From 7bb277ec5f952f83bac95f2cc109e2e0007e3a7f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Peter=20=C5=BDu=C5=BEek?= <peter@codeplay.com>
Date: Thu, 24 Jan 2019 18:33:46 +0000
Subject: [PATCH 1/2] Default constructed accessors

---
 placeholder_accessors/index.md | 50 ++++++++++++++++++++++++++++++++--
 1 file changed, 48 insertions(+), 2 deletions(-)

diff --git a/placeholder_accessors/index.md b/placeholder_accessors/index.md
index ed8fe92..8326be2 100644
--- a/placeholder_accessors/index.md
+++ b/placeholder_accessors/index.md
@@ -9,7 +9,7 @@
 | Reply-to | Ruyman Reyes <ruyman@codeplay.com> |
 | Original author | Ruyman Reyes <ruyman@codeplay.com> |
 | Requirements | CP001 |
-| Contributors | Gordon Brown <gordon@codeplay.com>, Victor Lomuller <victor@codeplay.com>, Mehdi Goli <mehdi.goli@codeplay.com>, Peter Zuzek <peter@codeplay.com>, Luke Iwanski <luke@codeplay.com> |
+| Contributors | Gordon Brown <gordon@codeplay.com>, Victor Lomuller <victor@codeplay.com>, Mehdi Goli <mehdi.goli@codeplay.com>, Peter Žužek <peter@codeplay.com>, Luke Iwanski <luke@codeplay.com>, Michael Haidl <michael.haidl@uni-muenster.de> |
 
 ## Overview
 
@@ -172,7 +172,8 @@ except for the aforementioned modifications.
 
 ### When `is_placeholder` returns true
 
-The accessor API features constructors that don't take the handler parameter.
+The accessor API features constructors that don't take the handler parameter
+and/or memory object as a constructor.
 
 In addition, a new method to obtain a normal accessor from the placeholder
 accessor is provided.
@@ -192,6 +193,51 @@ The handler gains a new method,
 that registers the requirements of the placeholder accessor on the given
 command group.
 
+### Placeholder `accessor` without a buffer
+
+Not providing either the handler or the memory object
+makes the accessor default constructible,
+creating a "null" accessor.
+A null accessor can be bound to a buffer
+by constructing a new placeholder accessor with the buffer
+and assigning it to the null accessor.
+Example:
+
+```cpp
+accessor<T, dim, mode, target, access::placeholer::true_t> acc0;
+
+acc0 = accessor<T, dim, mode, target, access::placeholer::true_t>(buf);
+```
+
+To simplify placeholder accessor construction,
+a new free function `cl::sycl::codeplay::make_placeholder_accessor` is provided,
+which takes a buffer as an argument:
+
+```cpp
+template <access::mode mode, access::target target, typename T, int dim,
+          typename AllocatorT>
+accessor<T, dim, mode, target, access::placeholder::true_t>
+make_placeholder_accessor(buffer<T, dim, AllocatorT>& bufferRef);
+```
+
+A null accessor does not need to be registered with the command group,
+i.e. there is no need to call `require()` on it.
+It is allowed to pass a null accessor to a kernel,
+as long as it's not dereferenced inside the kernel,
+otherwise there an out-of-bounds access will occur,
+likely leading to a segmentation fault or similar.
+If the null accessor has been bound to a buffer,
+it is no longer a null accessor,
+which means it has to be registered with the command group
+and can be dereferenced inside the kernel.
+
+An accessor can be checked for the existence of an associated buffer
+by calling the member function `is_null`.
+
+|Member function      |Description                                              |
+|---------------------|---------------------------------------------------------|
+|bool is_null() const |Returns true if the accessor is associated with a buffer, false otherwise.|
+
 
 [1]: https://github.com/codeplaysoftware/sycl-blas "SYCL-BLAS"
 [2]: https://github.com/lukeiwanski/tensorflow "TensorFlow/Eigen"

From b8a9189d73daf1c089ac0585aba2d7acda6b84e6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Peter=20=C5=BDu=C5=BEek?= <peter@codeplay.com>
Date: Fri, 25 Jan 2019 11:48:16 +0000
Subject: [PATCH 2/2] Minor fixes

---
 placeholder_accessors/index.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/placeholder_accessors/index.md b/placeholder_accessors/index.md
index 8326be2..2289250 100644
--- a/placeholder_accessors/index.md
+++ b/placeholder_accessors/index.md
@@ -224,7 +224,7 @@ A null accessor does not need to be registered with the command group,
 i.e. there is no need to call `require()` on it.
 It is allowed to pass a null accessor to a kernel,
 as long as it's not dereferenced inside the kernel,
-otherwise there an out-of-bounds access will occur,
+otherwise an out-of-bounds access will occur,
 likely leading to a segmentation fault or similar.
 If the null accessor has been bound to a buffer,
 it is no longer a null accessor,
@@ -236,7 +236,7 @@ by calling the member function `is_null`.
 
 |Member function      |Description                                              |
 |---------------------|---------------------------------------------------------|
-|bool is_null() const |Returns true if the accessor is associated with a buffer, false otherwise.|
+|bool is_null() const |Returns false if the accessor is associated with a buffer, true otherwise.|
 
 
 [1]: https://github.com/codeplaysoftware/sycl-blas "SYCL-BLAS"