add SkVptr()

I was wondering how feasible using this to make downcasts safe would be.
These tests would need to build and pass on all our bots, at least.

Change-Id: I1753ba58841bf6c17d6ac3af7374518356e1bb05
Reviewed-on: https://skia-review.googlesource.com/81180
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

gn/tests.gni

@@ -274,6 +274,7 @@ tests_sources = [
   "$_tests/VkHeapTests.cpp",
   "$_tests/VkUploadPixelsTests.cpp",
   "$_tests/VkWrapTests.cpp",
+  "$_tests/VptrTest.cpp",
   "$_tests/WindowRectanglesTest.cpp",
   "$_tests/WritePixelsTest.cpp",
   "$_tests/Writer32Test.cpp",

src/core/SkVptr.h

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2017 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkVptr_DEFINED
+#define SkVptr_DEFINED
+
+#include <string.h>
+#include <type_traits>
+
+// Experimentally, see if we can get at the vptr of objects with one.
+
+template <typename T>
+static inline void* SkVptr(const T& object) {
+    static_assert(std::has_virtual_destructor<T>::value, "");
+    void* vptr;
+    memcpy(&vptr, (const void*)&object, sizeof(vptr));
+    return vptr;
+}
+
+#endif//SkVptr_DEFINED

tests/VptrTest.cpp

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2017 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkMakeUnique.h"
+#include "SkVptr.h"
+#include "Test.h"
+
+namespace {
+
+    struct Base {
+        virtual ~Base() = default;
+        virtual size_t val() const = 0;
+    };
+
+    struct SubclassA : public Base {
+        SubclassA(size_t val) : fVal(val) {}
+
+        size_t val() const override { return fVal; }
+
+        size_t fVal;
+    };
+
+    struct SubclassB : public Base {
+        SubclassB() {}
+
+        size_t val() const override { return 42; }
+    };
+
+}
+
+DEF_TEST(Vptr, r) {
+    std::unique_ptr<Base> a = skstd::make_unique<SubclassA>(21),
+                          b = skstd::make_unique<SubclassB>(),
+                          c = skstd::make_unique<SubclassA>(22),
+                          d = skstd::make_unique<SubclassB>();
+
+    // These 4 objects all have unique identities.
+    REPORTER_ASSERT(r, a != b);
+    REPORTER_ASSERT(r, a != c);
+    REPORTER_ASSERT(r, a != d);
+    REPORTER_ASSERT(r, b != c);
+    REPORTER_ASSERT(r, b != d);
+    REPORTER_ASSERT(r, c != d);
+
+    // Only b and d have the same val().
+    REPORTER_ASSERT(r, a->val() != b->val());
+    REPORTER_ASSERT(r, a->val() != c->val());
+    REPORTER_ASSERT(r, a->val() != d->val());
+    REPORTER_ASSERT(r, b->val() != c->val());
+    REPORTER_ASSERT(r, b->val() == d->val());
+    REPORTER_ASSERT(r, c->val() != d->val());
+
+    // SkVptr() returns the same value for objects of the same concrete type.
+    REPORTER_ASSERT(r, SkVptr(*a) == SkVptr(*c));
+    REPORTER_ASSERT(r, SkVptr(*b) == SkVptr(*d));
+    REPORTER_ASSERT(r, SkVptr(*a) != SkVptr(*b));
+}