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

webrtc/base/array_view.h

+/*
+ *  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_(data), size_(size) {
+    // Check invariant: !data_ iff size_ == 0.
+    RTC_DCHECK_EQ(!data_, size_ == 0);
+  }
+
+  // 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>
+  // and rtc::Buffer to ArrayView<uint8_t>.
+  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, where std::vector
+  // has a data() method.
+  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>.)

[the sun, 2015/10/21 10:53:42]

Why not add clearly non-mutating data() and operator[] instead:

 T* data() { return data_; }
 T& operator[](size_t idx) { ...
 const T* data() const { return data_; }
 const T& operator[](size_t idx) const { ...

harder to misuse

[kwiberg-webrtc, 2015/10/21 12:08:55]

We discussed this face to face; the tl;dr is that representing an immutable
array with ArrayView<const T> is better than with const ArrayView<T>, since
there is no way to prevent users from making a non-const copy of the latter. So
I won't change anything.

+  size_t size() const { return size_; }
+  T* data() const { return data_; }
+  T& operator[](size_t idx) const {
+    RTC_DCHECK_LT(idx, size_);
+    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.
+  T* data_;
+  size_t size_;
+};
+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_ARRAY_VIEW_H_