FTL: Generalize StaticVector in-place construction
Replace the std::in_place_type constructor, which is limited to one
argument per element, with construction from InitializerList, which
forwards argument tuples for each element.
SmallMap will use this syntax for its key-value pair constructor.
Bug: 160012986
Test: ftl_test
Change-Id: I9331f20268532cb09e980475d68768ba891aca1f
diff --git a/include/ftl/ArrayTraits.h b/include/ftl/ArrayTraits.h
index 28f717a..ff685c5 100644
--- a/include/ftl/ArrayTraits.h
+++ b/include/ftl/ArrayTraits.h
@@ -19,6 +19,7 @@
#include <algorithm>
#include <iterator>
#include <new>
+#include <type_traits>
#define FTL_ARRAY_TRAIT(T, U) using U = typename ArrayTraits<T>::U
@@ -40,11 +41,16 @@
using const_iterator = const_pointer;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;
- // TODO: Replace with std::construct_at in C++20.
template <typename... Args>
static pointer construct_at(const_iterator it, Args&&... args) {
void* const ptr = const_cast<void*>(static_cast<const void*>(it));
- return new (ptr) value_type{std::forward<Args>(args)...};
+ if constexpr (std::is_constructible_v<value_type, Args...>) {
+ // TODO: Replace with std::construct_at in C++20.
+ return new (ptr) value_type(std::forward<Args>(args)...);
+ } else {
+ // Fall back to list initialization.
+ return new (ptr) value_type{std::forward<Args>(args)...};
+ }
}
};
diff --git a/include/ftl/InitializerList.h b/include/ftl/InitializerList.h
new file mode 100644
index 0000000..204f79b
--- /dev/null
+++ b/include/ftl/InitializerList.h
@@ -0,0 +1,69 @@
+/*
+ * Copyright 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <tuple>
+#include <utility>
+
+namespace android::ftl {
+
+// Compile-time counterpart of std::initializer_list<T> that stores per-element constructor
+// arguments with heterogeneous types. For a container with elements of type T, given Sizes
+// (S0, S1, ..., SN), N elements are initialized: the first element is initialized with the
+// first S0 arguments, the second element is initialized with the next S1 arguments, and so
+// on. The list of Types (T0, ..., TM) is flattened, so M is equal to the sum of the Sizes.
+//
+// The InitializerList is created using ftl::init::list, and is consumed by constructors of
+// containers. The function call operator is overloaded such that arguments are accumulated
+// in a tuple with each successive call. For instance, the following calls initialize three
+// strings using different constructors, i.e. string literal, default, and count/character:
+//
+// ... = ftl::init::list<std::string>("abc")()(3u, '?');
+//
+// WARNING: The InitializerList returned by an ftl::init::list expression must be consumed
+// immediately, since temporary arguments are destroyed after the full expression. Storing
+// an InitializerList results in dangling references.
+//
+template <typename T, typename Sizes = std::index_sequence<>, typename... Types>
+struct InitializerList;
+
+template <typename T, size_t... Sizes, typename... Types>
+struct InitializerList<T, std::index_sequence<Sizes...>, Types...> {
+ // Creates a superset InitializerList by appending the number of arguments to Sizes, and
+ // expanding Types with forwarding references for each argument.
+ template <typename... Args>
+ [[nodiscard]] constexpr auto operator()(Args&&... args) && -> InitializerList<
+ T, std::index_sequence<Sizes..., sizeof...(Args)>, Types..., Args&&...> {
+ return {std::tuple_cat(std::move(tuple),
+ std::forward_as_tuple(std::forward<Args>(args)...))};
+ }
+
+ // The temporary InitializerList returned by operator() is bound to an rvalue reference in
+ // container constructors, which extends the lifetime of any temporary arguments that this
+ // tuple refers to until the completion of the full expression containing the construction.
+ std::tuple<Types...> tuple;
+};
+
+namespace init {
+
+template <typename T, typename... Args>
+[[nodiscard]] constexpr auto list(Args&&... args) {
+ return InitializerList<T>{}(std::forward<Args>(args)...);
+}
+
+} // namespace init
+} // namespace android::ftl
diff --git a/include/ftl/SmallVector.h b/include/ftl/SmallVector.h
index cecec7f..2f05a9b 100644
--- a/include/ftl/SmallVector.h
+++ b/include/ftl/SmallVector.h
@@ -64,6 +64,16 @@
// assert(vector == (ftl::SmallVector{'h', 'i', '\0'}));
// assert(!vector.dynamic());
//
+// ftl::SmallVector strings = ftl::init::list<std::string>("abc")
+// ("123456", 3u)
+// (3u, '?');
+// assert(strings.size() == 3u);
+// assert(!strings.dynamic());
+//
+// assert(strings[0] == "abc");
+// assert(strings[1] == "123");
+// assert(strings[2] == "???");
+//
template <typename T, size_t N>
class SmallVector final : ArrayTraits<T>, ArrayComparators<SmallVector> {
using Static = StaticVector<T, N>;
@@ -365,8 +375,9 @@
SmallVector(T&&, Us&&...) -> SmallVector<V, 1 + sizeof...(Us)>;
// Deduction guide for in-place constructor.
-template <typename T, typename... Us>
-SmallVector(std::in_place_type_t<T>, Us&&...) -> SmallVector<T, sizeof...(Us)>;
+template <typename T, size_t... Sizes, typename... Types>
+SmallVector(InitializerList<T, std::index_sequence<Sizes...>, Types...>&&)
+ -> SmallVector<T, sizeof...(Sizes)>;
// Deduction guide for StaticVector conversion.
template <typename T, size_t N>
diff --git a/include/ftl/StaticVector.h b/include/ftl/StaticVector.h
index 457095d..c132556 100644
--- a/include/ftl/StaticVector.h
+++ b/include/ftl/StaticVector.h
@@ -17,6 +17,7 @@
#pragma once
#include <ftl/ArrayTraits.h>
+#include <ftl/InitializerList.h>
#include <algorithm>
#include <cassert>
@@ -32,7 +33,7 @@
// Fixed-capacity, statically allocated counterpart of std::vector. Akin to std::array, StaticVector
// allocates contiguous storage for N elements of type T at compile time, but stores at most (rather
// than exactly) N elements. Unlike std::array, its default constructor does not require T to have a
-// default constructor, since elements are constructed in-place as the vector grows. Operations that
+// default constructor, since elements are constructed in place as the vector grows. Operations that
// insert an element (emplace_back, push_back, etc.) fail when the vector is full. The API otherwise
// adheres to standard containers, except the unstable_erase operation that does not preserve order,
// and the replace operation that destructively emplaces.
@@ -63,6 +64,14 @@
// vector = ftl::StaticVector(array);
// assert(vector == (ftl::StaticVector{'h', 'i', '\0'}));
//
+// ftl::StaticVector strings = ftl::init::list<std::string>("abc")
+// ("123456", 3u)
+// (3u, '?');
+// assert(strings.size() == 3u);
+// assert(strings[0] == "abc");
+// assert(strings[1] == "123");
+// assert(strings[2] == "???");
+//
template <typename T, size_t N>
class StaticVector final : ArrayTraits<T>,
ArrayIterators<StaticVector<T, N>, T>,
@@ -153,15 +162,22 @@
static_assert(sizeof...(elements) < N, "Too many elements");
}
- // Constructs at most N elements. The template arguments T and N are inferred using the
- // deduction guide defined below. Element types must be convertible to the specified T:
+ // Constructs at most N elements in place by forwarding per-element constructor arguments. The
+ // template arguments T and N are inferred using the deduction guide defined below. The syntax
+ // for listing arguments is as follows:
//
- // ftl::StaticVector vector(std::in_place_type<std::string>, "red", "velvet", "cake");
+ // ftl::StaticVector vector = ftl::init::list<std::string>("abc")()(3u, '?');
+ //
// static_assert(std::is_same_v<decltype(vector), ftl::StaticVector<std::string, 3>>);
+ // assert(vector.full());
+ // assert(vector[0] == "abc");
+ // assert(vector[1].empty());
+ // assert(vector[2] == "???");
//
- template <typename... Es>
- explicit StaticVector(std::in_place_type_t<T>, Es... elements)
- : StaticVector(std::forward<Es>(elements)...) {}
+ template <typename U, size_t Size, size_t... Sizes, typename... Types>
+ StaticVector(InitializerList<U, std::index_sequence<Size, Sizes...>, Types...>&& init)
+ : StaticVector(std::index_sequence<0, 0, Size>{}, std::make_index_sequence<Size>{},
+ std::index_sequence<Sizes...>{}, init.tuple) {}
~StaticVector() { std::destroy(begin(), end()); }
@@ -292,6 +308,32 @@
template <size_t I>
explicit StaticVector(std::index_sequence<I>) : mSize(I) {}
+ // Recursion for in-place constructor.
+ //
+ // Construct element I by extracting its arguments from the InitializerList tuple. ArgIndex
+ // is the position of its first argument in Args, and ArgCount is the number of arguments.
+ // The Indices sequence corresponds to [0, ArgCount).
+ //
+ // The Sizes sequence lists the argument counts for elements after I, so Size is the ArgCount
+ // for the next element. The recursion stops when Sizes is empty for the last element.
+ //
+ template <size_t I, size_t ArgIndex, size_t ArgCount, size_t... Indices, size_t Size,
+ size_t... Sizes, typename... Args>
+ StaticVector(std::index_sequence<I, ArgIndex, ArgCount>, std::index_sequence<Indices...>,
+ std::index_sequence<Size, Sizes...>, std::tuple<Args...>& tuple)
+ : StaticVector(std::index_sequence<I + 1, ArgIndex + ArgCount, Size>{},
+ std::make_index_sequence<Size>{}, std::index_sequence<Sizes...>{}, tuple) {
+ construct_at(begin() + I, std::move(std::get<ArgIndex + Indices>(tuple))...);
+ }
+
+ // Base case for in-place constructor.
+ template <size_t I, size_t ArgIndex, size_t ArgCount, size_t... Indices, typename... Args>
+ StaticVector(std::index_sequence<I, ArgIndex, ArgCount>, std::index_sequence<Indices...>,
+ std::index_sequence<>, std::tuple<Args...>& tuple)
+ : mSize(I + 1) {
+ construct_at(begin() + I, std::move(std::get<ArgIndex + Indices>(tuple))...);
+ }
+
size_type mSize = 0;
std::aligned_storage_t<sizeof(value_type), alignof(value_type)> mData[N];
};
@@ -306,8 +348,9 @@
StaticVector(T&&, Us&&...) -> StaticVector<V, 1 + sizeof...(Us)>;
// Deduction guide for in-place constructor.
-template <typename T, typename... Us>
-StaticVector(std::in_place_type_t<T>, Us&&...) -> StaticVector<T, sizeof...(Us)>;
+template <typename T, size_t... Sizes, typename... Types>
+StaticVector(InitializerList<T, std::index_sequence<Sizes...>, Types...>&&)
+ -> StaticVector<T, sizeof...(Sizes)>;
template <typename T, size_t N>
template <typename E>