Abstract Contracts

Abstract contracts can't exist on their own. Their functionality can only be utilized by a contract that inherits from them. In this lesson, you'll learn how to create an abstract contract.

Objectives

By the end of this lesson you should be able to:

• Use the virtual, override, and abstract keywords to create and use an abstract contract

Abstract Contracts

Continue with your Inheritance.sol file. Add ContractD as an abstract contract. Add a virtual function called whoAreYou function, but do not add any implementation for that function.

abstract contract ContractD {
    function whoAreYou() public virtual view returns (string memory);
}

Inheriting from an Abstract Function

Update ContractA to inherit from ContractD.

You'll get a slightly confusing error that ContractA needs to be marked as abstract. Doing so is not the correct fix.

from solidity:
TypeError: Contract "ContractA" should be marked as abstract.
  --> contracts/Inheritance.sol:25:1:
   |
25 | contract ContractA is ContractB, ContractC, ContractD {
   | ^ (Relevant source part starts here and spans across multiple lines).
Note: Missing implementation:
 --> contracts/Inheritance.sol:6:5:
  |
6 |     function whoAreYou() public virtual view returns (string memory);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The clue for the correct solution is further down: Note: Missing implementation:

Only abstract contracts can declare functions that are not implemented. To fix this, provide an override implementation for whoAreYou in ContractA:

function whoAreYou() public override pure returns (string memory) {
    return "I'm a person!";
}

Conclusion

In this lesson, you've learned how to implement and inherit from an abstract contract.