How do I create a Multi-Index DB secondary index from a String?

Question

I'm developing a Smart Contract for EOSIO. I have a Multi-Index table with a uint64_t primary key. I'd like to add a secondary index to the table.

Ideally the data I'd be indexing on would be a std::string, but I'm under the impression that EOS does not allow that. I do see something in the documentation referred to as an idx256 described as a "256-bit fixed-size lexicographical key", though it is unclear how to construct one.

How do I create an idx256? What conversions must I do to a std::string to make it work (truncate or otherwise manipulate)? What are my limitations (for instance, is this a one-way conversion similar to hashing)? Any gotchas or other concerns I should be aware of?

Answer 1

As of eosio.cdt v1.5.0, you can not make a secondary index from a string.

The types of secondary indices allowed are:

    uint64_t    secondary_1;
    uint128_t   secondary_2;
    checksum256 secondary_3;
    double      secondary_4;
    long double secondary_5;

In principle, you could encode your string as a eosio::name and store it in a uint64_t, but this limits you to the 12 character-style names for eosio accounts.