What is the way to create a map from 256 bit integers to values?

Question

What is the way to create a map from 256 bit integers to values?

Is it std::map<>? or will std::map<> become slow as the data grows? If it is slow, what is faster?

Answer 1

My answer is too long to fit in comment, so leave here.

I came to know we are using different terms, but saying same things. eosio::multi_index is a table and you can add items (I called these 'rows'), but you said that you will make only one object. Then your code will be like this:

struct other {
  ...
};

struct [[eosio::table]] some {
  uint64_t id;
  std::map<checksum256,other> others; // checksum256 for 256-bit integer key

  uint64_t primary_key() const { return id; }
};

typedef eosio::multi_index<"some"_n, some> some_index;

If I understand you correctly, you will save all data into map (others above) in one row, not add multiple rows to multi_index. That is very inefficient when you save many data into one map. All data in that map need to be copied from internal db whenever you access this map.

struct [[eosio::table]] other {
  uint64_t id;
  checksum256 key;
  ...

  uint64_t primary_key() const { return id; }
  checksum256 secondary_key() const { return key; }
};

typedef eosio::multi_index<"other"_n,other,
  indexed_by<"secondary"_n, const_mem_fun<other, checksum256, &other::secondary_key>
> other_index;

The second example I attached shows a better way to save data with 256-bit integer key in eosio.