How-To Create An Action Which Requires The User To Pay

Overview

This guide provides instructions which show you how to create an action which requires the user to pay for the resources needed to store data in a Key-Value Map (kv map).

Make sure you have the following prerequisites in place:

• An EOSIO development environment, for details consult the Get Started Guide
• A smart contract named smrtcontract
• A user defined type, struct or class, which defines the data stored in the map, named person
• A kv map object, name my_map, which stores objects of type person, with unique keys of type int.

Refer to the following reference implementation for your starting point:

smartcontract.hpp file

struct person {
  eosio::name account_name;
  std::string first_name;
  std::string last_name;
};

class [[eosio::contract]] smartcontract : public eosio::contract {

   using my_map_t = eosio::kv::map<"kvmap"_n, int, person>;

   public:
      using contract::contract;
      smartcontract(eosio::name receiver, eosio::name code, eosio::datastream<const char*> ds)
         : contract(receiver, code, ds) {}

   private:
      my_map_t my_map{};
};

Procedure

Complete the following steps to create an action which requires the person's account, which is stored in the map, to pay for the resources needed to store the person object in the my_map object.

1. Create a new action in your contract, named upsert, which takes as input parameters the person id, an account_name, a first_name and a last_name.
2. Create an instance of the person class, named person_upsert, based on the input parameters: account_name, first_name and last_name.
3. Use the [] operator defined for the kv::map type. Pass as the input parameter for the [] operator an std::pair<int, person> instance with its first parameter the unique id and the second parameter the account which is paying for the resources account_name.

Refer to the following reference implementation to insert a new person object, and then update it, in the kv map:

smartcontract.hpp file

struct person {
  eosio::name account_name;
  std::string first_name;
  std::string last_name;
};

class [[eosio::contract]] smartcontract : public eosio::contract {

   using my_map_t = eosio::kv::map<"kvmap"_n, int, person>;

   public:
      using contract::contract;
      smartcontract(eosio::name receiver, eosio::name code, eosio::datastream<const char*> ds)
         : contract(receiver, code, ds) {}

      // inserts a person if not exists, or updates it if already exists.
      // the payer is the account_name, specified as input parameter.
      [[eosio::action]]
      void upsert(int id,
         eosio::name account_name,
         std::string first_name,
         std::string last_name);

   private:
      my_map_t my_map{};
};

smartcontract.cpp file

// inserts if not exists, or updates if already exists, a person
[[eosio::action]]
void smartcontract::upsert(
      int id,
      eosio::name account_name,
      std::string first_name,
      std::string last_name) {

   // create the person object which will be stored in kv::map
   const person& person_upsert = person{
      account_name = account_name,
      first_name = first_name,
      last_name = last_name};

   // upsert into kv::map and set the payer to be the account_name
   my_map[std::pair<int, eosio::name>(id, account_name)] = person_upsert;
}

Summary

In conclusion, the above instructions show how to require the user to pay for the resources needed to store data in a Key-Value Map (kv map).

Next Steps

• Verify if the newly inserted person actually exists in the map. To accomplish this task use the find() function of the kv_map.
• Delete the newly created or updated person from the map. To accomplish this task, use the erase() function of the kv map.