使用Mongocxx C++构建查询时遭遇不完整类型错误求助

我使用MongoDB C++驱动（mongocxx）查询数据库中是否存在字段等于特定字符串的文档。连接并遍历Users集合的代码可正常运行，但编写查询特定username字段的函数时，出现编译错误：incomplete type 'bsoncxx::v_noabi::builder::stream::value_context<bsoncxx::v_noabi::builder::stream::key_context<>>' used in type trait expression。以下是相关代码、头文件、include路径及错误详情，以及解决方法：

可正常运行的连接遍历代码 #

void viewMongoDBServer()
{
    cout <<"Check Mongo Connection" << endl;
    // 初始化mongocxx实例
    mongocxx::instance instance{};

    // 连接MongoDB服务器
    mongocxx::uri uri("mongodb://localhost:27017");
    mongocxx::client client(uri);
    
    // 访问UserData数据库
    mongocxx::database db = client["UserData"];

    // 访问Users集合
    mongocxx::collection collection = db["Users"];

    // 查询并遍历所有文档
    auto cursor = collection.find({});
    
    for (auto&& doc : cursor) {
        // 打印每个文档
        std::cout << bsoncxx::to_json(doc) << std::endl;
    }
}

文档示例输出 #

{
"_id": {
"$oid": "665d09b70296028f85120934"
},
"username": "Jake",
"password": "Hello"
}

出现错误的查询代码 #

void searchDataInMongoDBServer()
{
    cout << "Search MongoDB Connection" << endl;

    // 初始化mongocxx实例
    mongocxx::instance instance{};

    // 连接MongoDB服务器
    mongocxx::uri uri("mongodb://localhost:27017");
    mongocxx::client client(uri);

    // 访问UserData数据库
    mongocxx::database db = client["UserData"];

    // 访问Users集合
    mongocxx::collection myCollection = db["Users"];

    // 构建查询条件
    bsoncxx::builder::stream::document query_builder;
    query_builder << "username" << "Jake";
    
    // 执行查询
    auto cursor = myCollection.find(query_builder.view());

    // 遍历查询结果
    for (auto&& doc : cursor) {
        std::cout << bsoncxx::to_json(doc) << std::endl;
    }
}

错误信息 #

/Library/Developer/CommandLineTools/SDKs/MacOSX14.4.sdk/usr/include/c++/v1/__type_traits/is_base_of.h:22:94: error: incomplete type 'bsoncxx::v_noabi::builder::stream::value_context<bsoncxx::v_noabi::builder::stream::key_context<>>' used in type trait expression
struct _LIBCPP_TEMPLATE_VIS is_base_of : public integral_constant<bool, __is_base_of(_Bp, _Dp)> {};

引入的头文件 #

#include <mongocxx/client.hpp>
#include <bsoncxx/builder/stream/document.hpp>
#include <bsoncxx/json.hpp>
#include <mongocxx/uri.hpp>
#include <mongocxx/instance.hpp>
#include <algorithm>
#include <iostream>
#include <vector>

include路径 #

"includePath": [
                "${workspaceFolder}/**",
                "/opt/homebrew/include/mongocxx/v_noabi/",
                "/opt/homebrew/include/bsoncxx/v_noabi/",
                "/opt/homebrew/Cellar/mongo-cxx driver/3.10.1/include/bsoncxx/v_noabi/bsoncxx/third_party/mnmlstc",
                "/opt/homebrew/Cellar/mongo-cxx-driver/3.10.1/include/bsoncxx/v_noabi/bsoncxx/builder/stream"
]

解决方案 #

编译错误的核心原因是stream builder interior ^{原始初始其后终止 =>将 procedural幸福brownMar Bibliotecached通常�builder未完成文档构建：执行query_builder << "username" << "Jake";后，builder仍处于value_context状态（等待后续操作），并非完整的document对象，此时调用view()会触发不完整类型的错误。

方法1：添加finalize完成文档构建 #

修改查询builder的代码，添加finalize标记结束文档构建：

bsoncxx::builder::stream::document query_builder;
query_builder << "username" << "Jake" << bsoncxx::builder::stream::finalize;

方法2：使用基础builder或初始化列表简化代码 #

无需使用stream builder，直接用基础builder构建查询文档：

auto cursor = myCollection.find(
    bsoncxx::builder::basic::make_document(
        bsoncxx::builder::basic::kvp("username", "Jake")
    )
);

或者用更简洁的C++初始化列表（驱动支持的情况下）：

auto cursor = myCollection.find({{"username", "Jake"}});

优化include路径 #

现有include路径存在冗余，/opt/homebrew/include/mongocxx/v_noabi/和/opt/homebrew/include/bsoncxx/v_noabi/已包含所有子目录，可简化为：

"includePath": [
                "${workspaceFolder}/**",
                "/opt/homebrew/include/mongocxx/v_noabi/",
                "/opt/homebrew/include/bsoncxx/v_noabi/",
                "/opt/homebrew/Cellar/mongo-cxx-driver/3.10.1/include/bsoncxx/v_noabi/bsoncxx/third_party/mnmlstc"
]

内容的提问来源于stack exchange，提问作者Dtal