Documentation of memory

( foonathan/memory)

memory

The C++ STL allocator model has various flaws. For example, they are fixed to a certain type, because they are almost necessarily required to be templates. So you can't easily share a single allocator for multiple types. In addition, you can only get a copy from the containers and not the original allocator object. At least with C++11 they are allowed to be stateful and so can be made object not instance based. But still, the model has many flaws. Over the course of the years many solutions have been proposed. for example EASTL. This library is another. But instead of trying to change the STL, it works with the current implementation.

Features

New allocator concepts:

  • a RawAllocator that is similar to an Allocator but easier to use and write
  • a BlockAllocator that is an allocator for huge memory blocks

Several implementations:

  • heap_/malloc_/new_allocator
  • virtual memory allocators
  • allocator using a static memory block located on the stack
  • memory stack, iteration_allocator
  • different memory pools
  • a portable, improved alloca() in the form of temporary_allocator
  • facilities for joint memory allocations: share a big memory block for the object and all dynamic memory allocations for its members

Adapters, wrappers and storage classes:

  • incredible powerful allocator_traits allowing Allocators as RawAllocators
  • std_allocator to make a RawAllocator an Allocator again
  • adapters for the memory resource TS
  • allocator_deleter classes for smart pointers
  • (optionally type-erased) allocator_reference and other storage classes
  • memory tracking wrapper

In addition:

  • container node size debuggers that obtain information about the node size of an STL container at compile-time to specify node sizes for pools
  • debugging options for leak checking, double-free checks or buffer overflows
  • customizable error handling routines that can work with exceptions disabled
  • everything except the STL adapters works on a freestanding environment

Basic example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <foonathan/memory/container.hpp> // vector, list, list_node_size
#include <foonathan/memory/memory_pool.hpp> // memory_pool
#include <foonathan/memory/smart_ptr.hpp> // allocate_unique
#include <foonathan/memory/static_allocator.hpp> // static_allocator_storage, static_block_allocator
#include <foonathan/memory/temporary_allocator.hpp> // temporary_allocator
// alias namespace foonathan::memory as memory for easier access
template <typename BiIter>
void merge_sort(BiIter begin, BiIter end);
int main()
{
using namespace memory::literals;
// a memory pool RawAllocator
// allocates a memory block - initially 4KiB - and splits it into chunks of list_node_size<int>::value big
// list_node_size<int>::value is the size of each node of a std::list
memory::memory_pool<> pool(memory::list_node_size<int>::value, 4_KiB);
// just an alias for std::list<int, memory::std_allocator<int, memory::memory_pool<>>
// a std::list using a memory_pool
// std_allocator stores a reference to a RawAllocator and provides the Allocator interface
list.push_back(3);
list.push_back(2);
list.push_back(1);
for (auto e : list)
std::cout << e << ' ';
std::cout << '\n';
merge_sort(list.begin(), list.end());
for (auto e : list)
std::cout << e << ' ';
std::cout << '\n';
// allocate a std::unique_ptr using the pool
// memory::allocate_shared is also available
auto ptr = memory::allocate_unique<int>(pool, *list.begin());
std::cout << *ptr << '\n';
// static storage of size 4KiB
// a memory pool again but this time with a BlockAllocator
// this controls the internal allocations of the pool itself
// we need to specify the first template parameter giving the type of the pool as well
// (node_pool is the default)
// we use a static_block_allocator that uses the static storage above
// all allocations will use a memory block on the stack
using static_pool_t = memory::memory_pool<memory::node_pool, memory::static_block_allocator>;
static_pool_t static_pool(memory::unordered_set_node_size<int>::value, 4096u, storage);
// again, just an alias for std::unordered_set<int, std::hash<int>, std::equal_to<int>, memory::std_allocator<int, static_pool_t>
// see why I wrote these? :D
// now we have a hash set that lives on the stack!
set.insert(3);
set.insert(2);
set.insert(3); // running out of stack memory is properly handled, of course
for (auto e : set)
std::cout << e << ' ';
std::cout << '\n';
}
// naive implementation of merge_sort using temporary memory allocator
template <typename BiIter>
void merge_sort(BiIter begin, BiIter end)
{
using value_type = typename std::iterator_traits<BiIter>::value_type;
auto distance = std::distance(begin, end);
if (distance <= 1)
return;
auto mid = begin;
std::advance(mid, distance / 2);
// an allocator for temporary memory
// is similar to alloca() but uses its own stack
// this stack is thread_local and created on the first call to this function
// as soon as the allocator object goes out of scope, everything allocated through it, will be freed
auto alloc = memory::make_temporary_allocator();
// alias for std::vector<value_type, memory::std_allocator<value_type, memory::temporary_allocator>>
// a std::vector using a temporary_allocator
second(mid, end, alloc);
merge_sort(first.begin(), first.end());
merge_sort(second.begin(), second.end());
std::merge(first.begin(), first.end(), second.begin(), second.end(), begin);
}

See example/ for more.

Installation

This library can be used as CMake subdirectory. It is tested on GCC 4.8-5.0, Clang 3.5 and Visual Studio 2013. Newer versions should work too.

  1. Fetch it, e.g. using git submodules git submodule add https://github.com/foonathan/memory ext/memory and git submodule update --init --recursive.
  2. Call add_subdirectory(ext/memory) or whatever your local path is to make it available in CMake.
  3. Simply call target_link_libraries(your_target PUBLIC foonathan_memory) to link this library and setups the include search path and compilation options.

Note: If during CMake you see an error message that compatibility is not on the newest version, run git submodule update --recursive --remote to force the compatiblity submodule of memory to update to the latest version.

You can also install the library:

  1. Run cmake -DCMAKE_BUILD_TYPE="buildtype" -DFOONATHAN_MEMORY_BUILD_EXAMPLES=OFF -DFOONATHAN_MEMORY_BUILD_TESTS=OFF . inside the library sources.
  2. Run cmake --build . -- install to install the library under ${CMAKE_INSTALL_PREFIX}.
  3. Repeat 1 and 2 for each build type/configuration you want to have (like Debug, RelWithDebInfo and Release or custom names).

To use an installed library:

  1. Call find_package(foonathan_memory major.minor REQUIRED) to find the library.
  2. Call target_link_libraries(your_target PUBLIC foonathan_memory) to link to the library and setup all required options.

See http://foonathan.github.io/doc/memory/md_doc_installation.html for a detailed guide.

Documentation

Full documentation can be found at http://foonathan.github.io/doc/memory.

A tutorial is also available at http://foonathan.github.io/doc/memory/md_doc_tutorial.html.

RawAllocator

Below is the interface required for a RawAllocator, everything optional is marked:

struct raw_allocator
{
using is_stateful = std::integral_constant<bool, Value>; // optional, defaults to std::is_empty
void* allocate_node(std::size_t size, std::size_t alignment); // required, allocation function
void deallocate_node(void *node, std::size_t size, std::size_t alignment) noexcept; // required, deallocation function
void* allocate_array(std::size_t count, std::size_t size, std::size_t alignment); // optional, forwards to node version
void deallocate_array(void *ptr, std::size_t count, std::size_t size, std::size_t alignment) noexcept; // optional, forwards to node version
std::size_t max_node_size() const; // optional, returns maximum value
std::size_t max_array_size() const; // optional, forwards to max_node_size()
std::size_t max_alignment() const; // optional, returns maximum fundamental alignment, i.e. alignof(std::max_align_t)
};

A RawAllocator only needs to be moveable, all Allocator classes are RawAllocators too. Classes not providing the interface can specialize the allocator_traits, read more about writing allocators here or about the technical details of the concept here.