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.
Reveal code
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
:
Reveal code
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.