Add Sk version of std::exchange

src/core/SkExchange.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkExchange_DEFINED
+#define SkExchange_DEFINED
+
+namespace skstd {
+
+// std::exchange is in C++14
+template <typename T, typename U> inline T exchange(T& obj, U&& newValue) {

[mtklein_C, 2016/09/29 18:11:02]

Any idea why the standard does <typename T, typename U=T> ?

Should probably make this static, no?

[csmartdalton, 2016/09/29 19:19:22]

I think it's to avoid ambiguous overloads, e.g.:

exchange(some_ptr, nullptr);
I understood that "static" and "inline" had no effect on template methods.
(Which should raise the question on why I marked it inline but not static :)

Is this suggestion just for stylistic reasons?

[mtklein_C, 2016/09/29 19:36:39]

Generally we make all methods declared in headers static, so that they're not
exported from every compilation unit that they're compiled into.  That's true of
templates too: if a template is marked static, all the concrete instances of
that template will be static.

+    T oldValue(obj);

[mtklein_C, 2016/09/29 18:11:02]

Shouldn't we std::move obj?

[csmartdalton, 2016/09/29 19:19:22]

Done.

+    obj = newValue;

[mtklein_C, 2016/09/29 18:11:02]

and std::forward newValue?

[csmartdalton, 2016/09/29 19:19:22]

Done.

+    return oldValue;
+}
+
+}
+
+#endif  // SkExchange_DEFINED