MongoDB c++ driver是同步还是异步操作，写下同步和异步增删改查接口的C++代码

MongoDB C++ driver支持同步和异步操作。下面分别给出同步和异步增删改查接口的C++代码示例：

同步操作：

#include <mongocxx/client.hpp>
#include <mongocxx/instance.hpp>
#include <mongocxx/options/find.hpp>
#include <mongocxx/stdx.hpp>
#include <bsoncxx/json.hpp>

mongocxx::instance instance{}; // 实例化驱动
mongocxx::client client{mongocxx::uri{}}; // 连接MongoDB数据库
mongocxx::database db = client["test"]; // 选择数据库
mongocxx::collection coll = db["test_collection"]; // 选择集合

bsoncxx::builder::stream::document document{};
document << "name" << "John Doe" << "age" << 42;
coll.insert_one(document.view()); // 插入一条文档

auto builder = bsoncxx::builder::stream::document{};
auto query = builder << "name" << "John Doe" << bsoncxx::builder::stream::finalize;
auto update = builder << "$set" << bsoncxx::builder::stream::open_document << "age" << 43 << bsoncxx::builder::stream::close_document << bsoncxx::builder::stream::finalize;
coll.update_one(query.view(), update.view()); // 更新一条文档

auto delete_result = coll.delete_one(query.view()); // 删除一条文档

异步操作：

#include <mongocxx/client.hpp>
#include <mongocxx/instance.hpp>
#include <mongocxx/pool.hpp>
#include <mongocxx/options/find.hpp>
#include <mongocxx/stdx.hpp>
#include <bsoncxx/json.hpp>

mongocxx::instance instance{}; // 实例化驱动
mongocxx::pool pool{mongocxx::uri{}}; // 连接池
mongocxx::database db = pool.acquire().database("test"); // 从连接池中获取连接并选择数据库
mongocxx::collection coll = db["test_collection"]; // 选择集合

auto insert_result = coll.insert_one(
    bsoncxx::builder::stream::document{} << "name" << "John Doe" << "age" << 42 << bsoncxx::builder::stream::finalize,
    mongocxx::options::insert{}); // 插入一条文档

auto update_result = coll.update_one(
    bsoncxx::builder::stream::document{} << "name" << "John Doe" << bsoncxx::builder::stream::finalize,
    bsoncxx::builder::stream::document{} << "$set" << bsoncxx::builder::stream::open_document << "age" << 43 << bsoncxx::builder::stream::close_document << bsoncxx::builder::stream::finalize,
    mongocxx::options::update{}); // 更新一条文档

auto delete_result = coll.delete_one(
    bsoncxx::builder::stream::document{} << "name" << "John Doe" << bsoncxx::builder::stream::finalize,
    mongocxx::options::delete{}); // 删除一条文档

需要注意的是，异步操作需要使用连接池，而同步操作不需要。此外，异步操作需要使用回调函数处理结果，但这里没有给出回调函数的示例。