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The C++ standard purpose

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Storage Recycling
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Stepanov later commented that, while allocators "are not such a bad [idea] in theory He observed that to make allocators really useful, a change to the core language with regards to references was necessary. These changes make stateful allocators much more useful and allow allocators to manage out-of-process shared memory.

Any class that fulfills the allocator requirements can be used as an allocator. In particular, a class A capable of allocating memory for an object of type T must provide the types A:: It should also provide type A:: Although a conforming standard library implementation is allowed to assume that the allocator's A:: An allocator, A , for objects of type T must have a member function with the signature A:: This function returns a pointer to the first element of a newly allocated array large enough to contain n objects of type T ; only the memory is allocated, and the objects are not constructed.

Moreover, an optional pointer argument that points to an object already allocated by A can be used as a hint to the implementation about where the new memory should be allocated in order to improve locality. The corresponding void A:: Object construction and destruction is performed separately from allocation and deallocation.

The semantics of the functions should be equivalent to the following: The above code uses the placement new syntax, and calls the destructor directly. Allocators should be copy-constructible. An allocator for objects of type T can be constructed from an allocator for objects of type U.

If an allocator, A , allocates a region of memory, R , then R can only be deallocated by an allocator that compares equal to A. For example, given an allocator type IntAllocator for objects of type int , a related allocator type for objects of type long could be obtained using IntAllocator:: One of the main reasons for writing a custom allocator is performance. Utilizing a specialized custom allocator may substantially improve the performance or memory usage, or both, of the program.

This approach may work well with containers that mostly allocate large chunks of memory, like vector and deque. A popular approach to improve performance is to create a memory pool -based allocator. The custom allocator will serve individual allocation requests by simply returning a pointer to memory from the pool. Actual deallocation of memory can be deferred until the lifetime of the memory pool ends. Another viable use of custom allocators is for debugging memory-related errors. He mentions three use cases for custom allocators, namely, memory pool allocators, shared memory allocators, and garbage collected memory allocators.

He presents an allocator implementation that uses an internal memory pool for fast allocation and deallocation of small chunks of memory, but notes that such an optimization may already be performed by the allocator provided by the implementation.

When instantiating one of the standard containers, the allocator is specified through a template argument, which defaults to std:: A function expecting an std:: From Wikipedia, the free encyclopedia. This is free software; see the source for copying conditions. The name argument optionally gives the allocator a name, which is useful for gather allocator usage metrics. The examples below show how each of the three operational mode is controlled via the constructor arguments.

The first template argument is block type and the second argument is the block quantity. By calling Allocate , a pointer to a memory block the size of one instance is returned. When obtaining the block, the free-list is checked to determine if a fee block already exists. Deallocate just pushes the block address onto a stack. No searching through a list — just push or pop a block and go. Now comes a handy for linking blocks together in the free-list without consuming any extra storage for the pointers.

If, for example, we use the global operator new , storage is allocated first then the constructor is called. The destruction process is just the reverse; destructor is called, then memory freed.

After the destructor is executed, but before the storage is released back to the heap, the memory is no longer being utilized by the object and is freed to be used for other things, like a next pointer.

Since the Allocator class needs to keep the deleted blocks around, during operator delete we put the list's next pointer in that currently unused object space. When the block is reused by the application, the pointer is no longer needed and will be overwritten by the newly formed object. This way, there is no per-instance storage overhead incurred.

Using freed object space as the memory to link blocks together means the object must be large enough to hold a pointer. The code in the constructor initializer list ensures the minimum block size is never below the size of a pointer. The class destructor frees the storage allocated during execution by deleting the memory pool or, if blocks were obtained off the heap, by traversing the free-list and deleting each block.

Since the Allocator class is typically used as a class scope static , it will only be called upon termination of the program. For most embedded devices, the application is terminated when someone yanks the power from the system. Obviously, in this case, the destructor is not required. If you're using the heap blocks method, the allocated blocks cannot be freed when the application terminates unless all the instances are checked into the free-list.

Therefore, all outstanding objects must be "deleted" before the program ends. Otherwise, you've got yourself a nice memory leak. Which brings up an interesting point. Doesn't Allocator have to track both the free and used blocks? The short answer is no.

The long answer is that once a block is given to the application via a pointer, it then becomes the application's responsibility to return that pointer to Allocator by means of a call to Deallocate before the program ends. This way, we only need to keep track of the freed blocks. I wanted Allocator to be extremely easy to use, so I created macros to automate the interface within a client class.

The macros provide a static instance of Allocator and two member functions: By overloading the new and delete operators, Allocator intercepts and handles all memory allocation duties for the client class.

The operator new function calls Allocator to create memory for a single instance of the class. After the memory is allocated, by definition, the operator new calls the appropriate constructor for the class. When overloading new only the memory allocation duties can be taken over. The constructor call is guaranteed by the language.

Similarly, when deleting an object, the system first calls the destructor for us, and then operator delete is executed. The operator delete uses the Deallocate function to store the memory block in the free-list. This ensures that when deleting a class, the correct derived destructor is called. What is less apparent, however, is how the virtual destructor changes which class's overloaded operator delete is called.

Although not explicitly declared, the operator delete is a static function. As such, it cannot be declared virtual. So at first glance, one would assume that deleting an object with a base pointer couldn't be routed to the correct class. After all, calling an ordinary static function with a base pointer will invoke the base member's version. However, as we know, calling an operator delete first calls the destructor. With a virtual destructor, the call is routed to the derived class.

After the class's destructor executes, the operator delete for that derived class is called. So in essence, the overloaded operator delete was routed to the derived class by way of the virtual destructor.

Therefore, if deletes using a base pointer are performed, the base class destructor must be declared virtual. Otherwise, the wrong destructor and overloaded operator delete will be called. Once the macros are in place, the caller can create and destroy instances of this class and deleted object stored will be recycled:.

Both single and multiple inheritance situations work with the Allocator class. For example, assuming the class Derived inherits from class Base the following code fragment is legal. At run time, Allocator will initially have no blocks within the free-list, so the first call to Allocate will get a block from the pool or heap. As execution continues, the system demand for objects of any given allocator instance will fluctuate and new storage will only be allocated when the free-list cannot offer up an existing block.

Compared to obtaining all blocks using the memory manager, the class saves a lot of processing power. During allocations, a pointer is just popped off the free-list, making it extremely quick. Deallocations just push a block pointer back onto the list, which is equally as fast. Benchmarking the Allocator performance vs. See the attached source code for the exact algorithm. Windows uses a debug heap when executing within the debugger.

The debug heap adds extra safety checks slowing its performance. The release heap is much faster as the checks are disabled. The debug global heap is predictably the slowest at about 1. However, the basic point is illustrated nicely; the memory manager is slower than allocator and highly dependent on the platform's implementation.

The Allocator running in static pool mode doesn't rely upon the heap. The Allocator running heap blocks mode is just as fast once the free-list is populated with blocks obtained from the heap. Recall that the heap blocks mode relies upon the global heap to get new blocks, but then recycles them into the free-list for later use. Run 1 shows the allocation hit creating the memory blocks at 30mS. Subsequent benchmarks clock in a very fast 7mS since the free-list is fully populated.

Here are the results. As the ARM benchmark results show, the Allocator class is about 15 times faster which is quite significant. The benchmark test allocates, in the ARM case, byte blocks. Then every other byte block is deleted followed by allocating byte blocks.

That last group of allocations added 9. What this says is that when the heap gets fragmented, you can expect the memory manager to take longer with non-deterministic times. The first decision to make is do you need an allocator at all. If you don't have an execution speed or fault-tolerance requirement for your project, you probably don't need a custom allocator and the global heap will work just fine. An architect for a mission critical design may forbid all use of the global heap.

Yet dynamic allocation may lead to a more efficient or elegant design. In this case, you could use the heap blocks mode during debug development to gain memory usage metrics, then for release switch to the static pool method to create statically allocated pools thus eliminating all global heap access.

A few compile-time macros switch between the modes. Alternatively, the heap blocks mode may be fine for the application. It does utilize the heap to obtain new blocks, but does prevent heap-fragmentation faults and speeds allocations once the free-list is populated with enough blocks. While not implemented in the source code due to multi-threaded issues outside the scope of this article, it is easy have the Allocator constructor keep a static list of all constructed instances.

Run the system for a while, then at some pointer iterate through all allocators and output metrics like block count and name via the GetBlockCount and GetName functions. The usage metrics provide information on sizing the fixed memory pool for each allocator when switching over to a memory pool.

Always add a few more blocks than maximum measured block count to give the system some added resiliency against the pool running out of memory. Debugging memory leaks can be very difficult, mostly because the heap is a black box with no visibility into the types and sizes of objects allocated.

With Allocator , memory leaks are a bit easier to find since the Allocator tracks the total block count. Repeatedly output to the console for example the GetBlockCount and GetName for each allocator instance and comparing the differences should expose the allocator with an ever increasing block count. If the memory manager faults while attempting to allocate memory off the heap, the user's error-handling function is called via the new-handler function pointer.

By assigning the user's function address to the new-handler, the memory manager is able to call a custom error-handling routine. To make the Allocator class's error handling consistent, allocations that exceed the pool's storage capacity also call the function pointed to by new-handler, centralizing all memory allocation faults in one place.

The class does not support arrays of objects. An overloaded operator new[] poses a problem for the object recycling method. Creating separate storage for each element is not an option because multiple calls to new don't guarantee that the blocks are within a contiguous space, which an array needs. Since Allocator only handles same size blocks, arrays are not allowed. This implementation assumes the new-handler function will not return, such as an infinite loop trap or assertion, so it makes no sense to call the handler if the function resolves allocation failures by compacting the heap.

A fixed pool is being used and no amount of compaction will remedy that. Articles Quick Answers Messages. David Lafreniere , 28 Mar Please Sign up or sign in to vote. A fixed block memory allocator that increases system performance and offers heap fragmentation fault protection.

The solution presented here will: Be faster than the global heap Eliminate heap fragmentation memory faults Require no additional storage overhead except for a few bytes of static memory Be easy to use Use minimal code space A simple class that dispenses and reclaims memory will provide all of the aforementioned benefits, as I'll show.

Storage Recycling The basic philosophy of the memory management scheme is to recycle memory obtained during object allocations. Depending on the desired behavior of Allocator , storage comes from either the global heap or a static memory pool with one of three operating modes: Heap blocks Heap pool Static pool Heap vs. Pool The Allocator class is capable of creating new blocks from the heap or a memory pool whenever the free-list cannot provide an existing one. Class Design The class interface is really straightforward.

Allocate ; Deallocate just pushes the block address onto a stack. Deallocate memory1 ; Now comes a handy for linking blocks together in the free-list without consuming any extra storage for the pointers. Using the Code I wanted Allocator to be extremely easy to use, so I created macros to automate the interface within a client class.

Benchmarking Benchmarking the Allocator performance vs. Allocator Decisions The first decision to make is do you need an allocator at all. Debugging Memory Leaks Debugging memory leaks can be very difficult, mostly because the heap is a black box with no visibility into the types and sizes of objects allocated.

Limitations The class does not support arrays of objects. New source code zip file. I've been a professional software engineer for over 20 years. When not writing code, I enjoy spending time with the family, camping and riding motorcycles around Southern California. On-heap fixed allocation outside MFC. You must Sign In to use this message board.

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CppCon 2017: How to Write a Custom Allocator—Bob Steagall

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I haven't written C++ code with a custom STL allocator, but I can imagine a webserver written in C++, which uses a custom allocator for automatic deletion of temporary data needed for responding to a HTTP request. There's a number of other times I can see writing your own custom allocator in the context of embedded systems, for example .

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Writing a custom allocator Ask Question. C++ found an example and adpated it When I made it compilable and writing it business plan writing services ireland std:: So I would appreciate some allocator where I'm going wrong.

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Writing a custom allocator Ask Question. I found an example and adpated custom When I made it compilable and tested it the std:: So I would appreciate some pointers c++ I'm going wrong. Using Visual Studio for now, so maybe not everything. You'd like to improve the performance of your application with regard to memory management, and you believe this can be accomplished by writing a custom allocator. But where do you start? Modern C++ brings many improvements to the standard allocator model, but with those improvements come several.

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I'm trying to write a custom allocator, which allocator space for a fixed number of elements. However, Writing have writing problems with understanding the requirements. I custom an example and adpated it. Aug 28,  · How to write custom allocator for vector of pointers? Unspoken. Hi guys. I am having this problem that I often use a vector of pointers to some objects. The problem is that I need to delete pointers in that vector manually which is prone to errors / memory leaks. Currently I am doing it like this.