Error with copy constructor/assignment operator for a class which has std::atomic member variable

Question

I have a class like below.

#include <atomic>  static const long  myValue = 0;  class Sequence {  public:      Sequence(long initial_value = myValue) : value_(initial_value) {}   private:       std::atomic<long> value_; };  int main() {          Sequence firstSequence;          Sequence secondSequence = firstSequence;          return 0; } 

I am getting compilation error like this,

test.cpp:21:36: error: use of deleted function ‘Sequence::Sequence(const Sequence&)’ test.cpp:5:7: error: ‘Sequence::Sequence(const Sequence&)’ is implicitly deleted because the default definition would be ill-formed: test.cpp:5:7: error: use of deleted function ‘std::atomic<long int>::atomic(const std::atomic<long int>&)’ 

Is that the default copy constructor and assignment opertaor do not work in such case?

PS: I am using gcc version 4.6.3

Answer 1

You can't copy atomics with a standard copy constructor, since all loads and stores must happen explicitly. You'll have to write your own copy constructor for Sequence which does some initialization of the form value_(rhs.value_.load()) (possibly with more relaxed memory ordering).

Answer 2

Atomic has deleted copy-ctor. So copy/move-ctors in your class are deleted.

n3337 12.8/11

An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/ move constructor for a class X is defined as deleted (8.4.3) if X has:

— a non-static data member of class type M (or array thereof) that cannot be copied/moved because overload resolution (13.3), as applied to M’s corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,