Inheritance is one of the most important features of the object-oriented programming language. It is a way of extending the functionality of a program, used to separate the code, reduces the dependency, and increases the re-usability of the existing code. Solidity supports inheritance between smart contracts, where multiple contracts can be inherited into a single contract. The contract from which other contracts inherit features is known as a base contract, while the contract which inherits the features is called a derived contract. Simply, they are referred to as parent-child contracts. The scope of inheritance in Solidity is limited to public and internal modifiers only. Some of the key highlights of Solidity are:
- A derived contract can access all non-private members including state variables and internal methods. But using this is not allowed.
- Function overriding is allowed provided function signature remains the same. In case of the difference of output parameters, the compilation will fail.
- We can call a super contract’s function using a super keyword or using a super contract name.
- In the case of multiple inheritances, function calls using super gives preference to most derived contracts.
Solidity provides different types of inheritance.
1. Single Inheritance
In Single or single level inheritance the functions and variables of one base contract are inherited to only one derived contract.
Example: In the below example, the contract parent is inherited by the contract child, to demonstrate Single Inheritance.
2. Multi-level Inheritance
It is very similar to single inheritance, but the difference is that it has levels of the relationship between the parent and the child. The child contract derived from a parent also acts as a parent for the contract which is derived from it.
Example: In the below example, the contract A is inherited by contract B, contract B is inherited by contract C, to demonstrate Multi-level Inheritance.
3. Hierarchical Inheritance
In Hierarchical inheritance, a parent contract has more than one child contracts. It is mostly used when a common functionality is to be used in different places.
Example: In the below example, the contract A is inherited by contract B, contract A is inherited by contract C, thus demonstrating Hierarchical Inheritance.
4. Multiple Inheritance
In Multiple Inheritance, a single contract can be inherited from many contracts. A parent contract can have more than one child while a child contract can have more than one parent.
Example: In the below example, the contract A is inherited by contract B, contract C is inheriting contract A and contract B, thus demonstrating Multiple Inheritance.