Question

I have a boost multi index container thus.

using namespace boost::multi_index;
template < typename O >
class Container
{
public:
    multi_index_container<
        O,
        indexed_by<
            ordered_unique<
                const_mem_fun< O, std::string, &O::name >
            >
        >
    > _container;
};

As you can see, by this design every object I use to create this container has to have a member function returning a string with the name “name”.

This is obviously not ideal. I tried a couple of ways of passing in the “key” but I can’t get any of them to work..

I tried this..

using namespace boost::multi_index;
template < typename O, typename KT, typename KM >
class Container
{
public:
    multi_index_container<
        O,
        indexed_by<
            ordered_unique<
                const_mem_fun< O, KT, &KM >
            >
        >
    > _container;
};

int main( int c, char *v[] )
{
    Container< Object, std::string, Object::name > container;
}

but no joy..

the compiler complains that Object::name isn’t a type but I’m not sure how to correct this. And even if I work out how to supply a type to the template, I’ll still need a concrete instance of “Object::name” to be used by the container..

maybe I have to hand in the types and then and in the member function at construction? but then how do I construct the container .. My head hurts!?!

Alexy, below, kindly offered this solution

using namespace boost::multi_index;
template < typename O, typename KT, KT (O::* KM)() >
class Container
{
public:
    multi_index_container<
        O,
        indexed_by<
            ordered_unique<
                const_mem_fun< O, KT, KM >
            >
        >
    > _container;
};

int main( int c, char *v[] )
{
    Container< Object, std::string, &Object::name > container;  // <<---- ERROR HERE
}

However, this produded the following compiler error.

Template parameter KM requires an expression of type std::string (Object::*)().

at the line marked..

Ok. It turns out this was my fault by handing in an incorrectly signatured “&Object::name” parameter… I have fixed this..

Answer 1

Change class definition.

template < typename O, typename KT, KT (O::* KM)() >
class Container 
//...

and use KM instead of &KM.