api(memory.h): default_init_allocator/default_init_vector (#5169)

Move this from internal-only in imageio_pvt.h to be public in memory.h.
It's useful and we've used it in limited ways for a while without
needing any changes. It seems ready for prime-time (for example,
allowing it to be used in reader/writer plugins). None of the logic
changes, it's just moving from a private header to a public header and
out of the 'pvt' sub-namespace.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/include/OpenImageIO/memory.h

@@ -127,6 +127,79 @@ footprint(const std::vector<T>& vec)
 }  // namespace pvt
 
 
+/// Allocator adaptor that interposes construct() calls to convert value
+/// initialization into default initialization.
+///
+/// This is a way to achieve a std::vector whose resize does not force a value
+/// initialization of every allocated element. Put in more plain terms, the
+/// following:
+///
+///     std::vector<int> v(Nlarge);
+///
+/// will zero-initialize the Nlarge elements, which may be a cost we do not
+/// wish to pay, particularly when allocating POD types that we are going to
+/// write over anyway. Sometimes we do the following instead:
+///
+///     std::unique_ptr<int[]> v (new int[Nlarge]);
+///
+/// which does not zero-initialize the elements. But it's more awkward, and
+/// lacks the methods you get automatically with vectors.
+///
+/// But you will get a lack of forced value initialization if you use a
+/// std::vector with a special allocator that does default initialization.
+/// This is such an allocator, so the following:
+///
+///    std::vector<T, default_init_allocator<T>> v(Nlarge);
+///
+/// will have the same performance characteristics as the new[] version.
+///
+/// For details:
+/// https://stackoverflow.com/questions/21028299/is-this-behavior-of-vectorresizesize-type-n-under-c11-and-boost-container/21028912#21028912
+///
+template<typename T, typename A = std::allocator<T>>
+class default_init_allocator : public A {
+    typedef std::allocator_traits<A> a_t;
+
+public:
+    template<typename U> struct rebind {
+        using other
+            = default_init_allocator<U, typename a_t::template rebind_alloc<U>>;
+    };
+
+    using A::A;
+
+    template<typename U>
+    void
+    construct(U* ptr) noexcept(std::is_nothrow_default_constructible<U>::value)
+    {
+        ::new (static_cast<void*>(ptr)) U;
+    }
+    template<typename U, typename... Args>
+    void construct(U* ptr, Args&&... args)
+    {
+        a_t::construct(static_cast<A&>(*this), ptr,
+                       std::forward<Args>(args)...);
+    }
+};
+
+
+/// Type alias for a std::vector that uses the default_init_allocator.
+///
+/// Consider using a `default_init_vector<T>` instead of `std::vector<T>` when
+/// all of the following are true:
+///
+/// * The use is entirely internal to OIIO (since at present, this type is
+///   not defined in any public header files).
+/// * The type T is POD (plain old data) or trivially constructible.
+/// * The vector is likely to be large enough that the cost of default
+///   initialization is worth trying to avoid.
+/// * After allocation, the vector will be filled with data before any reads
+///   are attempted, so the default initialization is not needed.
+///
+template<typename T>
+using default_init_vector = std::vector<T, default_init_allocator<T>>;
+
+
 OIIO_NAMESPACE_3_1_END
 
 
@@ -137,5 +210,7 @@ namespace pvt {
 using v3_1::pvt::footprint;
 using v3_1::pvt::heapsize;
 }  // namespace pvt
+using v3_1::default_init_allocator;
+using v3_1::default_init_vector;
 #endif
 OIIO_NAMESPACE_END

src/include/imageio_pvt.h

@@ -282,80 +282,6 @@ device_free(void* mem);
 
 
 
-/// Allocator adaptor that interposes construct() calls to convert value
-/// initialization into default initialization.
-///
-/// This is a way to achieve a std::vector whose resize does not force a value
-/// initialization of every allocated element. Put in more plain terms, the
-/// following:
-///
-///     std::vector<int> v(Nlarge);
-///
-/// will zero-initialize the Nlarge elements, which may be a cost we do not
-/// wish to pay, particularly when allocating POD types that we are going to
-/// write over anyway. Sometimes we do the following instead:
-///
-///     std::unique_ptr<int[]> v (new int[Nlarge]);
-///
-/// which does not zero-initialize the elements. But it's more awkward, and
-/// lacks the methods you get automatically with vectors.
-///
-/// But you will get a lack of forced value initialization if you use a
-/// std::vector with a special allocator that does default initialization.
-/// This is such an allocator, so the following:
-///
-///    std::vector<T, default_init_allocator<T>> v(Nlarge);
-///
-/// will have the same performance characteristics as the new[] version.
-///
-/// For details:
-/// https://stackoverflow.com/questions/21028299/is-this-behavior-of-vectorresizesize-type-n-under-c11-and-boost-container/21028912#21028912
-///
-template<typename T, typename A = std::allocator<T>>
-class default_init_allocator : public A {
-    typedef std::allocator_traits<A> a_t;
-
-public:
-    template<typename U> struct rebind {
-        using other
-            = default_init_allocator<U, typename a_t::template rebind_alloc<U>>;
-    };
-
-    using A::A;
-
-    template<typename U>
-    void
-    construct(U* ptr) noexcept(std::is_nothrow_default_constructible<U>::value)
-    {
-        ::new (static_cast<void*>(ptr)) U;
-    }
-    template<typename U, typename... Args>
-    void construct(U* ptr, Args&&... args)
-    {
-        a_t::construct(static_cast<A&>(*this), ptr,
-                       std::forward<Args>(args)...);
-    }
-};
-
-
-/// Type alias for a std::vector that uses the default_init_allocator.
-///
-/// Consider using a `default_init_vector<T>` instead of `std::vector<T>` when
-/// all of the following are true:
-///
-/// * The use is entirely internal to OIIO (since at present, this type is
-///   not defined in any public header files).
-/// * The type T is POD (plain old data) or trivially constructible.
-/// * The vector is likely to be large enough that the cost of default
-///   initialization is worth trying to avoid.
-/// * After allocation, the vector will be filled with data before any reads
-///   are attempted, so the default initialization is not needed.
-///
-template<typename T>
-using default_init_vector = std::vector<T, default_init_allocator<T>>;
-
-
-
 OIIO_NAMESPACE_END
 
 #endif  // OPENIMAGEIO_IMAGEIO_PVT_H