headless: Add a Maybe-type

Add a Maybe-type which will be used by the C++ DevTools client API for
representing optional values.

BUG=595353

Review URL: https://codereview.chromium.org/1866063002

Cr-Commit-Position: refs/heads/master@{#385797}

headless/BUILD.gn

@@ -84,6 +84,7 @@ static_library("headless_lib") {
     "public/headless_browser.h",
     "public/headless_export.h",
     "public/headless_web_contents.h",
+    "public/util/maybe.h",
   ]
 
   deps = [
@@ -110,6 +111,19 @@ group("headless_tests") {
 
   deps = [
     ":headless_browsertests",
+    ":headless_unittests",
+  ]
+}
+
+test("headless_unittests") {
+  sources = [
+    "public/util/maybe_unittest.cc",
+  ]
+
+  deps = [
+    "//base/test:run_all_unittests",
+    "//base/test:test_support",
+    "//testing/gtest",
   ]
 }
 

headless/public/util/maybe.h

@@ -0,0 +1,92 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef HEADLESS_PUBLIC_UTIL_MAYBE_H_
+#define HEADLESS_PUBLIC_UTIL_MAYBE_H_
+
+#include <algorithm>
+
+#include "base/logging.h"
+#include "base/macros.h"
+
+namespace headless {
+
+// A simple Maybe which may or may not have a value. Based on v8::Maybe.
+template <typename T>
+class Maybe {
+ public:
+  Maybe() : has_value_(false) {}
+
+  bool IsNothing() const { return !has_value_; }
+  bool IsJust() const { return has_value_; }
+
+  // Will crash if the Maybe<> is nothing.
+  T& FromJust() {
+    DCHECK(IsJust());
+    return value_;
+  }
+  const T& FromJust() const {
+    DCHECK(IsJust());
+    return value_;
+  }
+
+  T FromMaybe(const T& default_value) const {
+    return has_value_ ? value_ : default_value;
+  }
+
+  bool operator==(const Maybe& other) const {
+    return (IsJust() == other.IsJust()) &&
+           (!IsJust() || FromJust() == other.FromJust());
+  }
+
+  bool operator!=(const Maybe& other) const { return !operator==(other); }
+
+  Maybe& operator=(Maybe&& other) {
+    has_value_ = other.has_value_;
+    value_ = std::move(other.value_);
+    return *this;
+  }
+
+  Maybe& operator=(const Maybe& other) {
+    has_value_ = other.has_value_;
+    value_ = other.value_;
+    return *this;
+  }
+
+  Maybe(const Maybe& other) = default;
+  Maybe(Maybe&& other) = default;
+
+ private:
+  template <class U>
+  friend Maybe<U> Nothing();
+  template <class U>
+  friend Maybe<U> Just(const U& u);
+  template <class U>
+  friend Maybe<typename std::remove_reference<U>::type> Just(U&& u);
+
+  explicit Maybe(const T& t) : has_value_(true), value_(t) {}
+  explicit Maybe(T&& t) : has_value_(true), value_(std::move(t)) {}
+
+  bool has_value_;
+  T value_;
+};
+
+template <class T>
+Maybe<T> Nothing() {
+  return Maybe<T>();
+}
+
+template <class T>
+Maybe<T> Just(const T& t) {
+  return Maybe<T>(t);
+}
+
+template <class T>
+Maybe<typename std::remove_reference<T>::type> Just(T&& t) {
+  return Maybe<typename std::remove_reference<T>::type>(std::move(t));
+}
+
+}  // namespace headless
+
+#endif  // HEADLESS_PUBLIC_UTIL_MAYBE_H_

headless/public/util/maybe_unittest.cc

@@ -0,0 +1,83 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "headless/public/util/maybe.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace headless {
+namespace {
+
+class MoveOnlyType {
+ public:
+  MoveOnlyType() {}
+  MoveOnlyType(MoveOnlyType&& other) {}
+
+  void operator=(MoveOnlyType&& other) {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MoveOnlyType);
+};
+
+}  // namespace
+
+TEST(MaybeTest, Nothing) {
+  Maybe<int> maybe;
+  EXPECT_TRUE(maybe.IsNothing());
+  EXPECT_FALSE(maybe.IsJust());
+}
+
+TEST(MaybeTest, Just) {
+  Maybe<int> maybe = Just(1);
+  EXPECT_FALSE(maybe.IsNothing());
+  EXPECT_TRUE(maybe.IsJust());
+  EXPECT_EQ(1, maybe.FromJust());
+
+  const Maybe<int> const_maybe = Just(2);
+  EXPECT_EQ(2, const_maybe.FromJust());
+}
+
+TEST(MaybeTest, Equality) {
+  Maybe<int> a;
+  Maybe<int> b;
+  EXPECT_EQ(a, b);
+  EXPECT_EQ(b, a);
+
+  a = Just(1);
+  EXPECT_NE(a, b);
+  EXPECT_NE(b, a);
+
+  b = Just(2);
+  EXPECT_NE(a, b);
+  EXPECT_NE(b, a);
+
+  b = Just(1);
+  EXPECT_EQ(a, b);
+  EXPECT_EQ(b, a);
+}
+
+TEST(MaybeTest, Assignment) {
+  Maybe<int> a = Just(1);
+  Maybe<int> b = Nothing<int>();
+  EXPECT_NE(a, b);
+
+  b = a;
+  EXPECT_EQ(a, b);
+}
+
+TEST(MaybeTest, MoveOnlyType) {
+  MoveOnlyType value;
+  Maybe<MoveOnlyType> a = Just(std::move(value));
+  EXPECT_TRUE(a.IsJust());
+
+  Maybe<MoveOnlyType> b = Just(MoveOnlyType());
+  EXPECT_TRUE(b.IsJust());
+
+  Maybe<MoveOnlyType> c = Nothing<MoveOnlyType>();
+  c = std::move(a);
+  EXPECT_TRUE(c.IsJust());
+
+  MoveOnlyType d = std::move(b.FromJust());
+}
+
+}  // namespace headless