Introduce rtc::ArrayView<T>, which keeps track of an array that it doesn't own

The main intended use case is as a function argument, replacing the
harder-to-read and harder-to-use separate pointer and size arguments.
It's easier to read because it's just one argument instead of two, and
with clearly defined semantics; it's easier to use because it has
iterators, and will automatically figure out the size of arrays.

BUG=webrtc:5028
R=andrew@webrtc.org, solenberg@webrtc.org

Review URL: https://codereview.webrtc.org/1408403002 .

Cr-Commit-Position: refs/heads/master@{#10415}

webrtc/base/BUILD.gn

@@ -94,6 +94,7 @@ static_library("rtc_base_approved") {
   public_configs = [ "..:common_inherited_config" ]
 
   sources = [
+    "array_view.h",
     "atomicops.h",
     "bitbuffer.cc",
     "bitbuffer.h",

webrtc/base/array_view.h

@@ -0,0 +1,84 @@
+/*
+ *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_BASE_ARRAY_VIEW_H_
+#define WEBRTC_BASE_ARRAY_VIEW_H_
+
+#include <vector>
+
+#include "webrtc/base/checks.h"
+
+namespace rtc {
+
+// Keeps track of an array (a pointer and a size) that it doesn't own.
+// ArrayView objects are immutable except for assignment, and small enough to
+// be cheaply passed by value.
+//
+// Note that ArrayView<T> and ArrayView<const T> are distinct types; this is
+// how you would represent mutable and unmutable views of an array.
+template <typename T>
+class ArrayView final {
+ public:
+  // Construct an empty ArrayView.
+  ArrayView() : ArrayView(static_cast<T*>(nullptr), 0) {}
+
+  // Construct an ArrayView for a (pointer,size) pair.
+  template <typename U>
+  ArrayView(U* data, size_t size)
+      : data_(size == 0 ? nullptr : data), size_(size) {
+    CheckInvariant();
+  }
+
+  // Construct an ArrayView for an array.
+  template <typename U, size_t N>
+  ArrayView(U (&array)[N]) : ArrayView(&array[0], N) {}
+
+  // Construct an ArrayView for any type U that has a size() method whose
+  // return value converts implicitly to size_t, and a data() method whose
+  // return value converts implicitly to T*. In particular, this means we allow
+  // conversion from ArrayView<T> to ArrayView<const T>, but not the other way
+  // around. Other allowed conversions include std::vector<T> to ArrayView<T>
+  // or ArrayView<const T>, const std::vector<T> to ArrayView<const T>, and
+  // rtc::Buffer to ArrayView<uint8_t> (with the same const behavior as
+  // std::vector).
+  template <typename U>
+  ArrayView(U& u) : ArrayView(u.data(), u.size()) {}
+  // TODO(kwiberg): Remove the special case for std::vector (and the include of
+  // <vector>); it is handled by the general case in C++11, since std::vector
+  // has a data() method there.
+  template <typename U>
+  ArrayView(std::vector<U>& u)
+      : ArrayView(u.empty() ? nullptr : &u[0], u.size()) {}
+
+  // Indexing, size, and iteration. These allow mutation even if the ArrayView
+  // is const, because the ArrayView doesn't own the array. (To prevent
+  // mutation, use ArrayView<const T>.)
+  size_t size() const { return size_; }
+  T* data() const { return data_; }
+  T& operator[](size_t idx) const {
+    RTC_DCHECK_LT(idx, size_);
+    RTC_DCHECK(data_);  // Follows from size_ > idx and the class invariant.
+    return data_[idx];
+  }
+  T* begin() const { return data_; }
+  T* end() const { return data_ + size_; }
+  const T* cbegin() const { return data_; }
+  const T* cend() const { return data_ + size_; }
+
+ private:
+  // Invariant: !data_ iff size_ == 0.
+  void CheckInvariant() const { RTC_DCHECK_EQ(!data_, size_ == 0); }
+  T* data_;
+  size_t size_;
+};
+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_ARRAY_VIEW_H_

webrtc/base/array_view_unittest.cc

@@ -0,0 +1,217 @@
+/*
+ *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include <algorithm>
+#include <string>
+#include <vector>
+
+#include "webrtc/base/array_view.h"
+#include "webrtc/base/buffer.h"
+#include "webrtc/base/checks.h"
+#include "webrtc/base/gunit.h"
+
+namespace rtc {
+
+namespace {
+template <typename T>
+void Call(ArrayView<T>) {}
+}  // namespace
+
+TEST(ArrayViewTest, TestConstructFromPtrAndArray) {
+  char arr[] = "Arrr!";
+  const char carr[] = "Carrr!";
+  Call<const char>(arr);
+  Call<const char>(carr);
+  Call<char>(arr);
+  // Call<char>(carr);  // Compile error, because can't drop const.
+  // Call<int>(arr);  // Compile error, because incompatible types.
+  ArrayView<int*> x;
+  EXPECT_EQ(0u, x.size());
+  EXPECT_EQ(nullptr, x.data());
+  ArrayView<char> y = arr;
+  EXPECT_EQ(6u, y.size());
+  EXPECT_EQ(arr, y.data());
+  ArrayView<const char> z(arr + 1, 3);
+  EXPECT_EQ(3u, z.size());
+  EXPECT_EQ(arr + 1, z.data());
+  ArrayView<const char> w(arr, 2);
+  EXPECT_EQ(2u, w.size());
+  EXPECT_EQ(arr, w.data());
+  ArrayView<char> q(arr, 0);
+  EXPECT_EQ(0u, q.size());
+  EXPECT_EQ(nullptr, q.data());
+#if RTC_DCHECK_IS_ON && GTEST_HAS_DEATH_TEST && !defined(WEBRTC_ANDROID)
+  // DCHECK error (nullptr with nonzero size).
+  EXPECT_DEATH(ArrayView<int>(static_cast<int*>(nullptr), 5), "");
+#endif
+  // These are compile errors, because incompatible types.
+  // ArrayView<int> m = arr;
+  // ArrayView<float> n(arr + 2, 2);
+}
+
+TEST(ArrayViewTest, TestCopyConstructor) {
+  char arr[] = "Arrr!";
+  ArrayView<char> x = arr;
+  EXPECT_EQ(6u, x.size());
+  EXPECT_EQ(arr, x.data());
+  ArrayView<char> y = x;  // Copy non-const -> non-const.
+  EXPECT_EQ(6u, y.size());
+  EXPECT_EQ(arr, y.data());
+  ArrayView<const char> z = x;  // Copy non-const -> const.
+  EXPECT_EQ(6u, z.size());
+  EXPECT_EQ(arr, z.data());
+  ArrayView<const char> w = z;  // Copy const -> const.
+  EXPECT_EQ(6u, w.size());
+  EXPECT_EQ(arr, w.data());
+  // ArrayView<char> v = z;  // Compile error, because can't drop const.
+}
+
+TEST(ArrayViewTest, TestCopyAssignment) {
+  char arr[] = "Arrr!";
+  ArrayView<char> x(arr);
+  EXPECT_EQ(6u, x.size());
+  EXPECT_EQ(arr, x.data());
+  ArrayView<char> y;
+  y = x;  // Copy non-const -> non-const.
+  EXPECT_EQ(6u, y.size());
+  EXPECT_EQ(arr, y.data());
+  ArrayView<const char> z;
+  z = x;  // Copy non-const -> const.
+  EXPECT_EQ(6u, z.size());
+  EXPECT_EQ(arr, z.data());
+  ArrayView<const char> w;
+  w = z;  // Copy const -> const.
+  EXPECT_EQ(6u, w.size());
+  EXPECT_EQ(arr, w.data());
+  // ArrayView<char> v;
+  // v = z;  // Compile error, because can't drop const.
+}
+
+TEST(ArrayViewTest, TestStdVector) {
+  std::vector<int> v;
+  v.push_back(3);
+  v.push_back(11);
+  Call<const int>(v);
+  Call<int>(v);
+  // Call<unsigned int>(v);  // Compile error, because incompatible types.
+  ArrayView<int> x = v;
+  EXPECT_EQ(2u, x.size());
+  EXPECT_EQ(v.data(), x.data());
+  ArrayView<const int> y;
+  y = v;
+  EXPECT_EQ(2u, y.size());
+  EXPECT_EQ(v.data(), y.data());
+  // ArrayView<double> d = v;  // Compile error, because incompatible types.
+  const std::vector<int> cv;
+  Call<const int>(cv);
+  // Call<int>(cv);  // Compile error, because can't drop const.
+  ArrayView<const int> z = cv;
+  EXPECT_EQ(0u, z.size());
+  EXPECT_EQ(nullptr, z.data());
+  // ArrayView<int> w = cv;  // Compile error, because can't drop const.
+}
+
+TEST(ArrayViewTest, TestRtcBuffer) {
+  rtc::Buffer b = "so buffer";
+  Call<const uint8_t>(b);
+  Call<uint8_t>(b);
+  // Call<int8_t>(b);  // Compile error, because incompatible types.
+  ArrayView<uint8_t> x = b;
+  EXPECT_EQ(10u, x.size());
+  EXPECT_EQ(b.data(), x.data());
+  ArrayView<const uint8_t> y;
+  y = b;
+  EXPECT_EQ(10u, y.size());
+  EXPECT_EQ(b.data(), y.data());
+  // ArrayView<char> d = b;  // Compile error, because incompatible types.
+  const rtc::Buffer cb = "very const";
+  Call<const uint8_t>(cb);
+  // Call<uint8_t>(cb);  // Compile error, because can't drop const.
+  ArrayView<const uint8_t> z = cb;
+  EXPECT_EQ(11u, z.size());
+  EXPECT_EQ(cb.data(), z.data());
+  // ArrayView<uint8_t> w = cb;  // Compile error, because can't drop const.
+}
+
+TEST(ArrayViewTest, TestSwap) {
+  const char arr[] = "Arrr!";
+  const char aye[] = "Aye, Cap'n!";
+  ArrayView<const char> x(arr);
+  EXPECT_EQ(6u, x.size());
+  EXPECT_EQ(arr, x.data());
+  ArrayView<const char> y(aye);
+  EXPECT_EQ(12u, y.size());
+  EXPECT_EQ(aye, y.data());
+  using std::swap;
+  swap(x, y);
+  EXPECT_EQ(12u, x.size());
+  EXPECT_EQ(aye, x.data());
+  EXPECT_EQ(6u, y.size());
+  EXPECT_EQ(arr, y.data());
+  // ArrayView<char> z;
+  // swap(x, z);  // Compile error, because can't drop const.
+}
+
+TEST(ArrayViewTest, TestIndexing) {
+  char arr[] = "abcdefg";
+  ArrayView<char> x(arr);
+  const ArrayView<char> y(arr);
+  ArrayView<const char> z(arr);
+  EXPECT_EQ(8u, x.size());
+  EXPECT_EQ(8u, y.size());
+  EXPECT_EQ(8u, z.size());
+  EXPECT_EQ('b', x[1]);
+  EXPECT_EQ('c', y[2]);
+  EXPECT_EQ('d', z[3]);
+  x[3] = 'X';
+  y[2] = 'Y';
+  // z[1] = 'Z';  // Compile error, because z's element type is const char.
+  EXPECT_EQ('b', x[1]);
+  EXPECT_EQ('Y', y[2]);
+  EXPECT_EQ('X', z[3]);
+#if RTC_DCHECK_IS_ON && GTEST_HAS_DEATH_TEST && !defined(WEBRTC_ANDROID)
+  EXPECT_DEATH(z[8], "");  // DCHECK error (index out of bounds).
+#endif
+}
+
+TEST(ArrayViewTest, TestIterationEmpty) {
+  ArrayView<std::vector<std::vector<std::vector<std::string>>>> av;
+  EXPECT_FALSE(av.begin());
+  EXPECT_FALSE(av.cbegin());
+  EXPECT_FALSE(av.end());
+  EXPECT_FALSE(av.cend());
+  for (auto& e : av) {
+    EXPECT_TRUE(false);
+    EXPECT_EQ(42u, e.size());  // Dummy use of e to prevent unused var warning.
+  }
+}
+
+TEST(ArrayViewTest, TestIteration) {
+  char arr[] = "Arrr!";
+  ArrayView<char> av(arr);
+  EXPECT_EQ('A', *av.begin());
+  EXPECT_EQ('A', *av.cbegin());
+  EXPECT_EQ('\0', *(av.end() - 1));
+  EXPECT_EQ('\0', *(av.cend() - 1));
+  char i = 0;
+  for (auto& e : av) {
+    EXPECT_EQ(arr + i, &e);
+    e = 's' + i;
+    ++i;
+  }
+  i = 0;
+  for (auto& e : ArrayView<const char>(av)) {
+    EXPECT_EQ(arr + i, &e);
+    // e = 'q' + i;  // Compile error, because e is a const char&.
+    ++i;
+  }
+}
+
+}  // namespace rtc

webrtc/base/base.gyp

@@ -29,6 +29,7 @@
       'target_name': 'rtc_base_approved',
       'type': 'static_library',
       'sources': [
+        'array_view.h',
         'atomicops.h',
         'basictypes.h',
         'bitbuffer.cc',

webrtc/base/base_tests.gyp

@@ -45,6 +45,7 @@
       'type': 'none',
       'direct_dependent_settings': {
         'sources': [
+          'array_view_unittest.cc',
           'atomicops_unittest.cc',
           'autodetectproxy_unittest.cc',
           'bandwidthsmoother_unittest.cc',

webrtc/base/checks.h

@@ -164,6 +164,7 @@ DEFINE_RTC_CHECK_OP_IMPL(GT, > )
 // code in debug builds. It does reference the condition parameter in all cases,
 // though, so callers won't risk getting warnings about unused variables.
 #if (!defined(NDEBUG) || defined(DCHECK_ALWAYS_ON))
+#define RTC_DCHECK_IS_ON 1
 #define RTC_DCHECK(condition) RTC_CHECK(condition)
 #define RTC_DCHECK_EQ(v1, v2) RTC_CHECK_EQ(v1, v2)
 #define RTC_DCHECK_NE(v1, v2) RTC_CHECK_NE(v1, v2)
@@ -172,6 +173,7 @@ DEFINE_RTC_CHECK_OP_IMPL(GT, > )
 #define RTC_DCHECK_GE(v1, v2) RTC_CHECK_GE(v1, v2)
 #define RTC_DCHECK_GT(v1, v2) RTC_CHECK_GT(v1, v2)
 #else
+#define RTC_DCHECK_IS_ON 0
 #define RTC_DCHECK(condition) RTC_EAT_STREAM_PARAMETERS(condition)
 #define RTC_DCHECK_EQ(v1, v2) RTC_EAT_STREAM_PARAMETERS((v1) == (v2))
 #define RTC_DCHECK_NE(v1, v2) RTC_EAT_STREAM_PARAMETERS((v1) != (v2))