答案:若一個類對外提供的接口的是原子的,或者說多線程編程時不用對該類的接口考慮同步問題,那麼該類就是線程安全的類。
栗子:
#include <iostream>
#include <string.h>
#include <atomic>
#include <pthread.h>
#include <unistd.h>
class Array
{
public:
Array()
{
memset(data_, 0, sizeof(int) * 10);
size = 0;
}
public:
void push_back(const int &data)
{
//pthread_mutex_lock(&mtx_);
data_[size] = data;
usleep(10);
size++;
//pthread_mutex_unlock(&mtx_);
}
void print()
{
for (size_t i = 0; i < 30; i++)
{
std::cout << data_[i] << " ";
}
std::cout << std::endl;
}
private:
int data_[30];
std::atomic<size_t> size;
pthread_mutex_t mtx_;
};
Array g_array;
using ADDDATAFUNC = void *(*)(void *);
void *AddData1(void *arg);
void *AddData2(void *arg);
void *AddData3(void *arg);
int main()
{
ADDDATAFUNC funcs[3] = {AddData1, AddData2, AddData3};
pthread_t pid[3] = {0};
for (size_t i = 0; i < 3; i++)
{
pthread_create(pid + 1, nullptr, funcs[i], nullptr);
}
for (size_t i = 0; i < 3; i++)
{
pthread_join(*(pid + 1), nullptr);
}
g_array.print();
return 0;
}
void *AddData1(void *arg)
{
for (size_t i = 0; i < 10; i++)
{
g_array.push_back(10 + i);
usleep(100);
}
return 0;
}
void *AddData2(void *arg)
{
for (size_t i = 0; i < 10; i++)
{
g_array.push_back(20 + i);
usleep(100);
}
return 0;
}
void *AddData3(void *arg)
{
for (size_t i = 0; i < 10; i++)
{
g_array.push_back(30 + i);
usleep(100);
}
return 0;
}
上述的處理結果爲:
30 0 0 31 0 0 32 0 23 0 0 14 0 0 15 0 0 26 0 16 0 0 28 0 17 0 0 18 0 19
如果將 push_back 的代碼改爲如下所示:
void push_back(const int &data)
{
pthread_mutex_lock(&mtx_);
data_[size] = data;
usleep(10);
size++;
pthread_mutex_unlock(&mtx_);
}
則結果如下所示:
10 20 30 11 21 12 31 13 22 32 23 14 33 15 24 16 25 34 26 17 35 27 36 18 28 19 37 29 38 39
該結果是正確的,所以後面的代碼中 push_back 是線程安全的。
(SAW:Game Over!)