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// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include <stddef.h>
#include <vector>
#include "base/macros.h"
#include "build/build_config.h"
namespace base {
// This allocator can be used with STL containers to provide a stack buffer
// from which to allocate memory and overflows onto the heap. This stack buffer
// would be allocated on the stack and allows us to avoid heap operations in
// some situations.
// STL likes to make copies of allocators, so the allocator itself can't hold
// the data. Instead, we make the creator responsible for creating a
// StackAllocator::Source which contains the data. Copying the allocator
// merely copies the pointer to this shared source, so all allocators created
// based on our allocator will share the same stack buffer.
// This stack buffer implementation is very simple. The first allocation that
// fits in the stack buffer will use the stack buffer. Any subsequent
// allocations will not use the stack buffer, even if there is unused room.
// This makes it appropriate for array-like containers, but the caller should
// be sure to reserve() in the container up to the stack buffer size. Otherwise
// the container will allocate a small array which will "use up" the stack
// buffer.
template<typename T, size_t stack_capacity>
class StackAllocator : public std::allocator<T> {
typedef typename std::allocator<T>::pointer pointer;
typedef typename std::allocator<T>::size_type size_type;
// Backing store for the allocator. The container owner is responsible for
// maintaining this for as long as any containers using this allocator are
// live.
struct Source {
Source() : used_stack_buffer_(false) {
// Casts the buffer in its right type.
T* stack_buffer() { return reinterpret_cast<T*>(stack_buffer_); }
const T* stack_buffer() const {
return reinterpret_cast<const T*>(&stack_buffer_);
// The buffer itself. It is not of type T because we don't want the
// constructors and destructors to be automatically called. Define a POD
// buffer of the right size instead.
alignas(T) char stack_buffer_[sizeof(T[stack_capacity])];
#if defined(__GNUC__) && !defined(ARCH_CPU_X86_FAMILY)
static_assert(alignof(T) <= 16, "");
// Set when the stack buffer is used for an allocation. We do not track
// how much of the buffer is used, only that somebody is using it.
bool used_stack_buffer_;
// Used by containers when they want to refer to an allocator of type U.
template<typename U>
struct rebind {
typedef StackAllocator<U, stack_capacity> other;
// For the straight up copy c-tor, we can share storage.
StackAllocator(const StackAllocator<T, stack_capacity>& rhs)
: std::allocator<T>(), source_(rhs.source_) {
// ISO C++ requires the following constructor to be defined,
// and std::vector in VC++2008SP1 Release fails with an error
// in the class _Container_base_aux_alloc_real (from <xutility>)
// if the constructor does not exist.
// For this constructor, we cannot share storage; there's
// no guarantee that the Source buffer of Ts is large enough
// for Us.
// TODO: If we were fancy pants, perhaps we could share storage
// iff sizeof(T) == sizeof(U).
template<typename U, size_t other_capacity>
StackAllocator(const StackAllocator<U, other_capacity>& other)
: source_(NULL) {
// This constructor must exist. It creates a default allocator that doesn't
// actually have a stack buffer. glibc's std::string() will compare the
// current allocator against the default-constructed allocator, so this
// should be fast.
StackAllocator() : source_(NULL) {
explicit StackAllocator(Source* source) : source_(source) {
// Actually do the allocation. Use the stack buffer if nobody has used it yet
// and the size requested fits. Otherwise, fall through to the standard
// allocator.
pointer allocate(size_type n, void* hint = 0) {
if (source_ != NULL && !source_->used_stack_buffer_
&& n <= stack_capacity) {
source_->used_stack_buffer_ = true;
return source_->stack_buffer();
} else {
return std::allocator<T>::allocate(n, hint);
// Free: when trying to free the stack buffer, just mark it as free. For
// non-stack-buffer pointers, just fall though to the standard allocator.
void deallocate(pointer p, size_type n) {
if (source_ != NULL && p == source_->stack_buffer())
source_->used_stack_buffer_ = false;
std::allocator<T>::deallocate(p, n);
Source* source_;
// A wrapper around STL containers that maintains a stack-sized buffer that the
// initial capacity of the vector is based on. Growing the container beyond the
// stack capacity will transparently overflow onto the heap. The container must
// support reserve().
// This will not work with std::string since some implementations allocate
// more bytes than requested in calls to reserve(), forcing the allocation onto
// the heap.
// WATCH OUT: the ContainerType MUST use the proper StackAllocator for this
// type. This object is really intended to be used only internally. You'll want
// to use the wrappers below for different types.
template<typename TContainerType, int stack_capacity>
class StackContainer {
typedef TContainerType ContainerType;
typedef typename ContainerType::value_type ContainedType;
typedef StackAllocator<ContainedType, stack_capacity> Allocator;
// Allocator must be constructed before the container!
StackContainer() : allocator_(&stack_data_), container_(allocator_) {
// Make the container use the stack allocation by reserving our buffer size
// before doing anything else.
// Getters for the actual container.
// Danger: any copies of this made using the copy constructor must have
// shorter lifetimes than the source. The copy will share the same allocator
// and therefore the same stack buffer as the original. Use std::copy to
// copy into a "real" container for longer-lived objects.
ContainerType& container() { return container_; }
const ContainerType& container() const { return container_; }
// Support operator-> to get to the container. This allows nicer syntax like:
// StackContainer<...> foo;
// std::sort(foo->begin(), foo->end());
ContainerType* operator->() { return &container_; }
const ContainerType* operator->() const { return &container_; }
#ifdef UNIT_TEST
// Retrieves the stack source so that that unit tests can verify that the
// buffer is being used properly.
const typename Allocator::Source& stack_data() const {
return stack_data_;
typename Allocator::Source stack_data_;
Allocator allocator_;
ContainerType container_;
// Range-based iteration support for StackContainer.
template <typename TContainerType, int stack_capacity>
auto begin(
const StackContainer<TContainerType, stack_capacity>& stack_container)
-> decltype(begin(stack_container.container())) {
return begin(stack_container.container());
template <typename TContainerType, int stack_capacity>
auto begin(StackContainer<TContainerType, stack_capacity>& stack_container)
-> decltype(begin(stack_container.container())) {
return begin(stack_container.container());
template <typename TContainerType, int stack_capacity>
auto end(StackContainer<TContainerType, stack_capacity>& stack_container)
-> decltype(end(stack_container.container())) {
return end(stack_container.container());
template <typename TContainerType, int stack_capacity>
auto end(const StackContainer<TContainerType, stack_capacity>& stack_container)
-> decltype(end(stack_container.container())) {
return end(stack_container.container());
// StackVector -----------------------------------------------------------------
// Example:
// StackVector<int, 16> foo;
// foo->push_back(22); // we have overloaded operator->
// foo[0] = 10; // as well as operator[]
template<typename T, size_t stack_capacity>
class StackVector : public StackContainer<
std::vector<T, StackAllocator<T, stack_capacity> >,
stack_capacity> {
StackVector() : StackContainer<
std::vector<T, StackAllocator<T, stack_capacity> >,
stack_capacity>() {
// We need to put this in STL containers sometimes, which requires a copy
// constructor. We can't call the regular copy constructor because that will
// take the stack buffer from the original. Here, we create an empty object
// and make a stack buffer of its own.
StackVector(const StackVector<T, stack_capacity>& other)
: StackContainer<
std::vector<T, StackAllocator<T, stack_capacity> >,
stack_capacity>() {
this->container().assign(other->begin(), other->end());
StackVector<T, stack_capacity>& operator=(
const StackVector<T, stack_capacity>& other) {
this->container().assign(other->begin(), other->end());
return *this;
// Vectors are commonly indexed, which isn't very convenient even with
// operator-> (using "->at()" does exception stuff we don't want).
T& operator[](size_t i) { return this->container().operator[](i); }
const T& operator[](size_t i) const {
return this->container().operator[](i);
} // namespace base