Hello World Contract

This tutorial introduces the concept of smart contracts; building smart contracts; deploying smart contracts to a blockchain and then uses cleos to call smart contract actions.

Create the Contract

Create a new directory called hello in the contracts directory you previously created and enter the newly created directory.:

cd CONTRACTS_DIR
mkdir hello
cd hello

Create a new file, hello.cpp, and open it in your preferred text editor:

touch hello.cpp

Below the eosio.hpp header file is included. The eosio.hpp file includes a few classes required to write a smart contract.

#include <eosio/eosio.hpp>

Using the eosio namespace will reduce clutter in your code. For example, by setting using namespace eosio;, eosio::print("foo") can be written print("foo")

using namespace eosio;

Create a standard C++11 class. The contract class needs to extend eosio::contract class which is included earlier from the eosio.hpp header:

#include <eosio/eosio.hpp>

using namespace eosio;

class [[eosio::contract]] hello : public contract {};

An empty contract is not enough. Add a public access specifier and a using-declaration. The using declaration will allow us to write more concise code:

#include <eosio/eosio.hpp>

using namespace eosio;

class [[eosio::contract]] hello : public contract {
  public:
    using contract::contract;
};

This contract needs to do something. In the spirit of hello world write an action that accepts a "name" parameter, and then prints that parameter out:

Actions implement the behavior of a contract

#include <eosio/eosio.hpp>

using namespace eosio;

class [[eosio::contract]] hello : public contract {
  public:
      using contract::contract;

      void hi( name user ) {
         print( "Hello, ", user);
      }
};

The above action accepts a parameter called user, that's a name type. EOSIO comes with a number of typedefs, one of the most common typedefs you'll encounter is name. Using the eosio::print library previously included, concatenate a string and print the user parameter. Use the braced initialization of name{user} to make the user parameter printable.

As is, the ABI generator in eosio.cdt won't know about the hi() action without an attribute. Add a C++11 style attribute above the action, this way the abi generator can produce more reliable output:

#include <eosio/eosio.hpp>

using namespace eosio;

class [[eosio::contract]] hello : public contract {
  public:
      using contract::contract;

      [[eosio::action]]
      void hi( name user ) {
         print( "Hello, ", user);
      }
};

Everything together, here's the completed hello world contract:

#include <eosio/eosio.hpp>

using namespace eosio;

class [[eosio::contract]] hello : public contract {
  public:
      using contract::contract;

      [[eosio::action]]
      void hi( name user ) {
         print( "Hello, ", user);
      }
};

Compile the Contract

Compile your code to web assembly (.wasm) as follows:

eosio-cpp hello.cpp -o hello.wasm

Deploy the Contract

When you deploy a contract, it is deployed to an account, and the account becomes the interface for the contract. As mentioned earlier these tutorials use the same public key for all of the accounts to keep things simple.

View the wallet keys by:

cleos wallet keys

Create an account for the contract using cleos create account:

cleos create account eosio hello YOUR_PUBLIC_KEY -p eosio@active

Deploy the compiled wasm to the blockchain with cleos set contract.

cleos set contract hello CONTRACTS_DIR/hello -p hello@active

Execute the Contract

Great! Now the contract is set. Push an action to it.

cleos push action hello hi '["bob"]' -p bob@active

executed transaction: 4c10c1426c16b1656e802f3302677594731b380b18a44851d38e8b5275072857  244 bytes  1000 cycles
#    hello.code <= hello.code::hi               {"user":"bob"}
>> Hello, bob

As written, the contract will allow any account to say hi to any user:

cleos push action hello hi '["bob"]' -p alice@active

executed transaction: 28d92256c8ffd8b0255be324e4596b7c745f50f85722d0c4400471bc184b9a16  244 bytes  1000 cycles
#    hello.code <= hello.code::hi               {"user":"bob"}
>> Hello, bob

As expected, the console output is Hello, bob

In this case "alice" is the one who authorized it and user is just an argument. Modify the contract so that the authorizing user, "alice" in this case, must be the same as the user the contract is responding "hi" to. Use the require_auth method. This method takes a name as a parameter, and will check if the user executing the action matches the provided parameter.

void hi( name user ) {
   require_auth( user );
   print( "Hello, ", name{user} );
}

Recompile the contract

eosio-cpp -abigen -o hello.wasm hello.cpp

And then update it

cleos set contract hello CONTRACTS_DIR/hello -p hello@active

Try to execute the action again, but this time with mismatched authorization.

cleos push action hello hi '["bob"]' -p alice@active

As expected, require_auth halted the transaction and threw an error.

Error 3090004: Missing required authority
Ensure that you have the related authority inside your transaction!;
If you are currently using 'cleos push action' command, try to add the relevant authority using -p option.

Now, with our change, the contract verifies the provided name user is the same as the authorising user. Try it again, but this time, with the authority of the "alice" account.

cleos push action hello hi '["alice"]' -p alice@active

executed transaction: 235bd766c2097f4a698cfb948eb2e709532df8d18458b92c9c6aae74ed8e4518  244 bytes  1000 cycles
#    hello <= hello::hi               {"user":"alice"}
>> Hello, alice

What's Next

• Deploy, Issue and Transfer Tokens: Learn how to deploy, issue and transfer tokens.