C++ 多线程 读锁之后不能跟写锁 shared

#include 
#include 
#include 
 
boost::shared_mutex mutex;
int count = 0;
 
void test(int j){
    // boost::shared_lock lock(mutex);
    boost::unique_lock lock(mutex);
 
    std::cout << "test == " << j << std::endl;
}

void Counter() {
    
  int i = count;
  std::cout << "count == " << i << std::endl;
  boost::shared_lock lock(mutex);
  test(count++);

}


 
int main() {
    
  boost::thread_group threads;
    
  for (int i = 0; i < 4; ++i) {
    threads.create_thread(&Counter);
  }
 
  threads.join_all();
  return 0;
}

写锁得等所有读锁释放之后，才能获得锁。可是每个子线程都持有读锁，并且不释放，所以造成死锁。

解决方案

用完锁就释放

#include 
#include 
#include 
 
boost::shared_mutex mutex;
int count = 0;
 
void test(int j){
    // boost::shared_lock lock(mutex);
    boost::unique_lock lock(mutex);
 
    std::cout << "test == " << j << std::endl;
}

void Counter() {
    
  int i = count;
  std::cout << "count == " << i << std::endl;
    {
        boost::shared_lock lock(mutex);
    }
  test(count++);

}


 
int main() {
    
  boost::thread_group threads;
    
  for (int i = 0; i < 4; ++i) {
    threads.create_thread(&Counter);
  }
 
  threads.join_all();
  return 0;
}