Improving flat containers interface.

base/containers/README.md

Index: base/containers/README.md
diff --git a/base/containers/README.md b/base/containers/README.md
index e64e8f3a637162d1b43b7d4df0f90e705289ecc9..5baa1d4bdcc6bf4f5f020ea3326dc4bbdc29f328 100644
--- a/base/containers/README.md
+++ b/base/containers/README.md
@@ -124,6 +124,59 @@ strategy and the memory access pattern. Assuming items are being linearly added,
 one would expect it to be 3/4 full, so per-item overhead will be 0.25 *
 sizeof(T).
 
+
+flat\_set/flat\_map support C++14 interface and a notion of transparent
+comparisons. Therefore you can, for example, lookup base::StringPiece in a set
+of std::strings without constructing a temporary std::string.
+You can find more information about transparent comparisons here:
+http://en.cppreference.com/w/cpp/utility/functional/less_void
+Example, smart pointer set:
+    // Define a custom comparator.
+    struct UniquePtrComparator {
+      // Mark your comparison as transparent.
+      using is_transparent = int;
+      template <typename T>
+      bool operator()(const std::unique_ptr<T>& lhs,
+                      const std::unique_ptr<T>& rhs) const {
+        return lhs < rhs;
+      }
+      template <typename T>
+      bool operator()(const T* lhs, const std::unique_ptr<T>& rhs) const {
+        return lhs < rhs.get();
+      }
+      template <typename T>
+      bool operator()(const std::unique_ptr<T>& lhs, const T* rhs) const {
+        return lhs.get() < rhs;
+      }
+    };
+    // Declare a typedef.
+    template <typename T>
+    using UniquePtrSet =
+        base::flat_set<std::unique_ptr<T>, UniquePtrComparator>;
+    // ...
+    // Collect data.
+    std::vector<std::unique_ptr<int>> ptr_vec;
+    ptr_vec.reserve(5);
+    std::generate_n(std::back_inserter(ptr_vec), 5, []{
+      return base::MakeUnique<int>(0);
+    });
+    // Construct a set.
+    UniquePtrSet<int> ptr_set(std::move(ptr_vec), base::KEEP_FIRST_OF_DUPES);
+    // Use raw pointers to lookup keys.
+    int* ptr = ptr_set.begin()->get();
+    EXPECT_TRUE(ptr_set.find(ptr) == ptr_set.begin());
+Example flat_map<std::string, int>:
+    base::flat_map<std::string, int> str_to_int({{"a", 1}, {"c", 2},{"b", 2}},
+                                                base::KEEP_FIRST_OF_DUPES);
+    // Does not construct temporary strings.
+    str_to_int.find("c")->second = 3;
+    str_to_int.erase("c");
+    EXPECT_EQ(str_to_int.end(), str_to_int.find("c")->second);
+    // NOTE: This does construct a temporary string. This happens since if the
+    // item is not in the container, then it needs to be constructed, which is
+    // something that transparent comparators don't have to guarantee.
+    str_to_int["c"] = 3;
+
 ### base::small\_map
 
 A small inline buffer that is brute-force searched that overflows into a full