CODEWITHSUNDEEP
c++c++11constantsatomicstdatomic
Teisman
Teisman

Reputation: 1358

How to initialize a static std::atomic data member

I would like to generate identifiers for a class named order in a thread-safe 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? Is there an alternative approach?

#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

Upvotes: 37

Views: 60992

Answers (4)

user7610
user7610

Reputation: 28869

Since std::atomic_init has been deprecated in C++20, here is a reimplementation which does not raise deprecation warnings, if you for some reason want to keep doing this.

static inline void sys_atomic_init(sys_atomic_t *ref, uint32_t value)
{
#ifdef __cplusplus
    // atomic_init is deprecated in C++20
    // use the equivalent definition from https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0883r2.pdf
    atomic_store_explicit(ref, value, std::memory_order_relaxed);
#else
    atomic_init(ref, value);
#endif
}

from https://github.com/skupperproject/skupper-router/blob/cfc8b4c6892dafc5dd0c86682b4db87a488f2451/include/qpid/dispatch/atomic.h#L41

Upvotes: 0

anhldbk
anhldbk

Reputation: 4587

Also you can use atomic_init:

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

Upvotes: 6

Mike Seymour
Mike Seymour

Reputation: 254631

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.

Upvotes: 71

Some programmer dude
Some programmer dude

Reputation: 409356

How about the definition

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

Upvotes: 34

Related Questions