simpledb.cpp

#include <eosiolib/eosio.hpp>

/*
 * this simpledb class is the contract itself
 * it inherits from eosio::contract class (that defines _self)
 */
class simpledb : public eosio::contract
{
    public:
/* this is the public constructor of the simpledb class
 * it has a single argument "account_name self", the account deploying it to the chain
 * as per the link below, we can initialize its fields in a list
 * those fields, or attributes, are defined by the structs "contract"
 * (inherited from the eosio::contract class) and "messages"
 * "contract" has its own constructor and it is used to re-pass the deployer account
 * "messages" is a multi_index and its constructor needs two arguments,
 * we pass the now defined _self inherited from the eosio::contract class
 * https://www.tutorialspoint.com/cplusplus/cpp_constructor_destructor.htm
 */
        simpledb(account_name self) : contract(self), messages(_self,_self) {}

/*
 * addmessage is a public action that a user can call from the contract using:
 * eosc push action simpledb addmessage "params"
 * these params are the method arguments (see simpledb.sh for more details)
 * for correct generation of the contract abi, we need to mark it for the interpreter
 * thus the use of "// @abi action"
 */
        // @abi action
        void addmessage( account_name sender, std::string text )
        {
/*
 * require_auth is self explanatory, a user can only be considered "sender" if
 * he has authority
 */
            require_auth(sender);

/*
 * here we define an iterator for the multi_index as the return value from the call
 * to its emplace method (that inserts a new row), see the link below for more details
 * https://www.boost.org/doc/libs/1_66_0/libs/multi_index/doc/index.html
 * for each column of the table, as defined in its struct, has to be filled
 */
            auto iter = messages.emplace( _self, [&](auto& message) {
                message.id = messages.available_primary_key();
                message.sender = sender;
                message.text = text;
            } );
        }

    private:
/*
 * this is the struct defining the actual table in the chain database,
 * with its columns and keys
 * analogous for defining public actions, we need to make use of "// @abi table"
 * to properly generate the contract abi
 */
        // @abi table
        struct messages
        {
            uint64_t id = 0;
            account_name sender;
            std::string text;
/*
 * the following line define the column id as the primary key
 * it is possible to define secondary keys and more
 */
            uint32_t primary_key() const { return id; }
/*
 * we need to serialize its data for internal use of the protocol library
 */
            EOSLIB_SERIALIZE( messages, (id)(sender)(text) );
        };
/*
 * this is the actual attribute of the contract that we initialized in the constructor
 * it provides a neat interface to interact with the database,
 * as seen in the addmessage method
 */
        eosio::multi_index<N(messages), messages> messages;
};

/*
 * this line is necessary to properly generate the contract abi
 */
EOSIO_ABI( simpledb, (addmessage) );