C++ ODB database mapper: Unable to use std::weak_ptr in relationship

Question

I'm trying to model a one-to-many relationship with ODB. I'm basically trying to recreate the example in https://www.codesynthesis.com/products/odb/doc/manual.xhtml#6.2.2

I have to use std::weak_ptr for one side of the relationship, to avoid circular ownership issues. However, my very simple example code does not compile, as ODB doesn't seem to play nicely with std::weak_ptr.

In my example, every Bar has exactly one Foo , while every Foo has multiple Bar s. This is my code:

#include <odb/core.hxx>
#include <string>
#include <memory>
#include <vector>

// Forward
class Foo;

#pragma db object
class Bar {
public:
    // A Bar has exactly *one* Foo
    #pragma db not_null
    std::shared_ptr<Foo> cfg;

private:
    #pragma db id auto
    unsigned long id_;
    friend class odb::access;
};

#pragma db object
class Foo {
public:
    // A Foo has multiple Bars
    // Using std::weak_ptr here instead of std::shared_ptr to avoid circular
    // ownership
    #pragma db value_not_null inverse(cfg)
    std::vector<std::weak_ptr<Bar>> entries;

private:
    #pragma db id auto
    unsigned long id_;
    friend class odb::access;
};

int main() {}

I generate the database code with:

odb --std c++11 --database sqlite --generate-query --generate-schema --at-once main.hpp

And I compile like this:

g++ --std=c++11 main.hpp main-odb.cxx

(I know that would crash at linking - I'm just trying to get it to compile.)

My compiler (GCC 7) tells me:

main-odb.cxx: In static member function ‘static void odb::access::object_traits_impl<Foo, (odb::database_id)1u>::entries_traits::init(odb::access::object_traits_impl<Foo, (odb::database_id)1u>::entries_traits::value_type&, const odb::access::object_traits_impl<Foo, (odb::database_id)1u>::entries_traits::data_image_type&, odb::database*)’:
main-odb.cxx:794:43: error: no matching function for call to ‘std::weak_ptr<Bar>::weak_ptr(odb::object_traits<Bar>::pointer_type)’
             obj_traits::object_type > (id));
                                           ^
In file included from /usr/include/c++/5/memory:82:0,
                 from main.hpp:3,
                 from main-odb.hxx:16,
                 from main-odb.cxx:7:
/usr/include/c++/5/bits/shared_ptr.h:492:2: note: candidate: template<class _Tp1, class> std::weak_ptr<_Tp>::weak_ptr(std::weak_ptr<_Tp1>&&)
  weak_ptr(weak_ptr<_Tp1>&& __r) noexcept
  ^
/usr/include/c++/5/bits/shared_ptr.h:492:2: note:   template argument deduction/substitution failed:
main-odb.cxx:794:43: note:   mismatched types ‘std::weak_ptr<_Tp>’ and ‘odb::object_traits<Bar>::pointer_type {aka Bar*}’
             obj_traits::object_type > (id));

There are three more candidates which I skipped. The important part: ODB tries somewhere to create a std::weak_ptr<Bar> from a Bar * , which is obviously not possible. It would have to create it from a std::shared_ptr<Bar> . However, the ODB documentation explicitly says one should (and in fact must) use std::weak_ptr in these cases.

What am I doing wrong?

Answer 1

Okay, I figured out a solution (not sure if it's the best solution):

You can force ODB to use std::shared_ptr<Bar> instead of Bar * everywhere. You do so by defining your class as:

#pragma db object pointer(std::shared_ptr)
class Bar {
...

This way, when a std::weak_ptr<Bar> is being created, it is created from a std::shared_ptr<Bar> , which works. You can also specify the pointer type to be used at a namespace or global scope, see https://www.codesynthesis.com/products/odb/doc/manual.xhtml#3.3