Unset document element in Mongo CXX

Question

I have a class that has a bsoncxx::document::view view as a private attribute, and a method that get a document from the Mongo database and save the result in the local bsoncxx::document::value value variable and then get the view from the value.view() and save it in the class bsoncxx::document::view view variable:

const bool User::search_one_by_id(const std::string &id = "")
{
    // The prototype of the method below: const std::tuple<bool, bsoncxx::document::value> search_one_by_id(mongocxx::collection &, const std::string &) const;
    auto status = Crud::search_one_by_id(this->collection, id);

    if (std::get<0>(status))
    {
        // Error
        return EXIT_FAILURE;
    }
    else
    {
        // Success
        bsoncxx::document::value value = std::get<1>(status);

        bsoncxx::document::view view = value.view();

        this->view = view;

        return EXIT_SUCCESS;
    }
}

The problem is that if I get some element from the view in the method above, i.e the code below before the return EXIT_SUCCESS, no errors are issued.

        bsoncxx::document::element element = this->view["first_name"];

        std::cout << "First Name: " << element.get_utf8().value.to_string();

But if I get a view, save it in the bsoncxx::document::view view variable and try to get some element from the view in another class method:

void get_element()
{
    bsoncxx::document::element element = this->view["first_name"];

    std::cout << "First Name: " << element.get_utf8().value.to_string();
}

I receive the error:

terminate called after throwing an instance of 'bsoncxx::v_noabi::exception'
  what():  unset document::element
Makefile:26: recipe for target 'run' failed
make: *** [run] Aborted (core dumped)

I had tried to use a pointer to save the reference to the view that I get in the search_one_by_id method. I had check if the type of the attribute (first_name) I'm getting is the right type to get the value of the element. I had check if the attribute exist in the document. I had tried to use the release() method from the view in the User::search_one_by_id. I had check if the view is empty through:

if (this->view.empty())
{
    std::cout << "Empty: " << this->view.empty();
}
else
{
    std::cout << "Loaded: " << this->view.empty() << " length " << this->view.length();
}

Inside the get_element method, and the output is:

# If I comment the call to search_one_by_id
$ Empty: 1

# If I enable the call to search_one_by_id
$ Loaded: 0 length 129

The GDB log to backtrace:

#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007ffff6fd7801 in __GI_abort () at abort.c:79
#2  0x00007ffff762c957 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3  0x00007ffff7632ab6 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4  0x00007ffff7632af1 in std::terminate() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#5  0x00007ffff7632d24 in __cxa_throw () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#6  0x00007ffff793985c in bsoncxx::v_noabi::document::element::type() const () from /usr/local/lib/libbsoncxx.so._noabi
#7  0x00007ffff7939abc in bsoncxx::v_noabi::document::element::get_utf8() const () from /usr/local/lib/libbsoncxx.so._noabi
#8  0x0000555555559353 in User::get_element() ()
#9  0x0000555555556668 in main ()

Some tips ? The project can be found on Github

References:

Unset Document::element, MongoCXX Find Option Projection

Document value

Document view

Document element

document::element::operator[] overloads should not throw when given an invalid element

How do I get the type of a variable?

Answer 1

i think you need to keep the pointer to buffer of value somewhere. view object is pointing to deleted memory i suppose. try something like this

class User
{
   bsoncxx::document::value _value;   <<< store at class level
};

const bool User::search_one_by_id(const std::string &id = "")
{
    // The prototype of the method below: const std::tuple<bool, bsoncxx::document::value> search_one_by_id(mongocxx::collection &, const std::string &) const;
    auto status = Crud::search_one_by_id(this->collection, id);

    if (std::get<0>(status))
    {
        // Error
        return EXIT_FAILURE;
    }
    else
    {
        // Success
        _value = std::get<1>(status);
        this->view = _value.view();

        return EXIT_SUCCESS;
    }
}