Introduction:
An example of thread-safe double-buffering implemented in C++
一个C++实现的线程安全的双缓冲实例
scenario: a multithreaded business system writes data to memory, while a synchronous thread reads data and synchronizes to redis.
场景：多线程的业务系统不断写入数据到内存中，同步线程不断读取数据并同步redis
Dsigning scheme：
For the business is well suited to the map structure, a two-layer map is adopted.
由于业务非常适合map的结构，因此采用了双层map
typedef std::map<int,int> EventCountMap;
typedef std::map<int,EventCountMap> RuleEventCountMap;
In order to achieve read/write separation, double buffering, one reading and one writing are adopted.
Since std::map is not thread safe, it needs to be locked when reading and writing.
为了实现读写分离，采用双缓冲，一个读，一个写，由于std::map并不是线程安全的，所以读写的时候需要加锁处理。
class BufferProc
{
public:
BufferProc();
~BufferProc();
int init();
RuleEventCountMap * get();
int set(int key, int field_key, int field_value);
void clear();
private:
MutexLock buffer_map_mutex;
RuleEventCountMap *buffer_map_A_ptr;
RuleEventCountMap *buffer_map_B_ptr;
RuleEventCountMap *buffer_map_producer_ptr;
RuleEventCountMap *buffer_map_consumer_ptr;
};
init() is used to initialize the double buffer
init用于初始化双缓冲
get() exchange the buffer_map_producer_ptr and buffer_map_consumer_ptr,then return RuleEventCountMap *
get() 交换缓冲区的指针并返回
set(int key, int field_key, int field_value) is used for producer insert data
set用于生产者插入数据，
celar() is used to clear the buffer.
celar用于清空缓冲区。