Simple Contract Header

Coding the Simple Contract's header file

Having created the SimpleContract files and registered them in CMake, let's implement the contract's header first, as most of the registration is done there.

Declaring the Contract Class

Open the header file (simplecontract.h) and add the following lines:

#ifndef SIMPLECONTRACT_H
#define SIMPLECONTRACT_H

#include "../dynamiccontract.h"
#include "../variables/safestring.h"
#include "../variables/safetuple.h"
#include "../../utils/utils.h" // SafeUintX_t aliases declared here

class SimpleContract : public DynamicContract {
  private:
    // ...
  public:
    // ...
};

#endif // SIMPLECONTRACT_H

This is a simple skeleton so we can start building the proper contract. From top to bottom:

  • We create include guards as a safety measure

  • We include the DynamicContract class and three SafeVariable classes for the contract's inner variables - SafeString, SafeUint256_t and SafeTuple, which represent a Solidity string, uint256 and tuple/struct types, respectively (check the src/contract/variables subfolder for all available variable abstractions)

    • For SafeUint (and SafeInt as well), we include src/utils/utils.h as all of the SafeUintX/SafeIntX aliases are declared there, if you want to use SafeUint_t<X>/SafeInt_t<X> then include src/contract/variables/safeuint.h (or safeint.h) directly instead

  • We create our SimpleContract class, inherit it from DynamicContract, and leave some space for the private and public members that will be coded next

Now we can declare the members of our contract. We have to pay attention to some rules described earlier, which we'll go through slowly, one part at a time.

Declaring the Contract Variables

Variables MUST be private and MUST inherit one of the SafeVariable classes. So our class declaration would start with something like this:

class SimpleContract : public DynamicContract {
  private:
    SafeString name_;
    SafeUint256_t number_;
    SafeTuple<std::string, uint256_t> tuple_;
  // ...
}

Our three variables name_, number_ and tuple_ are respectively declared as a SafeString, SafeUint256_t and SafeTuple<std::string, uint256_t>. Notice we can use primitive types inside the tuple just fine, as the contract's inner variables already ensure commit/revert safety due to their types being inherited from their SafeVariable counterparts.

Declaring the Contract Functions

Functions in general can return either void or an ABI-supported C++ type (see the correlation on Solidity ABI). The main difference between both is that view functions MUST be const (e.g. getName() const;), while non-view functions MUST NOT be const (e.g. setName(std::string name);).

For the two registering functions, registerContract() MUST be public static void, and registerContractFunctions() MUST be private void override. For the dump() function, it MUST be const override and return a DBBatch object.

class SimpleContract : public DynamicContract {
  private:
    SafeString name_;
    SafeUint256_t number_;
    SafeTuple<std::string, uint256_t> tuple_;
    void registerContractFunctions() override;

  public:
    std::string getName() const;
    uint256_t getNumber() const;
    std::tuple<std::string, uint256_t> getTuple() const;
    void setName(const std::string& argName);
    void setNumber(uint256_t argNumber);
    void setTuple(const std::tuple<std::string, uint256_t>& argTuple);

    static void registerContract() {
      ...
    }

    DBBatch dump() const override;
};

Just like with the tuple variable, we can use primitive types and returns on functions just fine, for the same reasons stated above. This also extends to constructors, whch we'll see below.

Declaring the Contract Constructor

Like any C++ derived class, we must call its base class constructor and pass the proper arguments to it (besides the arguments for the derived class itself) so it can be constructed properly. Any contract derived from DynamicContract MUST have two constructors - one for creating a new contract from scratch, and another for loading the contract from the database:

class SimpleContract : public DynamicContract {
  // ...
  public:
    // Constructor from scratch. Create new contract with given name and value.
    SimpleContract(
      const std::string& name,
      uint256_t number,
      const std::tuple<std::string, uint256_t>& tuple,
      const Address& address,
      const Address& creator,
      const uint64_t& chainId
    );

    // Constructor from load. Load contract from database.
    SimpleContract(
      const Address& address,
      const std::unique_ptr<DB> &db
    );

    // ...
}

Aside from our name, number and tuple variables, both constructors also take a few other arguments required by the base DynamicContract constructor:

// Constructor that creates the contract from scratch
DynamicContract(
  const std::string& contractName,
  const Address& address,
  const Address& creator,
  const uint64_t& chainId
) : BaseContract(contractName, address, creator, chainId) {};

// Constructor that loads the contract from the database
DynamicContract(
  const Address& address,
  const std::unique_ptr<DB> &db
) : BaseContract(address, db) {};

address, creator, chainId and db are internal variables used by the base class, and should always be declared last. They are equivalent to:

Keep in mind that, when calling the base class constructor later on, the contractName argument MUST be EXACTLY the same as your contract's class name. This is because contractName is used to load the contract type from the database, so incorrectly naming it will result in a segfault at load time.

Declaring the Contract Events

Events MUST be public, void, non-const, AND call an internal function named emitEvent(), which is available to all Dynamic Contracts and does the proper emission of the event.

class SimpleContract : public DynamicContract {
  private:
    SafeString name_; ///< The name of the contract.
    SafeUint256_t number_; ///< The number of the contract.
    void registerContractFunctions() override; ///< Register the contract functions.

  public:
    /// Event for when the name changes.
    void nameChanged(const EventParam<std::string, true>& name) {
      this->emitEvent(__func__, std::make_tuple(name));
    }

    /// Event for when the number changes.
    void numberChanged(const EventParam<uint256_t, false>& number) {
      this->emitEvent(__func__, std::make_tuple(number));
    }

    /// Event for when the name and number tuple changes.
    void tupleChanged(const EventParam<std::tuple<std::string, uint256_t>, true>& tuple) {
      this->emitEvent(__func__, std::make_tuple(tuple));
    }

  // ...
}

emitEvent() requires at most three arguments:

  • The event name (you can just pass __func__ which is the same as this->emitEvent("nameChanged", ...))

  • (Optional) A tuple of EventParam objects representing the event's arguments, where:

    • The first element is the argument type (e.g. name is a std::string, number is a uint256_t, tuple is a std::tuple<std::string, uint256_t>)

    • The second element is a bool that indicates whether the argument should be indexed or not.

    • If your event has no arguments at all you can omit it or pass an empty tuple instead (e.g. this->emitEvent(__func__, std::make_tuple()))

  • (Optional) A flag that indicates whether the event is anonymous or not. Events are non-anonymous by default, so if you wish you can omit this (which is the case for our example, it is equivalent to this->emitEvent(__func__, std::make_tuple(name), false) - if it was an anonymous event, the last flag would be true instead)

Registering the Contract Class

One last thing we have to do within our header is properly register the contract class. All Dynamic Contracts use templating to automate most of the hard work for both our contract and the BDK.

First, define a public tuple called ConstructorArguments with the contract's variable types from its first constructor (the one from scratch - in our example, name is const std::string&, number is const uint256_t&, tuple is const std::tuple<std::string, uint256_t>&):

class SimpleContract : public DynamicContract {
  // ...
  public:
    using ConstructorArguments = std::tuple<
      const std::string&, const uint256_t&, const std::tuple<std::string, uint256_t>&
    >;
    // ...
}

Then, implement the registerContract() function declared previously by calling another function from ContractReflectionInterface called registerContractMethods(), passing a few arguments to it, like this:

class SimpleContract : public DynamicContract {
  public:
    // ...
    static void registerContract() {
      ContractReflectionInterface::registerContractMethods<
        SimpleContract, const std::string&, const uint256_t&, const std::tuple<std::string, uint256_t>&,
        ContractManagerInterface&,
        const Address&, const Address&, const uint64_t&,
        const std::unique_ptr<DB>&
      >(
        std::vector<std::string>{"name_", "number_", "tuple_"},
        std::make_tuple("getName", &SimpleContract::getName, FunctionTypes::View
, std::vector<std::string>{}),
        std::make_tuple("getNumber", &SimpleContract::getNumber, FunctionTypes::View
, std::vector<std::string>{}),
        std::make_tuple("getTuple", &SimpleContract::getTuple, FunctionTypes::View, std::vector<std::string>{}),
        std::make_tuple("setName", &SimpleContract::setName, FunctionTypes::NonPayable
, std::vector<std::string>{"argName"}),
        std::make_tuple("setNumber", &SimpleContract::setNumber, FunctionTypes::NonPayable
, std::vector<std::string>{"argNumber"}),
        std::make_tuple("setTuple", &SimpleContract::setTuple, FunctionTypes::NonPayable, std::vector<std::string>{"argTuple"})
      );
    }
}

Inside the chevrons (registerContractMethods<...>()):

  • The first argument is the contract's class type (in this case, SimpleContract)

  • The following arguments are all the types of arguments inside its first constructor (from scratch - the same ones that were put inside ConstructorArguments) - you can copy-paste the constructor's arguments as-is and take out the names

    • It's important to remember that contract arguments should be declared before the internal arguments used by the base class constructor, as stated in the previous step

Inside the parentheses (registerContractMethods<>(...)):

  • The first argument is a string vector that is a list of all the exact names of the arguments in the constructor, each one separated by a comma - in this case, "name_", "number_" and "tuple_"

    • Note this does not include arguments used by the base class' constructor (e.g. interface, address, creator, chainId, db), only the ones inherent to the contract itself

  • The following arguments are tuples, one for each function from the contract, that contain respectively:

    • The exact name of the function ("getName")

    • A reference to the function itself (&SimpleContract::getName) - if you happen to have one or more overloads of the same function, you may need to specify which function is which using static_cast (e.g. getNumber() would be registered as static_cast<uint256_t(SimpleContract::*)() const>(&SimpleContract::getNumber), while getNumber(const uint256_t&) would be registered as static_cast<uint256_t(SimpleContract::*)(const uint256_t&) const>(&SimpleContract::getNumber))

    • The state mutability of said function, accessed by a FunctionTypes enum (available values are "View", "NonPayable" and "Payable")

    • A string vector that is the list of arguments that the function takes, if any (or a blank list if none)

Every contract MUST have both ConstructorArguments and registerContract() implemented in order to be registered, even if ConstructorArguments is empty (has no arguments at all).

If your contract has events, you should also register them so you can generate their ABI later on. Events are registered the same way as the contract class and its functions, but using a separate function from ContractReflectionInterface called registerContractEvents().

class SimpleContract : public DynamicContract {
  public:
    // ...
    static void registerContract() {
      // registerContract() called here
      ContractReflectionInterface::registerContractEvents<SimpleContract>(
        std::make_tuple("nameChanged", false, &SimpleContract::nameChanged, std::vector<std::string>{"name"}),
        std::make_tuple("numberChanged", false, &SimpleContract::numberChanged, std::vector<std::string>{"number"}),
        std::make_tuple("tupleChanged", false, &SimpleContract::tupleChanged, std::vector<std::string>{"tuple"})
      );
    }
}

Inside the chevrons (registerContractEvents<...>()), the only argument is the contract's class name (in this case, SimpleContract).

Inside the parentheses (registerContractEvents<>(...)):

  • The first argument is the event's name ("nameChanged")

  • The second argument is a flag indicating whether the event is anonymous or not

  • The third argument is a reference to the event itself (&SimpleContract::nameChanged)

  • The last argument is a string vector that is the list of parameters that the event takes, if any (or a blank list if none)

Finally, we go to the src/contract/customcontracts.h file, include our contract's header and add it to the ContractTypes tuple.

// ...some includes ...
#include "templates/simplecontract.h" // <--- Add this line

using ContractTypes = std::tuple<..., SimpleContract>; // <--- Add your contract here
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