介绍下c++中的ConcurrentHashMap

在C语言中，没有ConcurrentHashMap的内置实现。但是，可以通过使用多线程和锁来实现并发哈希表。下面是一个简单的示例：

#include <pthread.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

#define NUM_BUCKETS 256

typedef struct {
  void* key;
  void* value;
  pthread_mutex_t lock;
} hash_entry_t;

typedef struct {
  hash_entry_t* entries;
  pthread_mutex_t* locks;
} hash_table_t;

hash_table_t* hash_table_create() {
  hash_table_t* table = malloc(sizeof(hash_table_t));
  table->entries = calloc(NUM_BUCKETS, sizeof(hash_entry_t));
  table->locks = calloc(NUM_BUCKETS, sizeof(pthread_mutex_t));

  for (int i = 0; i < NUM_BUCKETS; i++) {
    pthread_mutex_init(&table->locks[i], NULL);
  }

  return table;
}

void hash_table_put(hash_table_t* table, void* key, void* value) {
  uint8_t hash = (uint8_t)key % NUM_BUCKETS;
  hash_entry_t* entry = &table->entries[hash];
  pthread_mutex_t* lock = &table->locks[hash];

  pthread_mutex_lock(lock);

  if (entry->key == NULL) {
    entry->key = key;
    entry->value = value;
  } else {
    while (true) {
      if (entry->key == key) {
        entry->value = value;
        break;
      } else if (entry->key == NULL) {
        entry->key = key;
        entry->value = value;
        break;
      } else {
        hash = (hash + 1) % NUM_BUCKETS;
        entry = &table->entries[hash];
        lock = &table->locks[hash];
        pthread_mutex_lock(lock);
      }
    }
  }

  pthread_mutex_unlock(lock);
}

void* hash_table_get(hash_table_t* table, void* key) {
  uint8_t hash = (uint8_t)key % NUM_BUCKETS;
  hash_entry_t* entry = &table->entries[hash];
  pthread_mutex_t* lock = &table->locks[hash];

  pthread_mutex_lock(lock);

  while (entry->key != NULL && entry->key != key) {
    hash = (hash + 1) % NUM_BUCKETS;
    entry = &table->entries[hash];
    lock = &table->locks[hash];
    pthread_mutex_lock(lock);
  }

  pthread_mutex_unlock(lock);

  if (entry->key == NULL) {
    return NULL;
  } else {
    return entry->value;
  }
}

void hash_table_destroy(hash_table_t* table) {
  for (int i = 0; i < NUM_BUCKETS; i++) {
    pthread_mutex_destroy(&table->locks[i]);
  }

  free(table->entries);
  free(table->locks);
  free(table);
}

在这个实现中，哈希表被分成了多个bucket，每个bucket都有一个锁来确保线程安全。当向哈希表中添加或获取元素时，线程会锁定相应的bucket，以防止其他线程同时访问它。虽然这个实现可能比Java或其他语言中的ConcurrentHashMap更慢，但它仍然是一个可行的并发哈希表实现。