folly::Try: add value_or() implementations mimicking std::optional::value_or API

Summary:
Add API equivalent to `std::optional::value_or`: https://en.cppreference.com/w/cpp/utility/optional/value_or

Returns the contained value if *this has a value, otherwise returns default_value.

  template< class U > constexpr T value_or( U&& default_value ) const&;

Equivalent to `bool(*this) ? **this : static_cast<T>(std::forward<U>(default_value))`

   template< class U > constexpr T value_or( U&& default_value ) &&;

Equivalent to `bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(default_value))`

Reviewed By: yfeldblum

Differential Revision: D27346319

fbshipit-source-id: 598b740cb9068cffb0cc0fbda3579e64fd125e8b

folly/Try-inl.h

@@ -168,6 +168,19 @@ const T&& Try<T>::value() const&& {
   return std::move(value_);
 }
 
+template <class T>
+template <class U>
+T Try<T>::value_or(U&& defaultValue) const& {
+  return hasValue() ? **this : static_cast<T>(static_cast<U&&>(defaultValue));
+}
+
+template <class T>
+template <class U>
+T Try<T>::value_or(U&& defaultValue) && {
+  return hasValue() ? std::move(**this)
+                    : static_cast<T>(static_cast<U&&>(defaultValue));
+}
+
 template <class T>
 void Try<T>::throwUnlessValue() const {
   switch (contains_) {

folly/Try.h

@@ -178,6 +178,17 @@ class Try {
    */
   const T&& value() const&&;
 
+  /*
+   * Returns a copy of the contained value if *this has a value,
+   * otherwise returns a value constructed from defaultValue.
+   *
+   * The selected constructor of the return value may throw exceptions.
+   */
+  template <class U>
+  T value_or(U&& defaultValue) const&;
+  template <class U>
+  T value_or(U&& defaultValue) &&;
+
   /*
    * [Re]throw if the Try contains an exception or is empty. Otherwise do
    * nothing.

folly/test/TryTest.cpp

@@ -192,6 +192,8 @@ TEST(Try, emplaceWithThrowingConstructor) {
 
     EXPECT_FALSE(t.hasValue());
     EXPECT_FALSE(t.hasException());
+
+    EXPECT_THROW(t.value_or(true), MyException);
   }
 
   {
@@ -202,6 +204,8 @@ TEST(Try, emplaceWithThrowingConstructor) {
     EXPECT_THROW(t.emplace(true), MyException);
     EXPECT_FALSE(t.hasValue());
     EXPECT_FALSE(t.hasException());
+
+    EXPECT_THROW(t.value_or(true), MyException);
   }
 }
 
@@ -376,6 +380,7 @@ TEST(Try, MoveDereference) {
   auto t = Try<std::unique_ptr<int>>{std::move(ptr)};
   auto result = *std::move(t);
   EXPECT_EQ(*result, 1);
+  EXPECT_TRUE(t.hasValue());
 }
 
 TEST(Try, MoveConstRvalue) {
@@ -429,6 +434,47 @@ TEST(Try, ValueOverloads) {
   }
 }
 
+TEST(Try, ValueOr) {
+  struct CopyableValue {
+    int x;
+  };
+  {
+    Try<CopyableValue> o{CopyableValue{42}};
+    CopyableValue defaultValue{17};
+    EXPECT_EQ(o.value_or(defaultValue).x, 42);
+    EXPECT_EQ(o.value_or(defaultValue).x, (*o).x);
+  }
+
+  {
+    Try<CopyableValue> empty;
+    EXPECT_FALSE(empty.hasValue());
+    CopyableValue defaultValue{17};
+    EXPECT_EQ(empty.value_or(defaultValue).x, defaultValue.x);
+  }
+
+  {
+    Try<std::unique_ptr<int>> o{std::make_unique<int>(42)};
+    std::unique_ptr<int> defaultValue = std::make_unique<int>(17);
+    std::unique_ptr<int> v = std::move(o).value_or(std::move(defaultValue));
+    ASSERT_TRUE(v);
+    EXPECT_EQ(*v, 42);
+    ASSERT_TRUE(defaultValue);
+    EXPECT_EQ(*defaultValue, 17);
+    EXPECT_TRUE(o.hasValue());
+    ASSERT_FALSE(*o);
+  }
+
+  {
+    Try<std::unique_ptr<int>> empty;
+    std::unique_ptr<int> defaultValue = std::make_unique<int>(17);
+    std::unique_ptr<int> v = std::move(empty).value_or(std::move(defaultValue));
+    ASSERT_TRUE(v);
+    EXPECT_EQ(*v, 17);
+    EXPECT_FALSE(defaultValue);
+    EXPECT_FALSE(empty.hasValue());
+  }
+}
+
 // Make sure we can copy Trys for copyable types
 TEST(Try, copy) {
   Try<int> t;