通过使用JSON Boost序列化创建JSON字符串时，如何获取名称/值对? [英] how get name-value pair when creating JSON string from using JSON boost serialization?

问题描述

在回答这个问题之前，我会请大家在这里先看看

Before answering this question I would request all you good people here to first take a look at this output that I am getting presently. The output is fetched from a sqlite3 table SiteCode which has two columns ID and Code.

尽管，我去了输出，但是我仍然没有在输出中看到字段名称. 我的意思是我需要一个看起来像以下(名称/值对)的输出

Although, I go the output but i still dont see the field name in the output. i mean I need an ouput which looks like the following (name-value pair)

{"ID":"7","Code":"786","ID":"8","Code":"78","ID":"9","Code":"785","ID":"10","Code":"998","ID":"11","Code":"656"}

那么我应该将boost-name-value-pair与解析器一起使用，还是需要对现有代码进行更改?怎么样?

So do I uses boost-name-value-pair along with parser or do I need to make changes to the existing code ? How ?

我的最终目标是使用HTTP POST方法通过网络发送此JSON名称-值字符串.

My final aim is to send this JSON name-value string over the network using HTTP POST method.

如果您能指导我完成这一工作，我将不胜感激.如果可能的话，请给我指出一些例子吗?

I would highly appreciate if you could guide me through this. If possible could you please point me to some examples ?

推荐答案

这是我现在编写该循环的方式:

Here's how I'd write that loop now:

for (auto &entry : _records) {
    ptree obj;
    obj.put("ID", entry.id);
    obj.put("CODE", entry.code);
    pt.insert(pt.end(), { "", obj });
}

替代拼写:

    pt.push_back(ptree::value_type("", obj));

编辑当然，如果您 确实 想要问题中的确切输出格式，则可以

Edit Of course, if you really want the exact output format from your question, you'd do

for (auto &entry : _records) {
    pt.add("ID", entry.id);
    pt.add("CODE", entry.code);
}

但是我看不到这种格式的用处，因为您会获得各种重复属性(不是标准JSON)，并且依赖属性的顺序(并非所有读者都支持).

But I can't see how this format is useful, since you get all kinds of duplicate properties (which are not standard JSON) and rely on the order of properties (which might not be supported by all readers).

再次测试:

$ sqlite3 database.db <<<"create table sitecode(id int primary key, code int);"
$ for a in {1..10}; do echo "insert into sitecode(id,code) values($a, $RANDOM);"; done | sqlite3 database.db
$ ./test

我得到:

before loading: 
The number of Records is: 0

==============[ AS JSON ]===============
{}
after loading: 
The number of Records is: 10
(1,24080) (2,9982) (3,3129) (4,5337) (5,23554) (6,3581) (7,32306) (8,12024) (9,9161) (10,27641) 
==============[ AS JSON ]===============
{"":{"ID":"1","CODE":"24080"},"":{"ID":"2","CODE":"9982"},"":{"ID":"3","CODE":"3129"},"":{"ID":"4","CODE":"5337"},"":{"ID":"5","CODE":"23554"},"":{"ID":"6","CODE":"3581"},"":{"ID":"7","CODE":"32306"},"":{"ID":"8","CODE":"12024"},"":{"ID":"9","CODE":"9161"},"":{"ID":"10","CODE":"27641"}}

完整演示

// FILE: some header
#include <ostream>

struct SiteCode {
    int id;
    int code;

    SiteCode(int id, int code) : id(id), code(code)
    { }

    friend inline std::ostream &operator<<(std::ostream &out, SiteCode const& site) {
        return out << "(" << site.id << "," << site.code << ")";
    }
};

#include <list> // I have deleted some header for sake of readability

// FILE: sqliteDB header
class sqliteDB {
    using Records = std::list<SiteCode>;
    Records _records;

  public:
    void load();
    Records const& get() const { return _records; }
    void printList() const;
    void writeJson(std::ostream& os) const;
};

// FILE: some sqlpp.hpp utility header (inline implementations only)
#include <memory>
#include <sqlite3.h>

namespace sqlpp {
    using database  = std::shared_ptr<::sqlite3>;

    void perror(int rc) {
        if (rc != SQLITE_OK) throw std::runtime_error(::sqlite3_errstr(rc));
    }

    struct statement {
        static statement prepare(database db, std::string const& sql) {
            ::sqlite3_stmt* stmt = nullptr;
            perror(::sqlite3_prepare_v2(db.get(), sql.c_str(), -1, &stmt, 0));

            return { handle(stmt, ::sqlite3_finalize), db };
        }

        int step()            { return ::sqlite3_step(_stmt.get()); }
        int column_int(int c) { return ::sqlite3_column_int(_stmt.get(), c); }
      private:
        using handle = std::shared_ptr<::sqlite3_stmt>;
        database _db; // keeping it around for the lifetime of _stmt
        handle _stmt;

        statement(handle&& h, database& db) : _db(db), _stmt(std::move(h)) { }
    };

    database open(char const* path) {
        ::sqlite3* db = nullptr;
        perror(::sqlite3_open(path, &db));

        return database(db, ::sqlite3_close);
    }

    statement prepare(database db, std::string const& sql) {
        return statement::prepare(db, sql);
    }
}

// FILE: sqliteDB implementation file
#include <boost/property_tree/json_parser.hpp>
#include <boost/property_tree/ptree.hpp>

void sqliteDB::load() {
    using namespace sqlpp;

    auto stmt = prepare(open("/tmp/database.db"), "SELECT ID, CODE FROM SiteCode;");

    while (stmt.step() == SQLITE_ROW)         
        _records.emplace_back(stmt.column_int(0), stmt.column_int(1));
}

void sqliteDB::writeJson(std::ostream& os) const {
    using namespace boost::property_tree;
    ptree pt;

    for (auto &entry : _records) {
        ptree obj;
        obj.put("ID", entry.id);
        obj.put("CODE", entry.code);
        pt.insert(pt.end(), { "", obj });
    }

    write_json(os, pt, false);
}

// FILE: main program
template <typename List>
static void printList(List const& list) {
    int s = list.size();
    std::cout << "The number of Records is: " << s << "\n";

    for (auto& r : list) std::cout << r << " ";
}

void dump(sqliteDB const& db) {
    printList(db.get());
    std::cout << "\n==============[ AS JSON ]===============\n";
    db.writeJson(std::cout);
}

int main() { 
    sqliteDB db;

    std::cout << "before loading: \n";
    dump(db);

    std::cout << "after loading: \n";
    db.load();
    dump(db);
}

只需将其编译为单个源

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