Developing Contracts in C++

C++ WASM Contract Development in C++

Much similar to EOS, developers can use C++ to develop smart contracts on the Ontology platform. Let us take a look at a sample Hello World app.

Sample Code

#include<ontiolib/ontio.hpp>
#include<stdio.h>

using namespace ontio;
class hello:public contract {
    public:
    using contract::contract:
    void sayHello(){
        printf("hello world!");
    }
};
ONTIO_DISPATCH(hello, (sayHello));

Smart Contract Entry Point

The Ontology WASM CDT Compiler encapsulates the required features for entry point and parameter processing. Thus, developers do not need to define entry point methods.

The entry point method can be called in the following manner:

ONTIO_DISPATCH(hello, (sayHello));

The next important part of the contract would be the external interface. This interface would make the contract's services available to external parties. In the sample code above we use the sayHello() method to demonstrate the same.

 printf("hello world!");

This "Hello World" will be printed out in the node log records at the respective "debug level". Practically speaking, the printf() method can only be used for debugging. A more realistic smart contract would need to implement many more complex features.

Smart Contract API

Ontology WASM provides an API that contains the following methods that allow communication with the blockchain system.

API	Parameter	Return Value	Description
timestamp	None	uint64	Current UNIX timestamp
block_height	None	uint32	Current block height
self_address	None	address	Contract address
caller_address	None	address	Invocation address (Same as self_address if invocation is not external)
entry_address	None	address	Entry contract address (Same as self_address if invocation is not external)
check_witness	address	bool	Check the signature of the incoming address
current_blockhash	None	H256	Current block hash
current_txhash	None	H256	Current transaction hash
notify	string	void	Send even notification
call_native	address, params, result	void	Invoke native contract
call_contract	address, params, result	void	Invoke ordinary contract (WASM/NeoVM)
storage_get	key, result	void	Fetch stored data
storage_put	key, value	void	Write data on the chain
storage_delete	key	void	Delete stored data
contract_create	code, vmtype, name, version, author, email, desc	address	Create new contract
contract_migrate	code, vmtype, name, version, author, email, desc	address	Migrate (upgrade) contract
contract_delete	address	void	Delete contract

Let us develop a slightly more complicated WASM contract to demonstrate how to use the API.

Red Envelope Smart Contract

Giving and receiving red envelopes is a part of China's tradition on important festivals and occasions. Red envelopes can now be sent and received using many different tools including social networking and IM platforms such as WeChat. The amount collected can also be deposited to bank accounts.

Let us try an create a smart contract that works the way WeChat's red envelope mechanism works. ONT, ONG, and other OEP-4 standard cryptocurrencies and tokenized assets can be transferred in the form of red envelopes. The amount received is transferred to the receiver's wallet account.

1. Creating a New Contract

#include<ontiolib/ontio.hpp>

using namespace ontio;

class redEnvlope: public contract{

}
ONTIO_DISPATCH(redEnvlope, (createRedEnvlope)(queryEnvlope)(claimEnvlope));

First, we need to create a new contract file and rename it to redEnvelope.cpp.

In this contract we will be implementing three API methods.

createRedEnvlope : To create a new red envelope

queryEnvlope : Query existing red envelope details

claimEnvlope : To claim a red envelope

    std::string rePrefix = "RE_PREFIX_";
    std::string sentPrefix = "SENT_COUNT_";
    std::string claimPrefix = "CLAIM_PREFIX_";

We need to store certain important data. Data is store in the form of key-value pairs withing the scope of the contract. The key to these data need to set the prefix to make querying more convenient.

    address ONTAddress = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1};
    address ONGAddress = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,2};

Since our contract supports the two native assets ONT and ONG, we can define the contract addresses of these these two assets.

Note: Unlike a standard smart contact where the contract address is calculated based on contract hash, the address of the native contract is fixed.

    struct receiveRecord{
        address account;   // user address
        asset amount;      // claimed amount  
        ONTLIB_SERIALIZE(receiveRecord,(account)(amount))
    };

    struct envlopeStruct{
        address tokenAddress;   // asset token address
        asset totalAmount;      // red envelope total amount
        asset totalPackageCount; // total no. of red envelopes
        asset remainAmount;      // remaining amount
        asset remainPackageCount; // remaining 
        std::vector<struct receiveRecord> records;  // claim records
        ONTLIB_SERIALIZE( envlopeStruct,  (tokenAddress)(totalAmount)(totalPackageCount)(remainAmount)(remainPackageCount)(records) )
    };

We need to store the red envelope information in the contract, such as asset related information (token contract address, red envelope total amount, no. of red envelopes, etc.)

ONTLIB_SERIALIZE(receiveRecord,(account)(amount))

The macros defined in CDT can be used to serialize the struct type data before storage.

The preparation is almost complete, and at this point we can start coding the specific API logic.

2. Creating a Red Envelope

bool createRedEnvlope(address owner,asset packcount, asset amount,address tokenAddr ){

        return true;
    }

A new red envelope is created by defining the creator's address, no. of red envelopes, red envelope amount, and the asset token address.

ontio_assert(check_witness(owner),"checkwitness failed");

Verify whether the creator has signed the action, and rollback the transaction if not.

With respect to ontio_assert(expr, errormsg) , when expr is false, an exception is thrown and the process is quit.

        if (isONTToken(tokenAddr)){
            ontio_assert(amount >= packcount,"ONT amount should greater than packcount");
        }

If the red envelope asset is ONT, the amount of ONT must be equal to or greater than the no. of red envelopes, since the asset is indivisible (cannot be smaller than 1). So this condition ensures that each red envelope contains at least 1 ONT.

        key sentkey = make_key(sentPrefix,owner.tohexstring());
        asset sentcount = 0;
        storage_get(sentkey,sentcount);
        sentcount += 1;
        storage_put(sentkey,sentcount);

For every red envelope creator, we need to maintain the record of the total number of red envelopes created and sent by them.

        H256 hash ;

        hash256(make_key(owner,sentcount),hash) ;

        key rekey = make_key(rePrefix,hash256ToHexstring(hash));

A red envelope hash is generated for each red envelope. This hash value serves as a unique identifier.

         address selfaddr = self_address();
        if (isONTToken(tokenAddr)){

            bool result = ont::transfer(owner,selfaddr ,amount);
            ontio_assert(result,"transfer native token failed!");

        }else if (isONGToken(tokenAddr)){

            bool result = ong::transfer(owner,selfaddr ,amount);
            ontio_assert(result,"transfer native token failed!");
        }else{
            std::vector<char> params = pack(std::string("transfer"),owner,selfaddr,amount);
            bool res; 
            call_contract(tokenAddr,params, res );

            ontio_assert(res,"transfer oep4 token failed!");
        }

A tokenized asset it imported to the contract based on its type. The self_address() method can be used to fetch the address of the invocation. The amount of token imported to the contract is based on the token type input by the user.

Note: The transfer operation for the two native assets ONT and ONG is carried out using the ont::transfer API method provided in the CDT. Other OEP-4 based tokens need to be transferred using the standard cross-contract invocation methods.

Note: A smart contract address can receive assets of any type, just as a wallet address can. However, the contract address is generated based on the binary code hash of the contract code, and so the assets stored in the contract address cannot be manipulated without the corresponding private key. Hence, If asset related operations are not set within the contract, there is no way to control the assets stored in it.

        struct envlopeStruct es ;
        es.tokenAddress = tokenAddr;
        es.totalAmount = amount;
        es.totalPackageCount = packcount;
        es.remainAmount = amount;
        es.remainPackageCount = packcount;
        es.records = {};
        storage_put(rekey, es);

The contract information is saved.

        char buffer [100];
        sprintf(buffer, "{\"states\":[\"%s\", \"%s\", \"%s\"]}","createEnvlope",owner.tohexstring().c_str(),hash256ToHexstring(hash).c_str());

        notify(buffer);
        return true;

Next, the red envelope creation event is sent to the chain since this is an asynchronous event with respect to contract invocation. Upon successful execution, the contract notifies the client regarding the event. The specific format of this notification can be defined by the developer.

With this a simple red envelope is created. The next step is to implement the method that can query the red envelope information.

3. Querying Red Envelope Information

   std::string queryEnvlope(std::string hash){
        key rekey = make_key(rePrefix,hash);
        struct envlopeStruct es;
        storage_get(rekey,es);
        return formatEnvlope(es);
    }

The query logic for the red envelope is simple. The stored red envelope data needs to be fetched, re-formatted, and returned.

And finally, the users can claim the red envelope based on the red envelope hash (an ID).

Note: For read-only operations of the smart contract, such as information query, the results can be fetched using pre-execution. Unlike the normal execution process, pre-execution does not require wallet authentication and does not consume ONG.

4. Claiming Red Envelope

We have successfully imported the assets to the smart contract. At this point, the ID can be shared with other users and they can start claiming the red envelope.

  bool claimEnvlope(address account, std::string hash){
      return true;
  }

Claiming a red envelope requires the claiming party's account address and the red envelope's hash.

ontio_assert(check_witness(account),"checkwitness failed");
key claimkey = make_key(claimPrefix,hash,account);
asset claimed = 0 ;
storage_get(claimkey,claimed);
ontio_assert(claimed == 0,"you have claimed this envlope!");

Similarly, the claiming party's signature needs to be verified to ensure that a person may only claim a red envelope themselves, and not by proxy. Also, each user is allowed to claim a red envelope only once.

        key rekey = make_key(rePrefix,hash);
        struct envlopeStruct es;
        storage_get(rekey,es);
        ontio_assert(es.remainAmount > 0, "the envlope has been claimed over!");
        ontio_assert(es.remainPackageCount > 0, "the envlope has been claimed over!");

Using the red envelope hash it's data can be fetched and can be determine whether a red envelope has been fully claimed.

        struct receiveRecord record ;
        record.account = account;
        asset claimAmount = 0;

The claim is added to the claim records.

        if (es.remainPackageCount == 1){
            claimAmount = es.remainAmount;
            record.amount = claimAmount;
        }else{
            H256 random = current_blockhash() ;
            char part[8];
            memcpy(part,&random,8);
            uint64_t random_num = *(uint64_t*)part;
            uint32_t percent = random_num % 100 + 1;

            claimAmount = es.remainAmount * percent / 100;
            //ont case
            if (claimAmount == 0){
                claimAmount = 1;
            }else if(isONTToken(es.tokenAddress)){
                if ( (es.remainAmount - claimAmount) < (es.remainPackageCount - 1)){
                    claimAmount = es.remainAmount - es.remainPackageCount + 1;
                }
            }

            record.amount = claimAmount;
        }
        es.remainAmount -= claimAmount;
        es.remainPackageCount -= 1;
        es.records.push_back(record);

This part of the program logic is a little lengthy. Here, the logic is to calculate the amount of asset claimed from the red envelope. If it is the last red packet, then the amount left is the amount in the last red packet. Otherwise, the remaining asset amount is calculated using a random number generated using the current block hash. This remaining asset amount is then updated in the red envelope information.

        address selfaddr = self_address();
        if (isONTToken(es.tokenAddress)){
            bool result = ont::transfer(selfaddr,account ,claimAmount);
            ontio_assert(result,"transfer ont token failed!");
        } else  if (isONGToken(es.tokenAddress)){
            bool result = ong::transfer(selfaddr,account ,claimAmount);
            ontio_assert(result,"transfer ong token failed!");
        } else{
            std::vector<char> params = pack(std::string("transfer"),selfaddr,account,claimAmount);

            bool res = false; 
            call_contract(es.tokenAddress,params, res );
            ontio_assert(res,"transfer oep4 token failed!");
        }

Based on the claimed asset, the calculated amount of the corresponding asset is transferred to the claim account address from the contract.

        storage_put(claimkey,claimAmount);
        storage_put(rekey,es);
        char buffer [100];        
        std::sprintf(buffer, "{\"states\":[\"%s\",\"%s\",\"%s\",\"%lld\"]}","claimEnvlope",hash.c_str(),account.tohexstring().c_str(),claimAmount);

        notify(buffer);
        return true;

The claim records are stored and the updated red envelope information is stored along with the notification of the event being sent out.

As stated above, the claimEnvlope() method is the only way to move an asset out of this contract. Hence, we establish that the assets stored in the contract are safe.

The simple logic for the red envelope system is complete. The entire smart contract sample code is available here for reference.

Testing a Smart Contract

Using the CLI

Please refer to： https://github.com/ontio/ontology-wasm-cdt-cpp/blob/master/How_To_Run_ontologywasm_node.md

Using the Golang SDK

Please refer to: https://github.com/ontio/ontology-wasm-cdt-cpp/blob/master/example/other/main.go

This example serves to demonstrate how a complete Ontology WASM contract uses API methods to interact with the blockchain. If a complete product is to be developed using this technology, there would be several other considerations such as privacy related concerns for the red envelope. Anyone can monitor a red envelope events to get the hash, and then claim it. This issue can be solved by fixing the addresses, and thereby the users who can claim a red packet. Interested developers can make the necessary changes to the code and test it out.