User-defined literals for YGValue

Summary:
@public

Adds `_pt` and `_percent` user defined literals to create `YGValue` instances.

This allows to create `YGValue`s in the following form:

```
use namespace facebook::yoga::literals;

auto a = 123_pt; // == YGValue{123.0f, YGUnitPoint}
auto b = -12.5_percent; // == YGValue{-12.5f, YGUnitPercent}
```

Reviewed By: SidharthGuglani

Differential Revision: D13942100

fbshipit-source-id: ce1e2f9431c3e2a99c6e11896a712539cc535e0d

tests/YGValueTest.cpp

@@ -0,0 +1,48 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the LICENSE
+ * file in the root directory of this source tree.
+ */
+#include <gtest/gtest.h>
+
+#include <yoga/YGValue.h>
+
+TEST(YGValue, supports_equality) {
+  ASSERT_EQ((YGValue{12.5f, YGUnitPercent}), (YGValue{12.5f, YGUnitPercent}));
+  ASSERT_NE((YGValue{12.5f, YGUnitPercent}), (YGValue{56.7f, YGUnitPercent}));
+  ASSERT_NE((YGValue{12.5f, YGUnitPercent}), (YGValue{12.5f, YGUnitPoint}));
+  ASSERT_NE((YGValue{12.5f, YGUnitPercent}), (YGValue{12.5f, YGUnitAuto}));
+  ASSERT_NE((YGValue{12.5f, YGUnitPercent}), (YGValue{12.5f, YGUnitUndefined}));
+
+  ASSERT_EQ(
+      (YGValue{12.5f, YGUnitUndefined}),
+      (YGValue{YGUndefined, YGUnitUndefined}));
+  ASSERT_EQ((YGValue{0, YGUnitAuto}), (YGValue{-1, YGUnitAuto}));
+}
+
+using namespace facebook::yoga::literals;
+
+TEST(YGValue, supports_double_point_literals) {
+  ASSERT_EQ(12.5_pt, (YGValue{12.5, YGUnitPoint}));
+}
+
+TEST(YGValue, supports_double_percent_literals) {
+  ASSERT_EQ(12.5_percent, (YGValue{12.5, YGUnitPercent}));
+}
+
+TEST(YGValue, supports_integral_point_literals) {
+  ASSERT_EQ(34_pt, (YGValue{34, YGUnitPoint}));
+}
+
+TEST(YGValue, supports_integral_percent_literals) {
+  ASSERT_EQ(125_percent, (YGValue{125, YGUnitPercent}));
+}
+
+TEST(YGValue, supports_negative_point_literals) {
+  ASSERT_EQ(-34.5_pt, (YGValue{-34.5, YGUnitPoint}));
+}
+
+TEST(YGValue, supports_negative_percent_literals) {
+  ASSERT_EQ(-34.5_percent, (YGValue{-34.5, YGUnitPercent}));
+}

yoga/YGValue.h

@@ -54,4 +54,30 @@ inline bool operator!=(const YGValue& lhs, const YGValue& rhs) {
   return !(lhs == rhs);
 }
 
+inline YGValue operator-(const YGValue& value) {
+  return {-value.value, value.unit};
+}
+
+namespace facebook {
+namespace yoga {
+namespace literals {
+
+inline YGValue operator"" _pt(long double value) {
+  return YGValue{static_cast<float>(value), YGUnitPoint};
+}
+inline YGValue operator"" _pt(unsigned long long value) {
+  return operator"" _pt(static_cast<long double>(value));
+}
+
+inline YGValue operator"" _percent(long double value) {
+  return YGValue{static_cast<float>(value), YGUnitPercent};
+}
+inline YGValue operator"" _percent(unsigned long long value) {
+  return operator"" _percent(static_cast<long double>(value));
+}
+
+} // namespace literals
+} // namespace yoga
+} // namespace facebook
+
 #endif