Use boost::container or boost::recursive_wrapper for creating containers of incomplete types

Question

I need to create a structure that has a vector<self> as a member variable. Boost offers 2 mechanisms that achieve this:

1 - using boost::recursive_wrapper i.e.:

struct filter
{
    uint32_t id;
    std::vector< boost::recursive_wrapper< filter > > childFilters; 
};

2 - using boost::container i.e.:

struct filter
{
    uint32_t id;
    boost::container::vector< filter > childFilters; 
};

Is there any advantage with each technique? The second boost::container option involves less syntax, and I guess it uses a technique similar to the boost::recursive_wrapper internally.

Answer 1

The better choice is to use boost::container::vector, because it allows direct access to the type stored in the vector.

The extra layer of indirection provided by boost::recursive_wrapper that allows the declaration of a std::vector<self> makes code manipulating the vector more cumbersome, because it is necessary get to access the actual type being held.

i.e.

struct filter
{
    uint32_t id;
    std::vector< boost::recursive_wrapper< filter > > childFilters; 
};

filter myFilter;

// fill myFilter .....

BOOST_FOREACH( boost::recursive_wrapper< filter > f, myFilter.childFilters )
{
    std::cout << f.get().id << "\n";
}

With boost::container::vector the example becomes:

struct filter
{
    uint32_t id;
    boost::container::vector< filter > childFilters; 
};

filter myFilter;

// fill myFilter .....

BOOST_FOREACH( boost::recursive_wrapper< filter > f, myFilter.childFilters )
{
    std::cout << f.id << "\n";    // no call to get()
}