads)
        th->join();
}

Answer 1

Your code only actually creates one child thread, and therefore it calls join() on that one thread 20 times.

To verify this, you could add the following loop right after you construct the vector:

for (int i=1; i<threads.size(); ++i)
    assert(threads[i - 1].get() == threads[i].get());

You likely want to create your vector with some form of:

std::vector<std::shared_ptr<std::thread>> threads(20);
for (auto & thread : threads)
    thread = std::make_shared<std::thread>(foo);