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ArrayView: Support compile-time constant sizes

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BUG=none

Review-Url: https://codereview.webrtc.org/2667383006
Cr-Commit-Position: refs/heads/master@{#16981}
This commit is contained in:
kwiberg 2017-03-02 12:33:50 -08:00 committed by Commit bot
parent 51ba2e2ed8
commit bfc7f02d79
2 changed files with 322 additions and 67 deletions
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214
webrtc/base/array_view.h
View File

@ -31,9 +31,9 @@ namespace rtc {
// std::vector, rtc::Buffer, ...), but it's error-prone because the caller has
// to correctly specify the array length:
//
// Contains17(arr, arraysize(arr)); // C array
// Contains17(&arr[0], arr.size()); // std::vector
// Contains17(arr, size); // pointer + size
// Contains17(arr, arraysize(arr)); // C array
// Contains17(arr.data(), arr.size()); // std::vector
// Contains17(arr, size); // pointer + size
// ...
//
// It's also kind of messy to have two separate arguments for what is
@ -60,6 +60,10 @@ namespace rtc {
// Contains17(nullptr); // nullptr -> empty ArrayView
// ...
//
// ArrayView<T> stores both a pointer and a size, but you may also use
// ArrayView<T, N>, which has a size that's fixed at compile time (which means
// it only has to store the pointer).
//
// One important point is that ArrayView<T> and ArrayView<const T> are
// different types, which allow and don't allow mutation of the array elements,
// respectively. The implicit conversions work just like you'd hope, so that
@ -68,81 +72,177 @@ namespace rtc {
// (ArrayView itself can be the source type in such conversions, so
// ArrayView<int> will convert to ArrayView<const int>.)
//
// Note: ArrayView is tiny (just a pointer and a count) and trivially copyable,
// so it's probably cheaper to pass it by value than by const reference.
// Note: ArrayView is tiny (just a pointer and a count if variable-sized, just
// a pointer if fix-sized) and trivially copyable, so it's probably cheaper to
// pass it by value than by const reference.
namespace impl {
// Magic constant for indicating that the size of an ArrayView is variable
// instead of fixed.
enum : std::ptrdiff_t { kArrayViewVarSize = -4711 };
// Base class for ArrayViews of fixed nonzero size.
template <typename T, std::ptrdiff_t Size>
class ArrayViewBase {
static_assert(Size > 0, "ArrayView size must be variable or non-negative");
public:
ArrayViewBase(T* data, size_t size) : data_(data) {}
static constexpr size_t size() { return Size; }
static constexpr bool empty() { return false; }
T* data() const { return data_; }
protected:
static constexpr bool fixed_size() { return true; }
private:
T* data_;
};
// Specialized base class for ArrayViews of fixed zero size.
template <typename T>
class ArrayView final {
class ArrayViewBase<T, 0> {
public:
explicit ArrayViewBase(T* data, size_t size) {}
static constexpr size_t size() { return 0; }
static constexpr bool empty() { return true; }
T* data() const { return nullptr; }
protected:
static constexpr bool fixed_size() { return true; }
};
// Specialized base class for ArrayViews of variable size.
template <typename T>
class ArrayViewBase<T, impl::kArrayViewVarSize> {
public:
ArrayViewBase(T* data, size_t size)
: data_(size == 0 ? nullptr : data), size_(size) {}
size_t size() const { return size_; }
bool empty() const { return size_ == 0; }
T* data() const { return data_; }
protected:
static constexpr bool fixed_size() { return false; }
private:
T* data_;
size_t size_;
};
} // namespace impl
template <typename T, std::ptrdiff_t Size = impl::kArrayViewVarSize>
class ArrayView final : public impl::ArrayViewBase<T, Size> {
public:
using value_type = T;
using const_iterator = const T*;
// Construct an empty ArrayView.
ArrayView() : ArrayView(static_cast<T*>(nullptr), 0) {}
ArrayView(std::nullptr_t) : ArrayView() {}
// Construct an ArrayView for a (pointer,size) pair.
// Construct an ArrayView from a pointer and a length.
template <typename U>
ArrayView(U* data, size_t size)
: data_(size == 0 ? nullptr : data), size_(size) {
CheckInvariant();
: impl::ArrayViewBase<T, Size>::ArrayViewBase(data, size) {
RTC_DCHECK_EQ(size == 0 ? nullptr : data, this->data());
RTC_DCHECK_EQ(size, this->size());
RTC_DCHECK_EQ(!this->data(),
this->size() == 0); // data is null iff size == 0.
}
// Construct an ArrayView for an array.
template <typename U, size_t N>
ArrayView(U (&array)[N]) : ArrayView(&array[0], N) {}
// Construct an empty ArrayView. Note that fixed-size ArrayViews of size > 0
// cannot be empty.
ArrayView() : ArrayView(nullptr, 0) {}
ArrayView(std::nullptr_t) : ArrayView() {}
ArrayView(std::nullptr_t, size_t size)
: ArrayView(static_cast<T*>(nullptr), size) {
static_assert(Size == 0 || Size == impl::kArrayViewVarSize, "");
RTC_DCHECK_EQ(0, size);
}
// 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).
// Construct an ArrayView from an array.
template <typename U, size_t N>
ArrayView(U (&array)[N]) : ArrayView(array, N) {
static_assert(Size == N || Size == impl::kArrayViewVarSize,
"Array size must match ArrayView size");
}
// (Only if size is fixed.) Construct an ArrayView from any type U that has a
// static constexpr size() method whose return value is equal to Size, and a
// data() method whose return value converts implicitly to T*. In particular,
// this means we allow conversion from ArrayView<T, N> to ArrayView<const T,
// N>, but not the other way around. We also don't allow conversion from
// ArrayView<T> to ArrayView<T, N>, or from ArrayView<T, M> to ArrayView<T,
// N> when M != N.
template <typename U,
typename std::enable_if<
Size != impl::kArrayViewVarSize &&
HasDataAndSize<U, T>::value>::type* = nullptr>
ArrayView(U& u) : ArrayView(u.data(), u.size()) {
static_assert(U::size() == Size, "Sizes must match exactly");
}
// (Only if size is variable.) Construct an ArrayView from 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
// ArrayView<T, N> to ArrayView<T> or ArrayView<const T>,
// std::vector<T> to ArrayView<T> or ArrayView<const T>,
// const std::vector<T> to ArrayView<const T>,
// rtc::Buffer to ArrayView<uint8_t> or ArrayView<const uint8_t>, and
// const rtc::Buffer to ArrayView<const uint8_t>.
template <
typename U,
typename std::enable_if<HasDataAndSize<U, T>::value>::type* = nullptr>
typename std::enable_if<Size == impl::kArrayViewVarSize &&
HasDataAndSize<U, T>::value>::type* = nullptr>
ArrayView(U& u) : ArrayView(u.data(), 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_; }
bool empty() const { return size_ == 0; }
T* data() const { return data_; }
// Indexing and iteration. These allow mutation even if the ArrayView is
// const, because the ArrayView doesn't own the array. (To prevent mutation,
// use a const element type.)
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];
RTC_DCHECK_LT(idx, this->size());
RTC_DCHECK(this->data());
return this->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_; }
T* begin() const { return this->data(); }
T* end() const { return this->data() + this->size(); }
const T* cbegin() const { return this->data(); }
const T* cend() const { return this->data() + this->size(); }
ArrayView subview(size_t offset, size_t size) const {
if (offset >= size_)
return ArrayView();
return ArrayView(data_ + offset, std::min(size, size_ - offset));
ArrayView<T> subview(size_t offset, size_t size) const {
return offset < this->size()
? ArrayView<T>(this->data() + offset,
std::min(size, this->size() - offset))
: ArrayView<T>();
}
ArrayView subview(size_t offset) const { return subview(offset, size_); }
// Comparing two ArrayViews compares their (pointer,size) pairs; it does
// *not* dereference the pointers.
friend bool operator==(const ArrayView& a, const ArrayView& b) {
return a.data_ == b.data_ && a.size_ == b.size_;
ArrayView<T> subview(size_t offset) const {
return subview(offset, this->size());
}
friend bool operator!=(const ArrayView& a, const ArrayView& b) {
return !(a == b);
}
private:
// Invariant: !data_ iff size_ == 0.
void CheckInvariant() const { RTC_DCHECK_EQ(!data_, size_ == 0); }
T* data_;
size_t size_;
};
// Comparing two ArrayViews compares their (pointer,size) pairs; it does *not*
// dereference the pointers.
template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
bool operator==(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
return a.data() == b.data() && a.size() == b.size();
}
template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
bool operator!=(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
return !(a == b);
}
// Variable-size ArrayViews are the size of two pointers; fixed-size ArrayViews
// are the size of one pointer. (And as a special case, fixed-size ArrayViews
// of size 0 require no storage.)
static_assert(sizeof(ArrayView<int>) == 2 * sizeof(int*), "");
static_assert(sizeof(ArrayView<int, 17>) == sizeof(int*), "");
static_assert(std::is_empty<ArrayView<int, 0>>::value, "");
template <typename T>
inline ArrayView<T> MakeArrayView(T* data, size_t size) {
return ArrayView<T>(data, size);

175
webrtc/base/array_view_unittest.cc
View File

@ -44,15 +44,27 @@ TEST(ArrayViewTest, TestConstructFromPtrAndArray) {
ArrayView<char> y = arr;
EXPECT_EQ(6u, y.size());
EXPECT_EQ(arr, y.data());
ArrayView<char, 6> yf = arr;
static_assert(yf.size() == 6, "");
EXPECT_EQ(arr, yf.data());
ArrayView<const char> z(arr + 1, 3);
EXPECT_EQ(3u, z.size());
EXPECT_EQ(arr + 1, z.data());
ArrayView<const char, 3> zf(arr + 1, 3);
static_assert(zf.size() == 3, "");
EXPECT_EQ(arr + 1, zf.data());
ArrayView<const char> w(arr, 2);
EXPECT_EQ(2u, w.size());
EXPECT_EQ(arr, w.data());
ArrayView<const char, 2> wf(arr, 2);
static_assert(wf.size() == 2, "");
EXPECT_EQ(arr, wf.data());
ArrayView<char> q(arr, 0);
EXPECT_EQ(0u, q.size());
EXPECT_EQ(nullptr, q.data());
ArrayView<char, 0> qf(arr, 0);
static_assert(qf.size() == 0, "");
EXPECT_EQ(nullptr, qf.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), "");
@ -62,7 +74,7 @@ TEST(ArrayViewTest, TestConstructFromPtrAndArray) {
// ArrayView<float> n(arr + 2, 2);
}
TEST(ArrayViewTest, TestCopyConstructor) {
TEST(ArrayViewTest, TestCopyConstructorVariable) {
char arr[] = "Arrr!";
ArrayView<char> x = arr;
EXPECT_EQ(6u, x.size());
@ -79,7 +91,38 @@ TEST(ArrayViewTest, TestCopyConstructor) {
// ArrayView<char> v = z; // Compile error, because can't drop const.
}
TEST(ArrayViewTest, TestCopyAssignment) {
TEST(ArrayViewTest, TestCopyConstructorFixed) {
char arr[] = "Arrr!";
ArrayView<char, 6> x = arr;
static_assert(x.size() == 6, "");
EXPECT_EQ(arr, x.data());
// Copy fixed -> fixed.
ArrayView<char, 6> y = x; // Copy non-const -> non-const.
static_assert(y.size() == 6, "");
EXPECT_EQ(arr, y.data());
ArrayView<const char, 6> z = x; // Copy non-const -> const.
static_assert(z.size() == 6, "");
EXPECT_EQ(arr, z.data());
ArrayView<const char, 6> w = z; // Copy const -> const.
static_assert(w.size() == 6, "");
EXPECT_EQ(arr, w.data());
// ArrayView<char, 6> v = z; // Compile error, because can't drop const.
// Copy fixed -> variable.
ArrayView<char> yv = x; // Copy non-const -> non-const.
EXPECT_EQ(6u, yv.size());
EXPECT_EQ(arr, yv.data());
ArrayView<const char> zv = x; // Copy non-const -> const.
EXPECT_EQ(6u, zv.size());
EXPECT_EQ(arr, zv.data());
ArrayView<const char> wv = z; // Copy const -> const.
EXPECT_EQ(6u, wv.size());
EXPECT_EQ(arr, wv.data());
// ArrayView<char> vv = z; // Compile error, because can't drop const.
}
TEST(ArrayViewTest, TestCopyAssignmentVariable) {
char arr[] = "Arrr!";
ArrayView<char> x(arr);
EXPECT_EQ(6u, x.size());
@ -100,6 +143,42 @@ TEST(ArrayViewTest, TestCopyAssignment) {
// v = z; // Compile error, because can't drop const.
}
TEST(ArrayViewTest, TestCopyAssignmentFixed) {
char arr[] = "Arrr!";
char init[] = "Init!";
ArrayView<char, 6> x(arr);
EXPECT_EQ(arr, x.data());
// Copy fixed -> fixed.
ArrayView<char, 6> y(init);
y = x; // Copy non-const -> non-const.
EXPECT_EQ(arr, y.data());
ArrayView<const char, 6> z(init);
z = x; // Copy non-const -> const.
EXPECT_EQ(arr, z.data());
ArrayView<const char, 6> w(init);
w = z; // Copy const -> const.
EXPECT_EQ(arr, w.data());
// ArrayView<char, 6> v(init);
// v = z; // Compile error, because can't drop const.
// Copy fixed -> variable.
ArrayView<char> yv;
yv = x; // Copy non-const -> non-const.
EXPECT_EQ(6u, yv.size());
EXPECT_EQ(arr, yv.data());
ArrayView<const char> zv;
zv = x; // Copy non-const -> const.
EXPECT_EQ(6u, zv.size());
EXPECT_EQ(arr, zv.data());
ArrayView<const char> wv;
wv = z; // Copy const -> const.
EXPECT_EQ(6u, wv.size());
EXPECT_EQ(arr, wv.data());
// ArrayView<char> v;
// v = z; // Compile error, because can't drop const.
}
TEST(ArrayViewTest, TestStdVector) {
std::vector<int> v;
v.push_back(3);
@ -146,7 +225,7 @@ TEST(ArrayViewTest, TestRtcBuffer) {
// ArrayView<uint8_t> w = cb; // Compile error, because can't drop const.
}
TEST(ArrayViewTest, TestSwap) {
TEST(ArrayViewTest, TestSwapVariable) {
const char arr[] = "Arrr!";
const char aye[] = "Aye, Cap'n!";
ArrayView<const char> x(arr);
@ -165,11 +244,28 @@ TEST(ArrayViewTest, TestSwap) {
// swap(x, z); // Compile error, because can't drop const.
}
TEST(FixArrayViewTest, TestSwapFixed) {
const char arr[] = "Arr!";
char aye[] = "Aye!";
ArrayView<const char, 5> x(arr);
EXPECT_EQ(arr, x.data());
ArrayView<const char, 5> y(aye);
EXPECT_EQ(aye, y.data());
using std::swap;
swap(x, y);
EXPECT_EQ(aye, x.data());
EXPECT_EQ(arr, y.data());
// ArrayView<char, 5> z(aye);
// swap(x, z); // Compile error, because can't drop const.
// ArrayView<const char, 4> w(aye, 4);
// swap(x, w); // Compile error, because different sizes.
}
TEST(ArrayViewTest, TestIndexing) {
char arr[] = "abcdefg";
ArrayView<char> x(arr);
const ArrayView<char> y(arr);
ArrayView<const char> z(arr);
ArrayView<const char, 8> z(arr);
EXPECT_EQ(8u, x.size());
EXPECT_EQ(8u, y.size());
EXPECT_EQ(8u, z.size());
@ -188,18 +284,26 @@ TEST(ArrayViewTest, TestIndexing) {
}
TEST(ArrayViewTest, TestIterationEmpty) {
// Variable-size.
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());
EXPECT_EQ(av.begin(), av.end());
EXPECT_EQ(av.cbegin(), av.cend());
for (auto& e : av) {
EXPECT_TRUE(false);
EXPECT_EQ(42u, e.size()); // Dummy use of e to prevent unused var warning.
}
// Fixed-size.
ArrayView<std::vector<std::vector<std::vector<std::string>>>, 0> af;
EXPECT_EQ(af.begin(), af.end());
EXPECT_EQ(af.cbegin(), af.cend());
for (auto& e : af) {
EXPECT_TRUE(false);
EXPECT_EQ(42u, e.size()); // Dummy use of e to prevent unused var warning.
}
}
TEST(ArrayViewTest, TestIteration) {
TEST(ArrayViewTest, TestIterationVariable) {
char arr[] = "Arrr!";
ArrayView<char> av(arr);
EXPECT_EQ('A', *av.begin());
@ -220,23 +324,57 @@ TEST(ArrayViewTest, TestIteration) {
}
}
TEST(ArrayViewTest, TestIterationFixed) {
char arr[] = "Arrr!";
ArrayView<char, 6> 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, 6>(av)) {
EXPECT_EQ(arr + i, &e);
// e = 'q' + i; // Compile error, because e is a const char&.
++i;
}
}
TEST(ArrayViewTest, TestEmpty) {
EXPECT_TRUE(ArrayView<int>().empty());
const int a[] = {1, 2, 3};
EXPECT_FALSE(ArrayView<const int>(a).empty());
static_assert(ArrayView<int, 0>::empty(), "");
static_assert(!ArrayView<int, 3>::empty(), "");
}
TEST(ArrayViewTest, TestCompare) {
int a[] = {1, 2, 3};
int b[] = {1, 2, 3};
EXPECT_EQ(ArrayView<int>(a), ArrayView<int>(a));
EXPECT_EQ((ArrayView<int, 3>(a)), (ArrayView<int, 3>(a)));
EXPECT_EQ(ArrayView<int>(a), (ArrayView<int, 3>(a)));
EXPECT_EQ(ArrayView<int>(), ArrayView<int>());
EXPECT_EQ(ArrayView<int>(), ArrayView<int>(a, 0));
EXPECT_EQ(ArrayView<int>(a, 0), ArrayView<int>(b, 0));
EXPECT_EQ((ArrayView<int, 0>(a, 0)), ArrayView<int>());
EXPECT_NE(ArrayView<int>(a), ArrayView<int>(b));
EXPECT_NE((ArrayView<int, 3>(a)), (ArrayView<int, 3>(b)));
EXPECT_NE((ArrayView<int, 3>(a)), ArrayView<int>(b));
EXPECT_NE(ArrayView<int>(a), ArrayView<int>());
EXPECT_NE(ArrayView<int>(a), ArrayView<int>(a, 2));
EXPECT_NE((ArrayView<int, 3>(a)), (ArrayView<int, 2>(a, 2)));
}
TEST(ArrayViewTest, TestSubView) {
TEST(ArrayViewTest, TestSubViewVariable) {
int a[] = {1, 2, 3};
ArrayView<int> av(a);
@ -253,4 +391,21 @@ TEST(ArrayViewTest, TestSubView) {
EXPECT_THAT(av.subview(1, 3), ElementsAre(2, 3));
}
TEST(ArrayViewTest, TestSubViewFixed) {
int a[] = {1, 2, 3};
ArrayView<int, 3> av(a);
EXPECT_EQ(av.subview(0), av);
EXPECT_THAT(av.subview(1), ElementsAre(2, 3));
EXPECT_THAT(av.subview(2), ElementsAre(3));
EXPECT_THAT(av.subview(3), IsEmpty());
EXPECT_THAT(av.subview(4), IsEmpty());
EXPECT_THAT(av.subview(1, 0), IsEmpty());
EXPECT_THAT(av.subview(1, 1), ElementsAre(2));
EXPECT_THAT(av.subview(1, 2), ElementsAre(2, 3));
EXPECT_THAT(av.subview(1, 3), ElementsAre(2, 3));
}
} // namespace rtc