Add conversion operator to std::array for StrSplit.

This is similar to the conversion operator to std::pair, but for N elements instead of two. Missing elements are filled with the empty string and extra elements are discarded.

PiperOrigin-RevId: 693727220
Change-Id: Icfee16613a4859b019ca043da712f20797386beb

Author: jakecobb

absl/strings/internal/str_split_internal.h

@@ -253,6 +253,10 @@ using ShouldUseLifetimeBoundForPair = std::integral_constant<
               (std::is_same<First, absl::string_view>::value ||
                std::is_same<Second, absl::string_view>::value)>;
 
+template <typename StringType, typename ElementType, std::size_t Size>
+using ShouldUseLifetimeBoundForArray = std::integral_constant<
+    bool, std::is_same<StringType, std::string>::value &&
+              std::is_same<ElementType, absl::string_view>::value>;
 
 // This class implements the range that is returned by absl::StrSplit(). This
 // class has templated conversion operators that allow it to be implicitly
@@ -344,7 +348,38 @@ class Splitter {
     return ConvertToPair<First, Second>();
   }
 
+  // Returns an array with its elements set to the first few strings returned by
+  // the begin() iterator.  If there is not a corresponding value the empty
+  // string is used.
+  template <typename ElementType, std::size_t Size,
+            std::enable_if_t<ShouldUseLifetimeBoundForArray<
+                                 StringType, ElementType, Size>::value,
+                             std::nullptr_t> = nullptr>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  operator std::array<ElementType, Size>() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
+    return ConvertToArray<ElementType, Size>();
+  }
+
+  template <typename ElementType, std::size_t Size,
+            std::enable_if_t<!ShouldUseLifetimeBoundForArray<
+                                 StringType, ElementType, Size>::value,
+                             std::nullptr_t> = nullptr>
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  operator std::array<ElementType, Size>() const {
+    return ConvertToArray<ElementType, Size>();
+  }
+
  private:
+  template <typename ElementType, std::size_t Size>
+  std::array<ElementType, Size> ConvertToArray() const {
+    std::array<ElementType, Size> a;
+    auto it = begin();
+    for (std::size_t i = 0; i < Size && it != end(); ++i, ++it) {
+      a[i] = ElementType(*it);
+    }
+    return a;
+  }
+
   template <typename First, typename Second>
   std::pair<First, Second> ConvertToPair() const {
     absl::string_view first, second;

absl/strings/str_split.h

@@ -444,8 +444,10 @@ using EnableSplitIfString =
 // behavior works for:
 //
 // 1) All standard STL containers including `std::vector`, `std::list`,
-//    `std::deque`, `std::set`,`std::multiset`, 'std::map`, and `std::multimap`
+//    `std::deque`, `std::set`,`std::multiset`, 'std::map`, and `std::multimap`.
 // 2) `std::pair` (which is not actually a container). See below.
+// 3) `std::array`, which is a container but has different behavior due to its
+//    fixed size. See below.
 //
 // Example:
 //
@@ -487,6 +489,21 @@ using EnableSplitIfString =
 //   std::pair<std::string, std::string> p = absl::StrSplit("a,b,c", ',');
 //   // p.first == "a", p.second == "b"       // "c" is omitted.
 //
+//
+// Splitting to `std::array` is similar to splitting to `std::pair`, but for
+// N elements instead of two; missing elements are filled with the empty string
+// and extra elements are discarded.
+//
+// Examples:
+//
+//   // Stores first two split strings as the elements in a std::array.
+//   std::array<std::string, 2> a = absl::StrSplit("a,b,c", ',');
+//   // a[0] == "a", a[1] == "b"   // "c" is omitted.
+//
+//   // The second element is empty.
+//   std::array<std::string, 2> a = absl::StrSplit("a,", ',');
+//   // a[0] == "a", a[1] == ""
+//
 // The `StrSplit()` function can be used multiple times to perform more
 // complicated splitting logic, such as intelligently parsing key-value pairs.
 //

absl/strings/str_split_test.cc

@@ -14,6 +14,7 @@
 
 #include "absl/strings/str_split.h"
 
+#include <array>
 #include <cstddef>
 #include <cstdint>
 #include <deque>
@@ -397,6 +398,12 @@ void TestPairConversionOperator(const Splitter& splitter) {
   EXPECT_EQ(p, (std::pair<FirstType, SecondType>("a", "b")));
 }
 
+template <typename StringType, typename Splitter>
+void TestArrayConversionOperator(const Splitter& splitter) {
+  std::array<StringType, 2> a = splitter;
+  EXPECT_THAT(a, ElementsAre("a", "b"));
+}
+
 TEST(Splitter, ConversionOperator) {
   auto splitter = absl::StrSplit("a,b,c,d", ',');
 
@@ -467,6 +474,10 @@ TEST(Splitter, ConversionOperator) {
   TestPairConversionOperator<absl::string_view, std::string>(splitter);
   TestPairConversionOperator<std::string, absl::string_view>(splitter);
   TestPairConversionOperator<std::string, std::string>(splitter);
+
+  // Tests conversion to std::array
+  TestArrayConversionOperator<std::string>(splitter);
+  TestArrayConversionOperator<absl::string_view>(splitter);
 }
 
 // A few additional tests for conversion to std::pair. This conversion is
@@ -511,6 +522,41 @@ TEST(Splitter, ToPair) {
   }
 }
 
+// std::array tests similar to std::pair tests above, testing fewer, exactly,
+// or more elements than the array size.
+TEST(Splitter, ToArray) {
+  {
+    // Empty string
+    std::array<std::string, 2> p = absl::StrSplit("", ',');
+    EXPECT_THAT(p, ElementsAre("", ""));
+  }
+
+  {
+    // Only first
+    std::array<std::string, 2> p = absl::StrSplit("a", ',');
+    EXPECT_THAT(p, ElementsAre("a", ""));
+  }
+
+  {
+    // Only second
+    std::array<std::string, 2> p = absl::StrSplit(",b", ',');
+    EXPECT_THAT(p, ElementsAre("", "b"));
+  }
+
+  {
+    // First and second.
+    std::array<std::string, 2> p = absl::StrSplit("a,b", ',');
+    EXPECT_THAT(p, ElementsAre("a", "b"));
+  }
+
+  {
+    // First and second and then more stuff that will be ignored.
+    std::array<std::string, 2> p = absl::StrSplit("a,b,c", ',');
+    EXPECT_THAT(p, ElementsAre("a", "b"));
+    // "c" is omitted.
+  }
+}
+
 TEST(Splitter, Predicates) {
   static const char kTestChars[] = ",a, ,b,";
   using absl::AllowEmpty;