How to check balance of dynamically determined token contract?

Question

If I am using eosio.token as my token contract (or I know my token contract during compile time), the following code will work.

accounts accountstable(N(eosio.token), account);
auto itr_a = accountstable.begin();
eosio::print (itr_a->balance);

However, I want to store the token contract account as a configuration within the contract. It looks like this:

struct config
    {
        account_name token_contract;
        account_name     primary_key() const { return token_contract; }
        EOSLIB_SERIALIZE(config, (token_contract));
    };

    typedef eosio::multi_index<N(configs), config> config_table;

When I replace the name of the token contract from the hard-coded token name in the initial example, it doesn't provide the correct information. It provides some random long integer that I think is from the wrong memory location.

accounts accountstable(N(itr->token_contract), account);
auto itr_a = accountstable.begin();
eosio::print (itr_a->balance);

How can I store the token contract name (and the symbol for that matter) dynamically in a config multi_index within the smart contract so that I can check the balance and perform transfers?

Answer 1

@Max your approach is good, but I think you can make it cleaner by using the token symbol directly doing something similar to what I did here: https://github.com/andresberrios/token_ram_recovery/blob/master/token_ram_recovery.cpp#L9

Specifically, getting the parameter as a string type and then converting it to a symbol_type:

symbol_type sym = string_to_symbol(0, symbol.c_str());

and then using it for comparisons with sym.name().

I would also recommend using a find or get instead of iterating through the whole index of the accounts table, since the primary key of the table is the symbol name anyway:

struct account {
    asset    balance;
    uint64_t primary_key()const { return balance.symbol.name(); }
};

Example using get:

auto balance = accountstable.get(sym.name(), "<Error message for when it's not found>");

This makes actual use of the index and you get much better performance.

Answer 2

I solved this by saving an example asset from the "init" method. And then, when I need to find that asset type in the token contract, I just loop until I find it. Then I know that I'm on the right asset. Ideally, I would just use the symbol characters, but I was unsure how to get this as a string.

struct config
    {
        account_name token_contract;
        asset        example_asset;
        account_name     primary_key() const { return token_contract; }

        EOSLIB_SERIALIZE(config, (token_contract)(example_asset));
    };



asset get_balance(const account_name account)
    {
        config_table config(_self, _self);
        auto itr = config.begin();
        eosio_assert(itr != config.end(), "token contract is not set");

        accounts accountstable(itr->token_contract, account);
        auto itr_a = accountstable.begin();

        while (itr_a != accountstable.end() && itr_a->balance.symbol != itr->example_asset.symbol) {
            itr_a++;
        }

        return itr_a->balance;
    }