Ethereum-Solidity Quiz Q21: How does function overriding work in Solidity?

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Function overriding in Solidity is the process by which a child contract provides a new implementation for a function already defined in its parent contract.

Since Solidity 0.6.0, this process is explicit and requires the use of two specific keywords(virtual & override) to prevent accidental overrides and improve code readability.

The Process

In the Parent Contract: Use the virtual keyword. This signals that the function is "open" for modification.
In the Child Contract: Use the override keyword. This signals that you are "intentionally" replacing the parent's logic.
Both virtual & override can be used in the same function in a multi level inheritance

Example:

contract Parent {
    // Marked virtual to allow overriding
    function message() public virtual pure returns (string memory) {
        return "Hello from Parent";
    }
}

contract Child is Parent {
    // Marked override to redefine logic
    // Marked virtual to allow overriding
    function message() public virtual override pure returns (string memory) {
        return "Hello from Child";
    }
}

contract Grandchild is Child {
    // Marked override to redefine logic
    function message() public override pure returns (string memory) {
        return "Hello from Grandchild";
    }
}

Constraints:

Visibility - Function visibility must be the same or more restrictive when overriding
State - State mutability can change to be more restrictive (but not less)
Interfaces - Functions in an interface are implicitly virtual. You only need the override keyword in the implementation.
Private - private functions cannot be virtual, so they can never be overridden.