Reland "[base] Implement std::size(), std::empty() and std::data()"

This is a reland of f6c12979

Original change's description:
> [base] Implement std::size(), std::empty() and std::data()
>
> This change provides C++14 implementations of C++17's std::size(),
> std::empty() and std::data().
>
> Bug: 828324
> Change-Id: If1b83826a23b26fd557f7854962bcbc771a7ffe0
> Reviewed-on: https://chromium-review.googlesource.com/992236
> Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
> Reviewed-by: kylechar <kylechar@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#550137}

TBR=dcheng@chromium.org,kylechar@chromium.org

Bug: 828324
Change-Id: I4c3e637129aaaf277d83ca07b4705dc3b815183b
Reviewed-on: https://chromium-review.googlesource.com/1011962Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550565}

base/stl_util.h

@@ -11,6 +11,7 @@
 #include <deque>
 #include <forward_list>
 #include <functional>
+#include <initializer_list>
 #include <iterator>
 #include <list>
 #include <map>
@@ -40,6 +41,69 @@ void IterateAndEraseIf(Container& container, Predicate pred) {
 
 }  // namespace internal
 
+// C++14 implementation of C++17's std::size():
+// http://en.cppreference.com/w/cpp/iterator/size
+template <typename Container>
+constexpr auto size(const Container& c) -> decltype(c.size()) {
+  return c.size();
+}
+
+template <typename T, size_t N>
+constexpr size_t size(const T (&array)[N]) noexcept {
+  return N;
+}
+
+// C++14 implementation of C++17's std::empty():
+// http://en.cppreference.com/w/cpp/iterator/empty
+template <typename Container>
+constexpr auto empty(const Container& c) -> decltype(c.empty()) {
+  return c.empty();
+}
+
+template <typename T, size_t N>
+constexpr bool empty(const T (&array)[N]) noexcept {
+  return false;
+}
+
+template <typename T>
+constexpr bool empty(std::initializer_list<T> il) noexcept {
+  return il.size() == 0;
+}
+
+// C++14 implementation of C++17's std::data():
+// http://en.cppreference.com/w/cpp/iterator/data
+template <typename Container>
+constexpr auto data(Container& c) -> decltype(c.data()) {
+  return c.data();
+}
+
+// std::basic_string::data() had no mutable overload prior to C++17 [1].
+// Hence this overload is provided.
+// Note: str[0] is safe even for empty strings, as they are guaranteed to be
+// null-terminated [2].
+//
+// [1] http://en.cppreference.com/w/cpp/string/basic_string/data
+// [2] http://en.cppreference.com/w/cpp/string/basic_string/operator_at
+template <typename CharT, typename Traits, typename Allocator>
+CharT* data(std::basic_string<CharT, Traits, Allocator>& str) {
+  return std::addressof(str[0]);
+}
+
+template <typename Container>
+constexpr auto data(const Container& c) -> decltype(c.data()) {
+  return c.data();
+}
+
+template <typename T, size_t N>
+constexpr T* data(T (&array)[N]) noexcept {
+  return array;
+}
+
+template <typename T>
+constexpr const T* data(std::initializer_list<T> il) noexcept {
+  return il.begin();
+}
+
 // Clears internal memory of an STL object.
 // STL clear()/reserve(0) does not always free internal memory allocated
 // This function uses swap/destructor to ensure the internal memory is freed.
@@ -72,6 +136,8 @@ STLCount(const Container& container, const T& val) {
 // (http://www.open-std.org/JTC1/SC22/WG21/docs/lwg-active.html#530)
 // proposes this as the method. According to Matt Austern, this should
 // already work on all current implementations.
+//
+// DEPRECATED(https://crbug.com/831499), use base::data(std::string&) instead.
 inline char* string_as_array(std::string* str) {
   // DO NOT USE const_cast<char*>(str->data())
   return str->empty() ? NULL : &*str->begin();

base/stl_util_unittest.cc

@@ -4,14 +4,17 @@
 
 #include "base/stl_util.h"
 
+#include <array>
 #include <deque>
 #include <forward_list>
 #include <functional>
+#include <initializer_list>
 #include <iterator>
 #include <list>
 #include <map>
 #include <set>
 #include <string>
+#include <type_traits>
 #include <unordered_map>
 #include <unordered_set>
 #include <vector>
@@ -94,6 +97,140 @@ struct HashByFirst {
 namespace base {
 namespace {
 
+TEST(STLUtilTest, Size) {
+  {
+    std::vector<int> vector = {1, 2, 3, 4, 5};
+    static_assert(
+        std::is_same<decltype(base::size(vector)),
+                     decltype(vector.size())>::value,
+        "base::size(vector) should have the same type as vector.size()");
+    EXPECT_EQ(vector.size(), base::size(vector));
+  }
+
+  {
+    std::string empty_str;
+    static_assert(
+        std::is_same<decltype(base::size(empty_str)),
+                     decltype(empty_str.size())>::value,
+        "base::size(empty_str) should have the same type as empty_str.size()");
+    EXPECT_EQ(0u, base::size(empty_str));
+  }
+
+  {
+    std::array<int, 4> array = {{1, 2, 3, 4}};
+    static_assert(
+        std::is_same<decltype(base::size(array)),
+                     decltype(array.size())>::value,
+        "base::size(array) should have the same type as array.size()");
+    static_assert(base::size(array) == array.size(),
+                  "base::size(array) should be equal to array.size()");
+  }
+
+  {
+    int array[] = {1, 2, 3};
+    static_assert(std::is_same<size_t, decltype(base::size(array))>::value,
+                  "base::size(array) should be of type size_t");
+    static_assert(3u == base::size(array), "base::size(array) should be 3");
+  }
+}
+
+TEST(STLUtilTest, Empty) {
+  {
+    std::vector<int> vector;
+    static_assert(
+        std::is_same<decltype(base::empty(vector)),
+                     decltype(vector.empty())>::value,
+        "base::empty(vector) should have the same type as vector.empty()");
+    EXPECT_EQ(vector.empty(), base::empty(vector));
+  }
+
+  {
+    std::array<int, 4> array = {{1, 2, 3, 4}};
+    static_assert(
+        std::is_same<decltype(base::empty(array)),
+                     decltype(array.empty())>::value,
+        "base::empty(array) should have the same type as array.empty()");
+    static_assert(base::empty(array) == array.empty(),
+                  "base::empty(array) should be equal to array.empty()");
+  }
+
+  {
+    int array[] = {1, 2, 3};
+    static_assert(std::is_same<bool, decltype(base::empty(array))>::value,
+                  "base::empty(array) should be of type bool");
+    static_assert(!base::empty(array), "base::empty(array) should be false");
+  }
+
+  {
+    constexpr std::initializer_list<int> il;
+    static_assert(std::is_same<bool, decltype(base::empty(il))>::value,
+                  "base::empty(il) should be of type bool");
+    static_assert(base::empty(il), "base::empty(il) should be true");
+  }
+}
+
+TEST(STLUtilTest, Data) {
+  {
+    std::vector<int> vector = {1, 2, 3, 4, 5};
+    static_assert(
+        std::is_same<decltype(base::data(vector)),
+                     decltype(vector.data())>::value,
+        "base::data(vector) should have the same type as vector.data()");
+    EXPECT_EQ(vector.data(), base::data(vector));
+  }
+
+  {
+    const std::string cstr = "const string";
+    static_assert(
+        std::is_same<decltype(base::data(cstr)), decltype(cstr.data())>::value,
+        "base::data(cstr) should have the same type as cstr.data()");
+
+    EXPECT_EQ(cstr.data(), base::data(cstr));
+  }
+
+  {
+    std::string str = "mutable string";
+    static_assert(std::is_same<decltype(base::data(str)), char*>::value,
+                  "base::data(str) should be of type char*");
+    EXPECT_EQ(str.data(), base::data(str));
+  }
+
+  {
+    std::string empty_str;
+    static_assert(std::is_same<decltype(base::data(empty_str)), char*>::value,
+                  "base::data(empty_str) should be of type char*");
+    EXPECT_EQ(empty_str.data(), base::data(empty_str));
+  }
+
+  {
+    std::array<int, 4> array = {{1, 2, 3, 4}};
+    static_assert(
+        std::is_same<decltype(base::data(array)),
+                     decltype(array.data())>::value,
+        "base::data(array) should have the same type as array.data()");
+    // std::array::data() is not constexpr prior to C++17, hence the runtime
+    // check.
+    EXPECT_EQ(array.data(), base::data(array));
+  }
+
+  {
+    constexpr int array[] = {1, 2, 3};
+    static_assert(std::is_same<const int*, decltype(base::data(array))>::value,
+                  "base::data(array) should be of type const int*");
+    static_assert(array == base::data(array),
+                  "base::data(array) should be array");
+  }
+
+  {
+    constexpr std::initializer_list<int> il;
+    static_assert(
+        std::is_same<decltype(il.begin()), decltype(base::data(il))>::value,
+        "base::data(il) should have the same type as il.begin()");
+    static_assert(il.begin() == base::data(il),
+                  "base::data(il) should be equal to il.begin()");
+  }
+}
+
 TEST(STLUtilTest, STLIsSorted) {
   {
     std::set<int> set;