MongoDB&C++开发 (三) C++ Driver 浅析（结合mongo-cxx-driver/examples中代码）

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MongoDB C++ Driver API 官方文档链接

在mongo-cxx-driver/examples文件夹下，有bsoncxx和mongocxx两个代码文件夹，对应于这两个命名空间。

1. bsoncxx

文件夹下包含的cpp

这里写图片描述

对应于API中的

这里写图片描述
从实用的角度讲，通过代码分析用法更为有效。
关于bsoncxx::builder中的basic,stream和core
builder_core.cpp中有这样一句注释

 // bsoncxx::builder::core is a low-level primitive that can be useful for building other    // BSON abstractions. Most users should just use builder::stream or builder::basic.

而我自己觉得，builder：：basic也是一种比较传统的创建builder::basic::document{}文档的方式，如果创建比较复杂的文档，还是太繁琐了，所以实用builder::stream流式的方式比较好。
现选择builder_stream.cpp和getting_values.cpp进行分析：

1.1 流式构造文档

#include <bsoncxx/builder/stream/document.hpp>#include <bsoncxx/builder/stream/array.hpp>#include <bsoncxx/builder/stream/helpers.hpp>#include <bsoncxx/types.hpp>using namespace bsoncxx;int main(int, char**) {    using builder::stream::document;    using builder::stream::array;    // bsoncxx::builder::stream presents an iostream like interface for succintly    // constructing complex BSON objects.    // stream::document builds a BSON document 构造一个BSON文档    auto doc = document{};    // stream::array builds a BSON array 构造一个BSON数组    auto arr = array{};    // We append keys and values to documents using the '<<' operator;使用<<运算符    // 连接key和value    doc << "myKey"        << "myValue";    // We can chain any number of keys and values    doc << "foo" << types::b_bool{false} << "baz" << types::b_int32{1234} << "quz"        << types::b_double{1.234};    // For arrays, everything just becomes a value， array里面的值都是value而不是key    arr << 1 << 2 << types::b_bool{true};    // The stream namespace includes some helpers that can be used similarly    // to the stream manipulators in <iomanip>    // To build a subdocument, use open_document and close_document     // 文档里面嵌套文档使用open_document和close_document    using builder::stream::open_document;    using builder::stream::close_document;    doc << "mySubDoc" << open_document << "subdoc key"        << "subdoc value" << close_document;    // To build a subarray, use open_array and close_array     // 数组里面嵌套数组，使用open_array和close_array    using builder::stream::open_array;    using builder::stream::close_array;    doc << "mySubArr" << open_array << 1 << types::b_bool{false} << "hello" << close_array;    // There is a special finalize helper that converts a stream to its underlying bson value    // this is useful for writing one-liners, for example    using builder::stream::finalize;    auto myQuery = document{} << "foo"                              << "bar" << finalize;    // There is a special concatenate helper to add all keys and corresponding values from one    //将一个文档中的内容加到另一个文档中（保持其key-value的对应关系），可以使用concatenate    // document into another.    using bsoncxx::builder::concatenate;    doc << concatenate(myQuery.view());    // `doc` now looks like:    // {    //   "myKey": "myValue",    //   ...    //   "mySubArr": [...],    //   "foo": "bar"    // }    // To nest an existing bsoncxx::document::value into a builder stream, you can create a    // types::b_document and append it. Alternatively you can open a new document and concatenate    // the value in. Or, simplest, is just to stream in the view.    bsoncxx::document::value nestedValue = document{} << "nested" << true << finalize;    document topLevelDoc{};    topLevelDoc << "subDoc1" << types::b_document{nestedValue.view()} << "subDoc2" << open_document                << concatenate(nestedValue.view()) << close_document << "subDoc3" << nestedValue                << finalize;    // `topLevelDoc` now looks like:    // {    //     "subDoc1": {    //         "nested": true    //     },    //     "subDoc2": {    //         "nested": true    //     },    //     "subDoc3": {    //         "nested": true    //     }    // }}

1.2 流式构造文档并通过[“key”] or [index]访问值

#include <bsoncxx/builder/stream/array.hpp>#include <bsoncxx/builder/stream/document.hpp>#include <bsoncxx/builder/stream/helpers.hpp>#include <bsoncxx/config/prelude.hpp>#include <bsoncxx/types.hpp>using namespace bsoncxx;int main(int, char**) {    using namespace builder::stream;    builder::stream::document build_doc;    // {    //     "_id" : 1,    //     "name" : { "first" : "John", "last" : "Backus" },    //     "contribs" : [ "Fortran", "ALGOL", "Backus-Naur Form", "FP" ],    //     "awards" : [    //                {    //                  "award" : "W.W. McDowell Award",    //                  "year" : 1967,    //                  "by" : "IEEE Computer Society"    //                },    //                { "award" : "Draper Prize",    //                  "year" : 1993,    //                  "by" : "National Academy of Engineering"    //                }    //     ]    // }    //name是key,它对应的value值是一个文件，里面有两个键值对，分别是"first" : "John"和 "last" : "Backus"。    //contribs是key,它对应的value是一个数组，数组里面有四个value:"Fortran", "ALGOL", "Backus-Naur Form", "FP"    //awards是key,它对应的value是一个数组，数组里面有两个文档，第一个文档里面有三个键值对，第二个文档里面也有三个键值对    //至此，使用builder::stream::document可以实现构造一个比较复杂的文件了。    build_doc << "_id" << 1 << "name" << open_document << "first"              << "John"              << "last"              << "Backus" << close_document << "contribs" << open_array << "Fortran"              << "ALGOL"              << "Backus-Naur Form"              << "FP" << close_array << "awards" << open_array << open_document << "award"              << "W.W. McDowell Award"              << "year" << 1967 << "by"              << "IEEE Computer Society" << close_document << open_document << "award"              << "Draper Prize"              << "year" << 1993 << "by"              << "National Academy of Engineering" << close_document << close_array;    auto doc = build_doc.view();    // Once we have the document view, we can use ["key"] or [index] notation to reach into nested    // documents or arrays.    //用流式方式构造的文件，可以通过类似下标的方式实现随机访问，这里使用auto关键字可以简化很多工作，也不容易出错。    auto awards = doc["awards"];//名为awards的key所对应的value---是一个包含两个document的array    auto first_award_year = awards[0]["year"];//awards对应的array里面的第一个array元素，即    //{    //                  "award" : "W.W. McDowell Award",    //                  "year" : 1967,    //                  "by" : "IEEE Computer Society"    //                },    //这个文档中名为year的key对应的value    auto second_award_year = doc["awards"][1]["year"];    auto last_name = doc["name"]["last"];    // If the key doesn't exist, or index is out of bounds, we get invalid elements.    //如果key不存在或者a要访问的rray的index越界，会得到无效的元素    auto invalid1 = doc["name"]["middle"];    auto invalid2 = doc["contribs"][1000];    if (invalid1 || invalid2) {        BSONCXX_UNREACHABLE;  // Not reached.    }    // Similarly, indexed access (either by string or numeric index) into a type that is not    // a document or an array yields invalid eleemnts.    auto invalid3 = doc["_id"]["invalid"];    auto invalid4 = doc["name"][3];    if (invalid3 || invalid4) {        BSONCXX_UNREACHABLE;  // Not reached.    }    // Make all variables used.    return (awards && first_award_year && second_award_year && last_name) ? EXIT_SUCCESS                                                                          : EXIT_FAILURE;}

2. mongocxx

mongocxx文件夹中的文件比bsoncxx中的多不少
见下篇博客吧，写在一起太长了。

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