Add constexpr + noexcept for atomic

[viboes]

Make boost::atomic behave as C++11 concerning constexpr and noexcept.

In particular

atomic() noexcept = default;
constexpr atomic(T) noexcept;

Note that the constructor from T is not explicit.

[timblechmann]

hm, seems that the codebase will require some refactoring to match the c++11 API. will have a look, but it might take me some time.

[viboes]

Hi,

The following patch worked for me.

svn diff boost/atomic
Index: boost/atomic/detail/base.hpp
===================================================================
--- boost/atomic/detail/base.hpp	(revision 82860)
+++ boost/atomic/detail/base.hpp	(working copy)
@@ -143,7 +143,7 @@
     typedef T value_type;
     typedef lockpool::scoped_lock guard_type;
 public:
-    base_atomic(void) {}
+    BOOST_CONSTEXPR base_atomic(void) {}
 
     explicit base_atomic(const value_type & v)
     {
@@ -230,8 +230,8 @@
     typedef T difference_type;
     typedef lockpool::scoped_lock guard_type;
 public:
-    explicit base_atomic(value_type v) : v_(v) {}
-    base_atomic(void) {}
+    explicit BOOST_CONSTEXPR base_atomic(value_type v) : v_(v) {}
+    BOOST_CONSTEXPR base_atomic(void) {}
 
     void
     store(value_type v, memory_order /*order*/ = memory_order_seq_cst) volatile
Index: boost/atomic/atomic.hpp
===================================================================
--- boost/atomic/atomic.hpp	(revision 82860)
+++ boost/atomic/atomic.hpp	(working copy)
@@ -88,8 +88,8 @@
     typedef T value_type;
     typedef atomics::detail::base_atomic<T, typename atomics::detail::classify<T>::type, atomics::detail::storage_size_of<T>::value, boost::is_signed<T>::value > super;
 public:
-    atomic(void) : super() {}
-    explicit atomic(const value_type & v) : super(v) {}
+    BOOST_CONSTEXPR atomic(void) : super() {}
+    explicit BOOST_CONSTEXPR atomic(const value_type & v) : super(v) {}
 
     atomic & operator=(value_type v) volatile
     {

But I suspect that we need to specialize boost::atomic<integral>.

BTW, why the construtor needs a memcpy?

    explicit base_atomic(const value_type & v)
    {
        memcpy(&v_, &v, sizeof(value_type));
    }

[timblechmann]

i've started working on a patch, but i'm not sure how complete it is.

• not sure about keeping the explicit. according to the standard, there is no explicit ctor.
• also the standard says: 'call by value', not 'call by const-reference'
• regarding the memcpy, i think it is better to cast to the storage type and let the compiler do the copying. memcpy might be slightly better as it probably has relaxed aliasing constraints (not sure if the compiler is smart enough that object and member are different objects). why memcpy was used in the first place, no idea ... the original author might know

[viboes]

Sorry, the preceding patch didn't worked at all. See attached patch that works for

static BOOST_CONSTEXPR boost::atomic<int> x(1);

[viboes]

Thanks for the change [82870] :)

[Andrey Semashev]

The memcpy is needed when the value type is a struct. The struct size may not match exactly the storage_type (e.g. struct s { char a[ 3 ]; };), so we have to zero-initialize the storage first and then copy the struct into it with memcpy. This also takes care of the alignment issue as a side effect.

[Andrey Semashev]

Should we close this ticket? In the latest library version everything is marked noexcept and constexpr (except for user-defined types and pointers).