Of what type is the return value of get_index?

Question

Every single example out there that shows how to use a secondary index in the EOSIO platform, including the scant documentation on the function found here: https://eosio.github.io/eosio.cdt/latest/group__multiindex/#function-get_index, declares its value locally using the keyword auto, like this:

void myaction() {
    ...
    auto zip_index = addresses.get_index<name("zip")>();

As I don't need to initialise this value in every method where I use it, I'd like to declare it in my contract class and initialise it in the constructor. However, I cannot declare it in the class using auto:

/Users/ekkis/dev/eos/dapp.cpp:39:5: error: 'auto' not allowed in non-static class member auto idxtst;

can anyone tell me how to properly declare the correct type?

Answer 1

I saw this question that is 2 months old already but still unanswered, so I thought I could reply to it for record, since it had quite a few visualisations...

Actually the function get_index<IndexName>() is a template function and depending on the template argument that specifies the index you want to get, then a specific type can be returned.

It will return a multi_index::index object, which is a private wrapper struct type for a multi_index, so it is not accessible outside of the multi_index namespace.

So the short answer is that you can't declare a member to hold an multi_index::index, since it's type isn't available outside of multi_index namespace, so only get_index<...>() is capable of returning those.