Initialize static atomic member variable

Question

I would like to generate identifiers for a class named order in a threadsafe manner. The code below does not compile. I know that the atomic types do not have copy constructors, and I assume that explains why this code does not work. Does anybody know a way to actually get this code to work? I'm still learning, so please also let me know if I'm on the wrong track (if so, I would appreciate it if you could point me to an alternative approach). Thanks!

#include <atomic>
#include <iostream>

class order {
public: 
    order() { id=c.fetch_add(1); }
    int id;
private:
    static std::atomic<int> c;
};

std::atomic<int> order::c = std::atomic<int>(0);

int main() {
    order *o1 = new order();
    order *o2 = new order();
    std::cout << o1->id << std::endl; // Expect 0
    std::cout << o2->id << std::endl; // Expect 1
}

Compiling the above results in the following error:

order.cpp:45:51: error: use of deleted function 
        ‘std::atomic<int>::atomic(const std::atomic<int>&)’
In file included from order.cpp:3:0:
/usr/include/c++/4.7/atomic:594:7: error: declared here

Solution 1

I know that the atomic types do not have copy constructors, and I assume that explains why this code does not work.

Yes, the error says that quite clearly.

Does anybody know a way to actually get this code to work?

Instead of copy-initialising from a temporary, which requires an accessible copy constructor:

std::atomic<int> order::c = std::atomic<int>(0);

use direct-initialisation, which doesn't:

std::atomic<int> order::c(0);   // or {0} for a more C++11 experience

You should probably prefer that anyway, unless you enjoy reading unnecessarily verbose code.

Solution 2

How about the definition

std::atomic<int> order::c{0}

Solution 3

Also you can use atomic_init:

std::atomic<int> data;
std::atomic_init(&data, 0);

Comments

[Teisman]

Thanks a lot Joachim! I've upvoted your answer but I'll accept Mike's as it is a bit more verbose. Hope you don't mind! ;)

[Sergei Krivonos]

this would not be initialized during compilation?

[anicicn]

Isn't static variable thread safe from c++11? I see the ++11 tag up in the question. So why do we need atomics on the first place? Why can't we just use regular int?