本文共 5690 字，大约阅读时间需要 18 分钟。

???????????????

?????????????????????????????????????????Race Condition??????????????????????????????????????????????

1. ???????

???????????????????????????????????????????????????????????????????????

2. ?????????????

????????????????????????

a. ??????Mutex?

??????????????????????????????????????????

• ????????????????????????Lock??
• ????????????????
• ???????????????????????????????

?????C++?

#include 
   
    
#include 
    
     
#include 
     
      
class MutexSample {
public:
    void add_to_list(int new_value);
    bool list_contains(int value_to_find);
private:
    std::list
      
        some_list;
    std::mutex some_mutex;
};
void MutexSample::add_to_list(int new_value) {
    std::lock_guard
       
         guard(some_mutex);
    some_list.push_back(new_value);
    std::cout << "Added value: " << new_value << std::endl;
}
bool MutexSample::list_contains(int value_to_find) {
    std::lock_guard
        
          guard(some_mutex); return std::find(some_list.begin(), some_list.end(), value_to_find) != some_list.end(); } int main() { MutexSample mtx; std::vector
         
           threads; for (int i = 0; i < 5; ++i) { threads.push_back(std::thread([=, &mtx] { mtx.add_to_list(i); })); } std::for_each(threads.begin(), threads.end(), std::mem_fn(&std::thread::join)); if (mtx.list_contains(3)) { std::cout << "Find the value" << std::endl; } }
         
        
       
      
     
    
   

??????????????????????????????????????????????????????????????????????????????

3. ?????????????

?????????????????????????

a. ???????????????????

????????????????????????????????????

b. ??????????

?????????????????std::stack ? pop ???????????????????????????????????????

• ? pop ??????
• ?? no-throw ???????? move ?????
• ? pop ????????
• ?? a ??? b ? c?

?????C++?

#include 
   
    
#include 
    
     
#include 
     
      
#include 
      
       
template
       
        
class ThreadSafeStack {
private:
    std::stack
        
          data; mutable std::mutex m; public: ThreadSafeStack() {} ThreadSafeStack(const ThreadSafeStack& other) { std::lock_guard
         
           lock(other.m); data = other.data; } ThreadSafeStack& operator=(const ThreadSafeStack& other) = delete; void push(T new_value) { std::lock_guard
          
            lock(m); data.push(new_value); } std::shared_ptr
           
             pop() { std::lock_guard
            
              lock(m); if (data.empty()) { throw empty_stack(); } std::shared_ptr
             
               res(std::make_shared
              
               (data.top())); data.pop(); return res; } void pop(T& value) { std::lock_guard
               
                 lock(m); if (data.empty()) { throw empty_stack(); } value = data.top(); data.pop(); } bool empty() const { std::lock_guard
                
                  lock(m); return data.empty(); } };
                
               
              
             
            
           
          
         
        
       
      
     
    
   

4. ??????????

?????

?????????????????????????????????????????????????????????

???????

?????C++?

#include 
   
    
#include 
    
     
class DeadLock {
private:
    std::mutex m;
public:
    void do_something() {
        std::lock_guard
     
       lock(m);
        std::cout << "do something!" << std::endl;
        do_something_else();
    }
    void do_something_else() {
        std::lock_guard
      
        lock(m);
        std::cout << "do something else" << std::endl;
    }
};
      
     
    
   

5. ?????????

a. ?????? std::lock ? std::lock_guard

std::lock ? std::lock_guard ????????????????????????

b. ????????????

???? std::unique_lock ?????? std::move ??????????

std::unique_lock
   
     get_lock() {
    extern std::mutex some_mutex;
    std::unique_lock
    
      lk(some_mutex);
    prepare_data();
    return lk;
}
void process_data() {
    std::unique_lock
     
       lk(get_lock());
    do_something();
}
     
    
   

c. ??????????

???????????????????????????

std::unique_ptr
   
     lk(some_mutex);
lk.lock();
// ?????...
lk.unlock();
lk.lock();
// ???????...
lk.unlock();
   

6. ?? std::unique_lock ??????

std::unique_lock ? std::lock_guard ??????????????????????

?????C++?

std::unique_lock
   
     lock_a(m1, std::deter_lock);
std::unique_lock
    
      lock_b(m2, std::deter_lock);
std::lock(lock_a, lock_b);
    
   

7. ???????????

???????????????????????????

?????C++?

#include 
   
    
#include 
    
     
#include 
     
      
std::shared_ptr
      
        resource_ptr;
std::once_flag resource_flag;
void init_resource() {
    resource_ptr.reset(new int(42));
}
void foo() {
    std::call_once(resource_flag, init_resource);
    resource_ptr->operator*()();
}
int main() {
    foo();
    foo();
    return 0;
}
      
     
    
   

8. ????????????

class X {
private:
    connection_info connection_details;
    connection_handle connection;
    std::once_flag connection_init_flag;
    void open_connection() {
        connection = connection_manager.open(connection_details);
    }
public:
    X(connection_info const& connection_details_) : 
        connection_details(connection_details_)
    {}
    void send_data(data_packet const& data) {
        std::call_once(connection_init_flag, &X::open_connection, this);
        connection.send_data(data);
    }
    data_packet receive_data() {
        std::call_once(connection_init_flag, &X::open_connection, this);
        return connection.receive_data();
    }
};

??????????????????????????????????????????????????