mongocxx bsoncxx new c++11 driver: Basic Question: Extracting string from document

[javantamt]

Hey everebody,

today the aim is to extract a string out of the database.
In

bsoncxx/test/bson_get_values.cpp

it's clearly described how to extract any other value out of the bson document. Int, bool, ...
But I realy would like to have a simpel string.

Doing something like this works perfectialy:

bsoncxx::document::value newDocument = document{}
    << "stuff" << TheJobView["source"]["url"].get_value()
    << finalize;

But I don't know how to get the string out of this.

  bsoncxx::stdx::optional<bsoncxx::document::value> vTheJob;
  GetADataset(&vTheJob);
  bsoncxx::document::view  TheJobView = vTheJob.value().view();
  std::string strLocationSource =  TheJobView["source"]["url"].get_value();

The compiler is telling me to stop my attempt immdiately:

 error: conversion from ‘bsoncxx::v_noabi::document::element’
 to non-scalar type
 ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’
 requested

My JSON looks like this:

{
  "_id" : ObjectId("5746db57b6a7d11cb64cbba1"),
  "source" : {
    "url" : "www.mongodb.com",
    "some": "more"
    }
}

[javantamt]

  bsoncxx::document::value ismaster = bsoncxx::builder::stream::document{} << "James" << "I will do my very best!" << finalize;
  bsoncxx::document::view  TheJobView = ismaster.view();
  std::string MyString = TheJobView["ismaster"].get_utf8().value.to_string();

  std::cout << MyString << std::endl;

[javantamt]

 
forgot to check the type. I dont't kinow how to make this ckeck more easy. But this works.
Maybe some of you having a better idea?

 bsoncxx::document::value ismaster = bsoncxx::builder::stream::document{} << "ismaster" << "hubs" << finalize;
  bsoncxx::document::view  TheJobView = ismaster.view();
  std::string MyString;
  auto MyType = TheJobView["ismaster"].type();

  auto hey = bsoncxx::types::b_utf8{"hello"};
 
  if (hey.type_id == TheJobView["ismaster"].type() )
  {

    MyString = TheJobView["ismaster"].get_utf8().value.to_string();

    // going to do the upper without prechecking will cause a SIGABTR,
    // a Signal generatet by this process send to cernal which is
    // sending same signal back. Recieving this signal is suicide.
    // Described in here as undefined behaviour:
    // https://mongodb.github.io/mongo-cxx-driver/classbsoncxx_1_1types_1_1value.html#a215272e9bca2780e88c036a59e51b90e
    // maybe this document could give a hint in the future how to check type?
   
    std::cout << MyString << std::endl;
  }

Am Donnerstag, 26. Mai 2016 16:58:45 UTC+2 schrieb javantamt:

[Andrew Morrow]

Hi -

You can get the type of a bsoncxx::document::element by calling the bsoncxx::document::element::type method, and then compare the returned value to one of the enumeration values from bsoncxx/types.h, like bsoncxx::type::k_utf8. As you noticed, each of the b_ prefixed types in the bsoncxx::types namespace has a public static constexpr member named type_id. Since the type_id member is static, you don't need to first create an object to obtain it. Here is a simpler version (not compiled) of your code:

auto TheJobView = ...;

auto ismasterElement = TheJobView["ismaster"];

if (!ismasterElement) {

    // we didn't get the field we expected

    throw std::logic_error("didn't find ismaster field in command response");

}

 

if (ismasterElement.type() != bsoncxx::types::b_utf8::type_id) {

    // we got the field, but it isn't the type we expected

    throw std::logic_error("ismaster field in command response was wrong type");

}

// OK, if we got here we have the field and we know it is of 

// type k_utf8, so it is safe to call get_utf8 on it.

auto MyString = isMasterElement.get_utf8().value.to_string();

std::cout << MyString << std::endl;

Can you try something more like this? I think you will find it works better.

Thanks,

Andrew

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[javantamt]

nice one!
I was looking for a solution like that.
thank you for the solution and the quick support!

Am Donnerstag, 26. Mai 2016 16:58:45 UTC+2 schrieb javantamt:

[sarthak agarwal]

Hello guys,

How do we handle array in this case. I want to store the value in a array and then get the size of that array and access its elements using [] operator. liek this 

bsoncxx::document::element edges_element = current_node_view["connects_to"];

auto = edges_element.get_array();

for(int i=0; i<edges.size(); i++){

     cout << edges[i] << "\t";

}cout << endl;

but it gives the following error - 

dijkstras.cpp: In function ‘int dijikstras(mongocxx::v_noabi::collection, int, int)’:

dijkstras.cpp:60:58: error: conversion from ‘bsoncxx::v_noabi::types::b_array’ to non-scalar type ‘bsoncxx::v_noabi::types::value’ requested

    bsoncxx::types::value edges = edges_element.get_array();

                                                          ^

dijkstras.cpp:61:25: error: ‘class bsoncxx::v_noabi::types::value’ has no member named ‘size’

    for(int i=0; i<edges.size(); i++){

                         ^

dijkstras.cpp:62:18: error: no match for ‘operator[]’ (operand types are ‘bsoncxx::v_noabi::types::value’ and ‘int’)

     cout << edges[i] << "\t";

can you please help me

[Wan Bachtiar]

I want to store the value in a array and then get the size of that array and access its elements using [] operator

Hi Sarthak,

Please open a new discussion thread instead of replying to an old thread from 2016.

Based on the information you provided, once you have queried a document, if it’s an array you can access the element(s) as the following example:

bsoncxx::document::element edges_elements = current_node_view["connects_to"];

if (edges_elements && edges_elements.type() == bsoncxx::type::k_array){
    bsoncxx::array::view subarray{edges_elements.get_array().value};
    for (bsoncxx::array::element element : subarray){
        // Assuming that the element value is String.
        std::cout<< element.get_utf8().value << "\t";
    }
}

If you have further questions, please open a new discussion thread with the following information:

• MongoDB C++ driver version
• The example document to show the structure and value types.

Regards,
Wan.

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