C++中内存池的简单原理及实现详解

目录

• 为什么要用内存池
• 内存池原理
• 内存池设计
• 内存池实现

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.每次释放一个对编程客栈象的空间，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 <stdint.h>
#include <mutex>

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编程客栈'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 编程客栈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 <IOStream>
#include "memory_pool.hpp"

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

	~MyObject()
	{
		//std::cout << "destruct www.devze.comobject" << 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 " << 编程客栈p4 << " " << p4->data << std::endl;

	MyObject* p5 = new MyObject(5);
	std::cout << "p5 " << 开发者_开发入门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

到此这篇关于C++中内存池的简单原理及实现详解的文章就介绍到这了,更多相关C++内存池内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！