I am using boost shared_ptr with my own memory manager like this (stripped down example, I hope there are no errors in it):
class MemoryManager
{
public:
/** Allocate some memory.*/
inline void* allocate(size_t nbytes)
{
return malloc(nbytes);
}
/** Remove memory agian.*/
inline void deallocate(void* p)
{
free(p);
}
};
MemoryManager globalMM;
// New operators
inline void* operator new(size_t nbytes, ogl2d::MemoryManagerImpl& mm)
{
return globalMM.allocate(nbytes);
}
// Corresponding delete operators
inline void operator delete(void *p, ogl2d::MemoryManagerImpl& mm)
{
globalMM.deallocate(p);
}
/** Class for smart pointers, to ensure
* correct deletion by the memory manger.*/
class Deleter
{
public:
void operator()(void *p) {
globalMM.deallocate(p);
}
};
And I am using it like this:
shared_ptr<Object>(new(globalMM) Object, Deleter);
But now I am realizing. If the shared_ptr deletes my onject, it calls Deleter::operator() and the objects gets deleted. But the destructor does not get called ...
How can I change this?
Because deleter should destroy the object:
class Deleter
{
public:
void operator()(Object *p) {
p->~Object();
globalMM.deallocate(p);
}
};
Edit: I was wrong in my deleter, fixed
You can explicitly call the destructor (which means that the Deleter should probably receive a T * instead of a void *). Note that the code you provided doesn't actually use placement new/delete, so my answer is only meaningful for this particular example.
Related
The following code is abstracted from the book << Hands-On Design Patterns with C++ >> by Fedor G. Pikus published by Packt.
Some confusions have been bugging me for weeks.
(1) How the char array mem_ is initialized?
(2) Is allocate used to allocate memory? How?
(3) Why does mem_ == p ? How was the memory delocated?
// 02_scoped_ptr.C
// Version 01 with deletion policy.
#include <cstdlib>
#include <cassert>
#include <iostream>
template <typename T, typename DeletionPolicy>
class SmartPtr {
public:
explicit SmartPtr(T* p = nullptr,
const DeletionPolicy& deletion_policy = DeletionPolicy() )
: p_(p), deletion_policy_(deletion_policy) {}
SmartPtr(const SmartPtr&) = delete;
SmartPtr& operator=(const SmartPtr&) = delete;
~SmartPtr() { deletion_policy_(p_); }
void release() { p_ = NULL; }
T* operator->() { return p_; }
const T* operator->() const { return p_; }
T& operator*() { return *p_; }
const T& operator*() const { return *p_; }
private:
T* p_;
DeletionPolicy deletion_policy_;
};
class SmallHeap {
public:
SmallHeap() {}
SmallHeap(const SmallHeap &) = delete;
SmallHeap &operator=(const SmallHeap &) = delete;
~SmallHeap() {}
void * allocate(size_t s) {
assert(s <= size_);
return mem_; // ------------------ is allocate used to allocate memory? how?
}
void deallocate(void *p) {
assert(mem_ == p); // ------------------- why does mem_ == p ? How was the memory delocated?
}
private:
static constexpr size_t size_ = 1024;
char mem_[size_]; // ------------------- how mem_ is initialized?
};
void * operator new(size_t s, SmallHeap *h)
{
return h->allocate(s);
}
template<typename T>
struct DeleteSmallHeap {
explicit DeleteSmallHeap(SmallHeap &heap)
: heap_(heap) {}
void operator()(T *p) const {
p->~T();
heap_.deallocate(p);
}
private:
SmallHeap &heap_;
};
int main() {
SmallHeap a_sh_obj;
SmartPtr<int, DeleteSmallHeap<int>> sh_sp{new(&a_sh_obj) int(42), DeleteSmallHeap<int>(a_sh_obj)};
std::cout << *sh_sp << std::endl;
}
------------------ Update 1 : how is char related to memory? --------------------
Thanks for the helpful explanations, and I need some time to them, especially the custom allocator.
But one thing that is really strange to me is that:
we are talking about memory stuff, but why do we need a char array here?
This code demonstrates a custom allocator which has a static fixed size of size (1024). There is no allocation, but it can be used as an allocator on a STL container on the assumption that you will never need more than 1024 bytes.
If you do need more, boom.
char mem_[size_];
This line initializes the static size and allocate() simply returns that without any call to new.
For the deallocation it uses a simple assert to ensure that the memory that is to be 'deleted' is the same than the one that was 'created'.
All these practises are practically non existant. If you do need a vector of a static size, use a std::array. If you need a vector of an unknown size, use the reserve() vector function to preallocate. If your vector's size is unknown but expected to be small, it's okay to leave it as it is for, in Windows (and I assume in other OSes), it eventually calls HeapAlloc and HeapFree which, for small allocations, are probably cheap, especially if the vector is within a limited scope.
If you need some flexible combination of stack/heap vector, you can use https://github.com/thelink2012/SmallVector.
How the char array mem_ is initialized?
mem_ is not initialized as in filled with values until the use of the custom new operator in new(&a_sh_obj) int(42). This only initializes a small portion of the memory though. Space is allocated on the stack however when you create the local SmallHeap a_sh_obj; variable in main.
Is allocate used to allocate memory? How?
Yes, it is used. The expression new(&a_sh_obj) int(42) uses
void * operator new(size_t s, SmallHeap *h)
{
return h->allocate(s);
}
which gets sizeof(int) passed as first parameter and &a_sh_obj as second parameter.
Why does mem_ == p? How was the memory delocated?
On destruction of sh_sp the DeleteSmallHeap<int> object is used to get rid of the object. The assert is just verification that the memory "freed" is actually the one expected. This doesn't actually deallocate anything, since the memory is still owned by a_sh_obj. It's leaving the main function that in fact releases the memory during when cleaning up a_sh_obj.
I was trying to understand the c++17 pmr.
So I did this and it is not working as I thought, what could go wrong?
template <typename T>
class Memory : public std::experimental::pmr::memory_resource {
public:
Memory() { this->memory = allocate(sizeof(T), alignof(T)); }
void *getMemory() { return this->memory; }
~Memory() { deallocate(this->memory, sizeof(T), alignof(T)); }
private:
void *do_allocate(std::size_t bytes, std::size_t alignment)
{
memory = ::operator new(bytes);
}
void do_deallocate(void *p, std::size_t bytes, std::size_t alignment)
{
::operator delete(memory);
}
bool do_is_equal(
const std::experimental::pmr::memory_resource& other) const noexcept
{
}
void *memory;
};
what can be going wrong with my implementation?
This is the client..
Memory<std::string> mem;
std::string * st = (std::string*)mem.getMemory();
st->assign("Pius");
std::cout << *st;
The polymorphic resource allocators allocate memory; that's all they do. Unlike container Allocators, they don't create objects. That's why they return void*s.
Memory resources are not meant to be used by themselves. That's why std::polymorphic_allocator<T> exists. You can also do the object creation/destruction yourself, using placement-new and manual destructor calls.
Also, your memory_resource implementation makes no sense. do_allocate should return the allocated memory, not store it internally. Your function provokes undefined behavior by returning nothing (which your compiler should have warned about).
I think this question might be a duplicate, but I don't know how to search for it.
I'm trying to overload operator new so that I can allow for a variable-length buffer after my class. Does my current design work as intended, or is it undefined behavior?
If the answer differs in C++98, C++03, and C++11 then please explain the differences.
struct POD { /* ...other POD members here... */ };
struct BufferedPOD : POD
{
size_t n;
BufferedPOD()
// Assume n is already initialized...
{
}
static void *operator new(size_t size)
{
return ::operator new(size);
}
static void *operator new(size_t size, size_t additional_size)
{
void *const p = operator new(size + additional_size);
static_cast<BufferedPOD *>(p)->n = additional_size;
return p;
}
static void operator delete(void *p)
{
return ::operator delete(p);
}
static void operator delete(void *p, size_t)
{
return operator delete(p);
}
};
int main()
{
std::auto_ptr<BufferedPOD> p(new (1000) BufferedPOD());
foo(p.get()); // do something with buffer
return 0;
}
First off, you are relying on undefined behavior, the memory is indeterminate upon calling the constructor.
In debug builds, it will often be filled with some marker-pattern for easier debugging, in release builds this freedom is generally just used to speed up the construction.
In both, reading indeterminate objects gets you UB, however that will play out in detail.
Anyway, you are going at it the wrong way (let's ignore violation of the "rule of three" for the time being:
Just declare matching overloads for ::operator new and ::operator delete, and a factory-function (which should be the only code with access to the only ctor you leave usable) which uses that and passes the extra-space on:
void* operator new(size_t a, size_t b) {
if(a+b< a || a+b < b)
throw new std::bad_alloc("::operator new(size_t, size_t) too big");
return operator new(a+b);
}
void operator delete(void* p, size_t a, size_t b) {return operator delete(p/*, a+b*/);}
struct Buffered : POD { // Not a pod due to inheritance
Buffered* make(size_t extra) {return new(extra) Buffered(extra);}
private:
size_t n;
Buffered(size_t extra) : n(extra) {}
Buffered(Buffered&) = delete;
void operator=(Buffered&) = delete;
};
I want to override new operator in C++, by following the instructions at http://www.cprogramming.com/tutorial/operator_new.html
class CMyclass
{
public:
CMyClass(BOOL bUseWhichMemManager);
void* operator new(size_t);
void operator delete(void*);
};
I create two memory manager called CMemManager1 and CMemMangaer2, using different algorithms to allocate buffer. Now I want to control which memory manager to be used, when calling new.
I try to add a parameter bUseWhichMemManager to the constructor, but in overrided new function, there are no way to access the parameter. Is there a way to pass more parameters to new operator, such as:
void* operator new(size_t size, BOOL bUseWhichManager);
Thanks
You can pass parameters to an allocation function (i.e. operator new) in the new-expression like this:
new (b) CMyClass // passes "b" to "bUseWhichManager"
assuming you had the following declaration of operator new as a member of CMyClass, as in your question:
void* operator new(size_t size, BOOL bUseWhichManager);
(Don't put the parameter in the constructor, as it doesn't make sense, probably.)
All you need is a placement syntax to be defined for you custom allocation. Like:
void *
operator new (std::size_t size, CMemManager1& manager)
{
return manger.allocate(size) ;
}
void
operator delete (void * p, CMemManager1& manager)
{
manager.deallocate(p) ;
}
void *
operator new (std::size_t size, CMemManager2& manager)
{
return manger.allocate(size) ;
}
void
operator delete (void * p, CMemManager2& manager)
{
manager.deallocate(p) ;
}
and then
CMemManager1 manager1;
CMemManager2 manager2;
CMyclass * p1 = new (manager1) CMyclass ;
CMyclass * p2 = new (manager2) CMyclass ;
// It is a placement syntax so you should call your destructor manually.
p1->~CMyclass();
p2->~CMyclass();
operator delete(p1, manager1);
operator delete(p2, manager2);
Using the placement operator
#include<new>
#include<memory>
class A {
// Some stuff
};
A* a = (A*) _mm_malloc(sizeof(A),16);
// Now call the ctor without allocation
new a A(/* some arguments */);
// Delete
_mm_free(a);
Suppose, we have hierarchy of classes and we want to make them allocate/deallocate their memory only throughout our memory manager. What is a classical C++ way to achieve this behavior?
Is it a MUST to have additional checks such as:
class Foo{
public:
virtual ~Foo(){}
void* operator new(size_t bytes)
{
if (bytes != sizeof(Foo)){
return ::operator new(bytes);
}
return g_memory_manager.alloc(bytes);
}
void operator delete(void *space, size_t bytes)
{
if (bytes != sizeof(Foo)){
return ::operator delete(space);
}
g_memory_manager.dealloc(space, bytes);
}
}
No the checks are unnecessary.
Have a look at Alexandrescu's Loki's SmallObject allocator, you just inherit from SmallObject and it does all the heavy lifting for you!
And do not forget to overload all versions of new and delete:
simple version
array version
placement version
Otherwise you might have some troubles.
my comments:
make creators to do the things, for example:
template<class t>
struct creator_new { // a creator using new()/delete()
static t *Create(void)
{
return new t;
}
static void Destroy(t *p)
{
delete p;
}
};
you could extend the creator to check memory leak, or use memory pool to manage your objects etc.