Question

I’m using boost::multi_index_container and am trying to get a modify operation working. My modification routine looks like this (roughly), using a function inner class:

void FooContainer::modifyAttribute(string key, int newValue) {
    struct ModifyFunc {
        int val;
        ModifyFunc(int val): val(val) {}
        void operator()(Foo &f) {
            foo.val = val;
        }
    };
    StorageContainer::index<keyTag>::type &idx = mContainer.get<keyTag>();
    StorageContainer::index<keyTag>::type::iterator iter = idx.find(key);
    idx.modify(iter, ModifyFunc(newValue));
}

When I try to compile this, I get a multi-page spew of compiler error like this (most of it omitted):

FooContainer.cpp:##: error: no matching function for call to [...]::modify([...]&, FooContainer::modifyAttribute(string,int)::ModifyFunc)’

What’s wrong with this invocation and how can I make it work?

Answer 1

The problem is that function inner classes aren’t recognized by the compiler as a valid typename for template parameters; it isn’t obvious, but the multi_index_container::index<T>::type::modify method uses the type of the modify parameter as a template argument which is a detail normally hidden from view. However, look at its declaration in, for example, boost/multi_index/hashed_index.hpp:

template<typename Modifier>
bool modify(iterator position,Modifier mod)

The easy fix is to make the ModifyFunc struct not a function inner class; trivially, make it inner to FooContainer rather than to the FooContainer::modifyAttribute method. Of course, this also means that you can reuse the ModifyFunc class elsewhere, if it turns out you need it in more than one place.