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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.

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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

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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);
}

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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!";
}

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Conclusion

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