Solidity supports multiple inheritance. Contracts can inherit other contract by using the is
keyword.
Function that is going to be overridden by a child contract must be declared as virtual
.
Function that is going to override a parent function must use the keyword override
.
Order of inheritance is important.
You have to list the parent contracts in the order from “most base-like” to “most derived”.
// SPDX-License-Identifier: MIT
pragma solidity ^0.7.3;
/* Graph of inheritance
A
/ \
B C
/ \ /
F D,E
*/
contract A {
function foo() public pure virtual returns (string memory) {
return "A";
}
}
// Contracts inherit other contracts by using the keyword 'is'.
contract B is A {
// Override A.foo()
function foo() public pure virtual override returns (string memory) {
return "B";
}
}
contract C is A {
// Override A.foo()
function foo() public pure virtual override returns (string memory) {
return "C";
}
}
// Contracts can inherit from multiple parent contracts.
// When a function is called that is defined multiple times in
// different contracts, parent contracts are searched from
// right to left, and in depth-first manner.
contract D is B, C {
// D.foo() returns "C"
// since C is the right most parent contract with function foo()
function foo() public pure override(B, C) returns (string memory) {
return super.foo();
}
}
contract E is C, B {
// E.foo() returns "B"
// since B is the right most parent contract with function foo()
function foo() public pure override(C, B) returns (string memory) {
return super.foo();
}
}
// Inheritance must be ordered from âmost base-likeâ to âmost derivedâ.
// Swapping the order of A and B will throw a compilation error.
contract F is A, B {
function foo() public pure override(A, B) returns (string memory) {
return super.foo();
}
}