I'm looking into a solution of building containers which track stored size of their elements in addition to basic functions.
So far I didn't saw a solution which doesn't create a huge amount of boilerplate code of each invalidating member of container. This also assumes that stored elements cannot change size after being stored.
Unless standard containers have some feature that allows to inject such behaviour. The following example should be working one, albeit abridged for brevity. The declarations used are:
typedef uint8_t Byte;
typedef Byte PacketId;
template <class T>
struct CollectionTraits {
typedef T collection_type;
typedef typename collection_type::value_type value_type;
typedef typename collection_type::size_type size_type;
typedef typename collection_type::iterator iterator;
typedef typename collection_type::reference reference;
typedef typename collection_type::const_iterator const_iterator;
const_iterator begin() const { return _collection.begin(); }
const_iterator end() const { return _collection.end(); }
iterator begin() { return _collection.begin(); }
iterator end() { return _collection.end(); }
size_type size() const { return _collection.size(); }
protected:
T _collection;
};
struct Packet : CollectionTraits<std::vector<Byte>>
{
PacketId id;
};
The container itself:
struct PacketList : CollectionTraits<std::deque<Packet>>
{
public:
typedef Packet::size_type data_size;
void clear() { _collection.clear(); _total_size = 0; }
data_size total_size() const { return _total_size; }
void push_back(const Packet& v) {
_collection.push_back(v);
_add(v);
}
void push_back(const Packet&& v) {
_collection.push_back(std::move(v));
_add(v);
}
void push_front(const Packet& v) {
_collection.push_front(v);
_add(v);
}
void push_front(const Packet&& v) {
_collection.push_front(std::move(v));
_add(v);
}
void pop_back() {
_remove(_collection.back());
_collection.pop_back();
}
void erase(const_iterator first, const_iterator last) {
for(auto it = first; it != last; ++it) _remove(*it);
_collection.erase(first, last);
}
PacketList() : _total_size(0) {}
PacketList(const PacketList& other) : _total_size(other._total_size) {}
private:
void _add(const Packet& v) { _total_size += v.size(); }
void _remove(const Packet& v) { _total_size -= v.size(); }
data_size _total_size;
};
The interface in result should similar to a standard container. Is there a way to avoid this amount of repeated code? Is there some standard solution for this problem?
I want to have a template function which accepts unary member-function pointers of an instance of some generic type.
My problem is that I must support both void(T val) and void(const T& val) member functions.
I have written one template function for each case and it works fine, but this leads to code duplication since the function logic is completely the same. (I found something completely similar here: Function taking both pointer to member-function and pointer to const member-function but I fail to see a definitive solution).
An example of the generic type mentioned above:
using UserAddress = std::string;
class User
{
private:
int mHeight;
UserAddress mAddress;
public:
void SetHeight(int height){mHeight = height;}
void SetAddress(const UserAddress& address){mAddress = address;}
};
Where UserAddress is some heavy type I want to pass by reference.
My templated function:
template <typename TPersistentObject>
class Persistence
{
private:
std::map<std::string, std::function<void(User*)>> mSetterOfProperty;
template <typename TPersistentObject, typename TPropertyValue>
void DefinePropertySettingMethod(const std::string& propertyName,
void (TPersistentObject::*propertySetter)(TPropertyValue), std::function<TPropertyValue(void)> dataReader)
{
mSetterOfProperty[propertyName] =
[propertySetter, columnDataReader](TPersistentObject* persistentObject)
{
(persistentObject->*propertySetter)(dataReader());
};
}
};
/// Const& implementation leading to code duplication
template <typename TPersistentObject, typename TPropertyValue>
void DefinePropertySettingMethod(const std::string& propertyName,
void (TPersistentObject::*propertySetter)(const TPropertyValue&), std::function<TPropertyValue(void)> dataReader)
{
...
}
};
Is there some way to define this function to support the following:
int main()
{
auto intDataReader = []() {
return 1;
};
auto stringDataReader = []() {
return UserAddress("Next Door");
};
Persistence p;
p.DefinePropertySettingMethod<User,int>("Height", &User::SetHeight, intDataReader);
p.DefinePropertySettingMethod<User,UserAddress>("Address", &User::SetAddress, stringDataReader);
}
Thanks to Igor Tandetnik 's tip I managed to compile a solution. std::enable_if is not what I needed though since I did not need to deactivate an overload (or at least I couldn't come to a solution using it).
std::conditional did the trick.
Here is the code:
#include <string>
#include <functional>
#include <map>
#include <string>
#include <type_traits>
using UserAddress = std::string;
class User
{
private:
int mHeight;
UserAddress mAddress;
public:
void SetHeight(int height){mHeight = height;}
void SetAddress(const UserAddress& address){mAddress = address;}
};
template <typename TPersistentObject>
class Persistence
{
public:
std::map<std::string, std::function<void(TPersistentObject*)>> mSetterOfProperty;
template <typename TPropertyValue>
void DefinePropertySettingMethod(const std::string& propertyName,
void (TPersistentObject::*propertySetter)(TPropertyValue),
std::function<
typename std::conditional<!std::is_same<TPropertyValue, typename std::decay<TPropertyValue>::type>::value,
typename std::decay<TPropertyValue>::type, TPropertyValue>::type
(void)> dataReader)
{
mSetterOfProperty[propertyName] =
[propertySetter, dataReader](TPersistentObject* persistentObject)
{
(persistentObject->*propertySetter)(dataReader());
};
}
};
int main()
{
std::function<int()> intDataReader = []() {
return 1;
};
std::function<std::string()> stringDataReader = []() {
return UserAddress("Next Door");
};
Persistence<User> p;
p.DefinePropertySettingMethod("Height", &User::SetHeight, intDataReader);
p.DefinePropertySettingMethod("Address", &User::SetAddress, stringDataReader);
}
My code runs ok when thread number is 15 or less, but when I run it with larger thread number (but still a very tiny number) say 50. I ran into following error when main function exits, seems like error occurs in the cleaning up process. I couldn't figure out where the bug is. My development tool is Visual Studio 2017. Here's my code:
threadsafe_queue class:
#pragma once
#include <memory>
#include <mutex>
template<typename T>
class threadsafe_queue
{
private:
struct Node {
std::shared_ptr<T> data;
std::unique_ptr<Node> next;
};
Node* tail;
std::unique_ptr<Node> head;
std::mutex head_mutex;
std::mutex tail_mutex;
std::condition_variable data_cond;
Node* get_tail();
std::unique_ptr<Node> pop_head();
std::unique_lock<std::mutex> wait_for_data();
public:
threadsafe_queue();
~threadsafe_queue();
threadsafe_queue(const threadsafe_queue& t) = delete;
threadsafe_queue operator = (const threadsafe_queue& t) = delete;
void push(T);
bool try_pop(T&);
std::shared_ptr<T> try_pop();
void wait_and_pop(T&);
std::shared_ptr<T> wait_and_pop();
bool empty();
};
using namespace std;
template<typename T>
threadsafe_queue<T>::threadsafe_queue() {
head = std::unique_ptr<Node>(new Node);
tail = head.get();
}
template<typename T>
threadsafe_queue<T>::~threadsafe_queue()
{
}
template<typename T>
typename threadsafe_queue<T>::Node* threadsafe_queue<T>::get_tail() {
lock_guard<mutex> lock(tail_mutex);
return tail;
}
template<typename T>
unique_ptr<typename threadsafe_queue<T>::Node> threadsafe_queue<T>::pop_head()
{
auto old_head = move(head);
head = move(old_head->next);
return old_head;
}
template<typename T>
unique_lock<mutex> threadsafe_queue<T>::wait_for_data()
{
unique_lock<mutex> headLock(head_mutex);
data_cond.wait(headLock, [&] {return head.get() != get_tail(); });
return std::move(headLock);
}
template<typename T>
void threadsafe_queue<T>::wait_and_pop(T & value)
{
unique_lock<mutex> lock(wait_for_data());
value = move(pop_head()->data);
}
template<typename T>
shared_ptr<T> threadsafe_queue<T>::wait_and_pop()
{
unique_lock<mutex> lock(wait_for_data());
return pop_head()->data;
}
template<typename T>
void threadsafe_queue<T>::push(T newValue)
{
shared_ptr<T> data(make_shared<T>(std::move(newValue)));
unique_ptr<Node> new_tail(new Node);
{
lock_guard<mutex> lock(tail_mutex);
tail->data = data;
Node* new_tail_ptr = new_tail.get();
tail->next = move(new_tail);
tail = new_tail_ptr;
}
data_cond.notify_one();
}
template<typename T>
bool threadsafe_queue<T>::try_pop(T & value)
{
lock_guard<mutex> headLock(head_mutex);
if (head == get_tail())
return false;
value = move(pop_head()->data);
return true;
}
template<typename T>
shared_ptr<T> threadsafe_queue<T>::try_pop()
{
lock_guard<mutex> headLock(head_mutex);
if (head == get_tail())
return shared_ptr<T>();
return pop_head()->data;
}
template<typename T>
bool threadsafe_queue<T>::empty()
{
lock_guard<mutex> lock(head_mutex);
return head.get() == get_tail();
}
main function:
#pragma once
#include "threadsafe_queue.h"
#include <assert.h>
#include <memory>
#include <atomic>
#include <vector>
#include <thread>
using namespace std;
void worker(threadsafe_queue<int>& queue, std::atomic<int>& count, int const & pushcount, int const & popcount) {
for (unsigned i = 0; i < pushcount; i++) {
queue.push(i);
count++;
}
for (unsigned i = 0; i < popcount; i++) {
queue.wait_and_pop();
count--;
}
}
int main() {
threadsafe_queue<int> queue;
std::atomic<int> item_count = 0;
std::vector<thread*> threads;
unsigned const THREAD_COUNT=50, PUSH_COUT=100, POP_COUNT=50;
for (unsigned i = 0; i < THREAD_COUNT; i++) {
threads.push_back(new thread(worker, ref(queue), ref(item_count), ref(PUSH_COUT), ref(POP_COUNT)));
}
for (auto thread : threads) {
thread->join();
}
for (auto thread : threads) {
delete thread;
}
assert(item_count == THREAD_COUNT * (PUSH_COUT-POP_COUNT));
return 0;
}
error message:
Unhandled exception at 0x00862899 in Sample.exe: 0xC00000FD: Stack overflow
(parameters: 0x00000001, 0x00E02FDC). occurred
The location of the error is in memory library code:
const pointer& _Myptr() const _NOEXCEPT
{ // return const reference to pointer
return (_Mypair._Get_second());
}
The answer is based on #IgorTandetnik 's comment above. Basically I needed to implement ~threadsafe_queue to destroy the nodes iteratively. The nodes are linked, so they will be destructed in recursive manner, which causes stack overflow when number of nodes remaining in the queue is relatively large. Below is the destructor code.
threadsafe_queue<T>::~threadsafe_queue(){
Node* current = head.release();
while (current != tail) {
Node* temp = (current->next).release();
delete current;
current = temp;
}
delete tail;
}
I am trying to create an stl container of a move-only type that uses its own allocator in VStudio 2012.
The trouble is: it seems as though I have to provide a construct function for the allocator which in turn needs access to a public copy constructor on the contained type.
I either get:
error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'
or
error C2039: 'construct' : is not a member of 'MyAllocator'
The same code works in clang so I suspect the problem is due to Microsoft but can anyone suggest a possible work around?
This is my code for minimal reproduction
#include <memory>
#include <vector>
using namespace std;
template< typename T>
struct MyAllocator
{
typedef T value_type;
typedef value_type* pointer;
typedef value_type& reference;
typedef const value_type* const_pointer;
typedef const value_type& const_reference;
typedef size_t size_type;
typedef ptrdiff_t difference_type;
template<class t_other>
struct rebind
{
typedef MyAllocator<t_other> other;
};
MyAllocator():m_id(0) {}
MyAllocator(int id):m_id(id){}
template <class T>
MyAllocator(const MyAllocator<T>& other)
:m_id(other.getId())
{
}
T* allocate(std::size_t n)
{
return reinterpret_cast<T*>(malloc(sizeof(T) * n));
}
void deallocate(T* p, std::size_t n)
{
free(p);
}
int getId() const{ return m_id;}
//Have to add these although should not be necessary
void construct(pointer mem, const_reference value)
{
std::_Construct(mem, value);
}
void destroy(pointer mem)
{
std::_Destroy(mem);
}
private:
int m_id;
};
template <class T1, class U>
bool operator==(const MyAllocator<T1>& lhs, const MyAllocator<U>& rhs)
{
return lhs.getId() == rhs.getId() ;
}
template <class T1, class U>
bool operator!=(const MyAllocator<T1>&, const MyAllocator<U>&)
{
return lhs.getId() != rhs.getId();
}
//define a move only type
typedef unique_ptr<uint32_t> MyIntPtr;
//define a container based on MyIntPtr and MyAllocator
typedef vector<MyIntPtr, MyAllocator<MyIntPtr> > MyVector;
int main(int argc, char* argv[])
{
MyAllocator<MyIntPtr> alloc1(1);
MyVector vec(alloc1);
uint32_t* rawPtr = new uint32_t;
*rawPtr = 18;
vec.emplace_back(rawPtr);
return 0;
}
The error you get is because you try to construct a std::unique_ptr from a constant reference to a std::unique_ptr of the same type - and there is no such constructor.
You can rework your construct method to take an an rvalue reference and then everything compiles nicely:
void construct(pointer mem, value_type&& value)
{
std::_Construct(mem, std::move(value));
}
In std::map, this ends up causing an error when the first object is constructed. I've checked the debugger, and I see that free_list::init() creates the consecutive memory addresses correctly. I'm aware this allocator cannot be used in vector or other related containers, but it's only meant to work with the nodular containers.
I get a run-time error from this in xutility (in VC12), at line 158:
_Container_proxy *_Parent_proxy = _Parent->_Myproxy;
Checking the debugger, it appears that _Parent was never initialized, bringing about the 0xC0000005 run-time error. Why or how it didn't get initialized and why this occurred when the first object was being constructed (after std::map did 3 separate allocations), I do not know.
I would like to have this work with std::map and std::list and the other nodular containers and am not worried about whether it can perform in std::vector, etc.
#include <algorithm>
class free_list {
public:
free_list() {}
free_list(free_list&& other)
: m_next(other.m_next) {
other.m_next = nullptr;
}
free_list(void* data, std::size_t num_elements, std::size_t element_size) {
init(data, num_elements, element_size);
}
free_list& operator=(free_list&& other) {
m_next = other.m_next;
other.m_next = nullptr;
}
void init(void* data, std::size_t num_elements, std::size_t element_size) {
union building {
void* as_void;
char* as_char;
free_list* as_self;
};
building b;
b.as_void = data;
m_next = b.as_self;
b.as_char += element_size;
free_list* runner = m_next;
for (std::size_t s = 1; s < num_elements; ++s) {
runner->m_next = b.as_self;
runner = runner->m_next;
b.as_char += element_size;
}
runner->m_next = nullptr;
}
free_list* obtain() {
if (m_next == nullptr) {
return nullptr;
}
free_list* head = m_next;
m_next = head->m_next;
return head;
}
void give_back(free_list* ptr) {
ptr->m_next = m_next;
m_next = ptr;
}
free_list* m_next;
};
template<class T>
class pool_alloc {
typedef pool_alloc<T> myt;
public:
typedef std::size_t size_type;
typedef std::ptrdiff_t difference_type;
typedef T value_type;
typedef T& reference;
typedef const T& const_reference;
typedef T* pointer;
typedef const T* const_pointer;
typedef std::false_type propagate_on_container_copy_assignment;
typedef std::true_type propagate_on_container_move_assignment;
typedef std::true_type propagate_on_container_swap;
template<class U> struct rebind {
typedef pool_alloc<U> other;
};
~pool_alloc() {
destroy();
}
pool_alloc() : data(nullptr), fl(), capacity(4096) {
}
pool_alloc(size_type capacity) : data(nullptr), fl(), capacity(capacity) {}
pool_alloc(const myt& other)
: data(nullptr), fl(), capacity(other.capacity) {}
pool_alloc(myt&& other)
: data(other.data), fl(std::move(other.fl)), capacity(other.capacity) {
other.data = nullptr;
}
template<class U>
pool_alloc(const pool_alloc<U>& other)
: data(nullptr), fl(), capacity(other.max_size()) {}
myt& operator=(const myt& other) {
destroy();
capacity = other.capacity;
}
myt& operator=(myt&& other) {
destroy();
data = other.data;
other.data = nullptr;
capacity = other.capacity;
fl = std::move(other.fl);
}
static pointer address(reference ref) {
return &ref;
}
static const_pointer address(const_reference ref) {
return &ref;
}
size_type max_size() const {
return capacity;
}
pointer allocate(size_type) {
if (data == nullptr) create();
return reinterpret_cast<pointer>(fl.obtain());
}
void deallocate(pointer ptr, size_type) {
fl.give_back(reinterpret_cast<free_list*>(ptr));
}
template<class... Args>
static void construct(pointer ptr, Args&&... args) {
::new (ptr) T(std::forward<Args>(args)...);
}
static void destroy(pointer ptr) {
ptr->~T();
}
bool operator==(const myt& other) const {
return reinterpret_cast<char*>(data) ==
reinterpret_cast<char*>(other.data);
}
bool operator!=(const myt& other) const {
return !operator==(other);
}
private:
void create() {
data = ::operator new(capacity * sizeof(value_type));
fl.init(data, capacity, sizeof(value_type));
}
void destroy() {
::operator delete(data);
data = nullptr;
}
void* data;
free_list fl;
size_type capacity;
};
template<>
class pool_alloc < void > {
public:
template <class U> struct rebind { typedef pool_alloc<U> other; };
typedef void* pointer;
typedef const void* const_pointer;
typedef void value_type;
};
The problem comes when std::pair is being constructed (in MSVC12 utility at line 214):
template<class _Other1,
class _Other2,
class = typename enable_if<is_convertible<_Other1, _Ty1>::value
&& is_convertible<_Other2, _Ty2>::value,
void>::type>
pair(_Other1&& _Val1, _Other2&& _Val2)
_NOEXCEPT_OP((is_nothrow_constructible<_Ty1, _Other1&&>::value
&& is_nothrow_constructible<_Ty2, _Other2&&>::value))
: first(_STD forward<_Other1>(_Val1)),
second(_STD forward<_Other2>(_Val2))
{ // construct from moved values
}
Even after stepping in, the run-time error occurs, the same as described above with _Parent not being initialized.
I was able to answer my own question through extensive debugging. Apparently, VC12's std::map implementation at least at times will cast an _Alnod (permanent allocator that stays in scope for the life of the map, which is used to allocate and deallocate the nodes in the map, what I'd expect to be what actually calls allocate() and deallocate()) as an _Alproxy, a temporary allocator which creates some sort of object called _Mproxy (or something like that) using allocate(). The problem, though, is that VC12's implementation then lets _Alproxy go out of scope while still expecting the pointer to the allocated object to remain valid, so it is clear then that I would have to use ::operator new and ::operator delete on an object like _Mproxy: using a memory pool that then goes out of scope while a pointer to a location in it remains is what causes the crash.
I came up with what I suppose could be called a dirty trick, a test that is performed when copy-constructing or copy-assigning an allocator to another allocator type:
template<class U>
pool_alloc(const pool_alloc<U>& other)
: data(nullptr), fl(), capacity(other.max_size()), use_data(true) {
if (sizeof(T) < sizeof(U)) use_data = false;
}
I added the bool member use_data to the allocator class, which if true means to use the memory pool and which if false means to use ::operator new and ::operator delete. By default, it is true. The question of its value arises when the allocator gets cast as another allocator type whose template parameter's size is smaller than that of the source allocator; in that case, use_data is set to false. Because this _Mproxy object or whatever it's called is rather small, this fix seems to work, even when using std::set with char as the element type.
I've tested this using std::set with type char in both VC12 and GCC 4.8.1 in 32-bit and have found that in both cases it works. When allocating and deallocating the nodes in both cases, the memory pool is used.
Here is the full source code:
#include <algorithm>
class free_list {
public:
free_list() {}
free_list(free_list&& other)
: m_next(other.m_next) {
other.m_next = nullptr;
}
free_list(void* data, std::size_t num_elements, std::size_t element_size) {
init(data, num_elements, element_size);
}
free_list& operator=(free_list&& other) {
if (this != &other) {
m_next = other.m_next;
other.m_next = nullptr;
}
return *this;
}
void init(void* data, std::size_t num_elements, std::size_t element_size) {
union building {
void* as_void;
char* as_char;
free_list* as_self;
};
building b;
b.as_void = data;
m_next = b.as_self;
b.as_char += element_size;
free_list* runner = m_next;
for (std::size_t s = 1; s < num_elements; ++s) {
runner->m_next = b.as_self;
runner = runner->m_next;
b.as_char += element_size;
}
runner->m_next = nullptr;
}
free_list* obtain() {
if (m_next == nullptr) {
return nullptr;
}
free_list* head = m_next;
m_next = head->m_next;
return head;
}
void give_back(free_list* ptr) {
ptr->m_next = m_next;
m_next = ptr;
}
free_list* m_next;
};
template<class T>
class pool_alloc {
typedef pool_alloc<T> myt;
public:
typedef std::size_t size_type;
typedef std::ptrdiff_t difference_type;
typedef T value_type;
typedef T& reference;
typedef const T& const_reference;
typedef T* pointer;
typedef const T* const_pointer;
typedef std::false_type propagate_on_container_copy_assignment;
typedef std::true_type propagate_on_container_move_assignment;
typedef std::true_type propagate_on_container_swap;
myt select_on_container_copy_construction() const {
return *this;
}
template<class U> struct rebind {
typedef pool_alloc<U> other;
};
~pool_alloc() {
clear();
}
pool_alloc() : data(nullptr), fl(), capacity(4096), use_data(true) {}
pool_alloc(size_type capacity) : data(nullptr), fl(),
capacity(capacity), use_data(true) {}
pool_alloc(const myt& other)
: data(nullptr), fl(), capacity(other.capacity),
use_data(other.use_data) {}
pool_alloc(myt&& other)
: data(other.data), fl(std::move(other.fl)), capacity(other.capacity),
use_data(other.use_data) {
other.data = nullptr;
}
template<class U>
pool_alloc(const pool_alloc<U>& other)
: data(nullptr), fl(), capacity(other.max_size()), use_data(true) {
if (sizeof(T) < sizeof(U)) use_data = false;
}
myt& operator=(const myt& other) {
if (*this != other) {
clear();
capacity = other.capacity;
use_data = other.use_data;
}
}
myt& operator=(myt&& other) {
if (*this != other) {
clear();
data = other.data;
other.data = nullptr;
capacity = other.capacity;
use_data = other.use_data;
fl = std::move(other.fl);
}
return *this;
}
template<class U>
myt& operator=(const pool_alloc<U>& other) {
if (this != reinterpret_cast<myt*>(&other)) {
capacity = other.max_size();
if (sizeof(T) < sizeof(U))
use_data = false;
else
use_data = true;
}
return *this;
}
static pointer address(reference ref) {
return &ref;
}
static const_pointer address(const_reference ref) {
return &ref;
}
size_type max_size() const {
return capacity;
}
pointer allocate(size_type) {
if (use_data) {
if (data == nullptr) create();
return reinterpret_cast<pointer>(fl.obtain());
} else {
return reinterpret_cast<pointer>(::operator new(sizeof(T)));
}
}
void deallocate(pointer ptr, size_type) {
if (use_data) {
fl.give_back(reinterpret_cast<free_list*>(ptr));
} else {
::operator delete(reinterpret_cast<void*>(ptr));
}
}
template<class... Args>
static void construct(pointer ptr, Args&&... args) {
::new ((void*)ptr) value_type(std::forward<Args>(args)...);
}
static void destroy(pointer ptr) {
ptr->~value_type();
}
bool operator==(const myt& other) const {
return reinterpret_cast<char*>(data) ==
reinterpret_cast<char*>(other.data);
}
bool operator!=(const myt& other) const {
return !operator==(other);
}
private:
void create() {
size_type size = sizeof(value_type) < sizeof(free_list*) ?
sizeof(free_list*) : sizeof(value_type);
data = ::operator new(capacity * size);
fl.init(data, capacity, size);
}
void clear() {
::operator delete(data);
data = nullptr;
}
void* data;
free_list fl;
size_type capacity;
bool use_data;
};
template<>
class pool_alloc < void > {
public:
template <class U> struct rebind { typedef pool_alloc<U> other; };
typedef void* pointer;
typedef const void* const_pointer;
typedef void value_type;
};
template<class Container, class Alloc>
void change_capacity(Container& c, typename Alloc::size_type new_capacity) {
Container temp(c, Alloc(new_capacity));
c = std::move(temp);
}
Since the allocator is not automatic-growing (don't know how to make such a thing), I have added the change_capacity() function.