C++内存池的简单原理及实现

为什么要用内存池

C++程序默认的内存管理（new，delete，malloc，free）会频繁地在堆上分配和释放内存，导致性能的损失，产生大量的内存碎片，降低内存的利用率。默认的内存管理因为被设计的比较通用，所以在性能上并不能做到极致。
因此，很多时候需要根据业务需求设计专用内存管理器，便于针对特定数据结构和使用场合的内存管理，比如：内存池。

内存池原理

内存池的思想是，在真正使用内存之前，预先申请分配一定数量、大小预设的内存块留作备用。当有新的内存需求时，就从内存池中分出一部分内存块，若内存块不够再继续申请新的内存，当内存释放后就回归到内存块留作后续的复用，使得内存使用效率得到提升，一般也不会产生不可控制的内存碎片。

内存池设计

算法原理：

1. 预申请一个内存区chunk，将内存中按照对象大小划分成多个内存块block
2. 维持一个空闲内存块链表，通过指针相连，标记头指针为第一个空闲块
3. 每次新申请一个对象的空间，则将该内存块从空闲链表中去除，更新空闲链表头指针
4. 每次释放一个对象的空间，则重新将该内存块加到空闲链表头
5. 如果一个内存区占满了，则新开辟一个内存区，维持一个内存区的链表，同指针相连，头指针指向最新的内存区，新的内存块从该区内重新划分和申请

如图所示：

内存池实现

memory_pool.hpp

#ifndef _MEMORY_POOL_H_
#define _MEMORY_POOL_H_

#include 
#include 

template<size_t BlockSize, size_t BlockNum = 10>
class MemoryPool
{
public:
	MemoryPool()
	{
		std::lock_guard<std::mutex> lk(mtx); // avoid race condition

		// init empty memory pointer
		free_block_head = NULL;
		mem_chunk_head = NULL;
	}

	~MemoryPool()
	{
		std::lock_guard<std::mutex> lk(mtx); // avoid race condition

		// destruct automatically
		MemChunk* p;
		while (mem_chunk_head)
		{
			p = mem_chunk_head->next;
			delete mem_chunk_head;
			mem_chunk_head = p;
		}
	}

	void* allocate()
	{
		std::lock_guard<std::mutex> lk(mtx); // avoid race condition

		// allocate one object memory

		// if no free block in current chunk, should create new chunk
		if (!free_block_head)
		{
			// malloc mem chunk
			MemChunk* new_chunk = new MemChunk;
			new_chunk->next = NULL;

			// set this chunk's first block as free block head
			free_block_head = &(new_chunk->blocks[0]);

			// link the new chunk's all blocks
			for (int i = 1; i < BlockNum; i++)
				new_chunk->blocks[i - 1].next = &(new_chunk->blocks[i]);
			new_chunk->blocks[BlockNum - 1].next = NULL; // final block next is NULL
			
			if (!mem_chunk_head)
				mem_chunk_head = new_chunk;
			else
			{
				// add new chunk to chunk list
				mem_chunk_head->next = new_chunk;
				mem_chunk_head = new_chunk;
			}
		}

		// allocate the current free block to the object
		void* object_block = free_block_head;
		free_block_head = free_block_head->next; 

		return object_block;
	}

	void* allocate(size_t size)
	{
		std::lock_guard<std::mutex> lk(array_mtx); // avoid race condition for continuous memory

		// calculate objects num
		int n = size / BlockSize;

		// allocate n objects in continuous memory
		
		// FIXME: make sure n > 0
		void* p = allocate();

		for (int i = 1; i < n; i++)
			allocate();

		return p;
	}

	void deallocate(void* p)
	{
		std::lock_guard<std::mutex> lk(mtx); // avoid race condition

		// free object memory
		FreeBlock* block = static_cast<FreeBlock*>(p);
		block->next = free_block_head; // insert the free block to head
		free_block_head = block;
	}

private:
	// free node block, every block size exactly can contain one object
	struct FreeBlock
	{
		unsigned char data[BlockSize];
		FreeBlock* next;
	};

	FreeBlock* free_block_head;

	// memory chunk, every chunk contains blocks number with fixed BlockNum
	struct MemChunk
	{
		FreeBlock blocks[BlockNum];
		MemChunk* next;
	};

	MemChunk* mem_chunk_head;

	// thread safe related
	std::mutex mtx;
	std::mutex array_mtx;
};

#endif // !_MEMORY_POOL_H_

main.cpp

#include 
#include "memory_pool.hpp"

class MyObject
{
public:
	MyObject(int x): data(x)
	{
		//std::cout << "contruct object" << std::endl;
	}

	~MyObject()
	{
		//std::cout << "destruct object" << std::endl;
	}

	int data;

	// override new and delete to use memory pool
	void* operator new(size_t size);
	void operator delete(void* p);
	void* operator new[](size_t size);
	void operator delete[](void* p);
};

// define memory pool with block size as class size
MemoryPool<sizeof(MyObject), 3> gMemPool;


void* MyObject::operator new(size_t size)
{
	//std::cout << "new object space" << std::endl;
	return gMemPool.allocate();
}

void MyObject::operator delete(void* p)
{
	//std::cout << "free object space" << std::endl;
	gMemPool.deallocate(p);
}

void* MyObject::operator new[](size_t size)
{
	// TODO: not supported continuous memoery pool for now
	//return gMemPool.allocate(size);
	return NULL;
}
void MyObject::operator delete[](void* p)
{
	// TODO: not supported continuous memoery pool for now
	//gMemPool.deallocate(p);
}

int main(int argc, char* argv[])
{
	MyObject* p1 = new MyObject(1);
	std::cout << "p1 " << p1 << " " << p1->data<< std::endl;

	MyObject* p2 = new MyObject(2);
	std::cout << "p2 " << p2 << " " << p2->data << std::endl;
	delete p2;

	MyObject* p3 = new MyObject(3);
	std::cout << "p3 " << p3 << " " << p3->data << std::endl;

	MyObject* p4 = new MyObject(4);
	std::cout << "p4 " << p4 << " " << p4->data << std::endl;

	MyObject* p5 = new MyObject(5);
	std::cout << "p5 " << p5 << " " << p5->data << std::endl;

	MyObject* p6 = new MyObject(6);
	std::cout << "p6 " << p6 << " " << p6->data << std::endl;

	delete p1;
	delete p2;
	//delete p3;
	delete p4;
	delete p5;
	delete p6;

	getchar();
	return 0;
}

运行结果

p1 00000174BEDE0440 1
p2 00000174BEDE0450 2
p3 00000174BEDE0450 3
p4 00000174BEDE0460 4
p5 00000174BEDD5310 5
p6 00000174BEDD5320 6

可以看到内存地址是连续，并且回收一个节点后，依然有序地开辟内存
对象先开辟内存再构造，先析构再释放内存

注意

• 在内存分配和释放的环节需要加锁来保证线程安全
• 还没有实现对象数组的分配和释放