Contract classes and instances

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As in object-oriented programming, Starknet distinguishes between a contract and its implementation by separating contracts into classes and instances.

Contract classes

A contract class is the definition of a contract. It includes Cairo byte code, hint information, entry point names, and everything that defines its semantics.

Each class is uniquely identified by its class hash, comparable to a class name in traditional object-oriented programming languages.

Contract instances

A contract instance is a deployed contract that corresponds to a class. Only contract instances act as true contracts, in that they have their own storage and can be called by transactions or other contracts.

A contract class does not necessarily have a deployed instance in Starknet.

A contract class does not necessarily require a deployed instance in Starknet.

A contract instance has a nonce, the value of which is the number of transactions originating from this address plus 1. For example, when you deploy an account with a DEPLOY_ACCOUNT transaction, the nonce of the account contract in the transaction is 0. After the DEPLOY_ACCOUNT transaction, until the account contract sends its next transaction, the nonce is 1.

Working with classes

Adding new classes

To introduce new classes to Starknet’s state, use the DECLARE transaction.

Deploying instances

To deploy a new instance of a previously declared class, use the deploy system call.

Using class functionality

To use the functionality of a declared class without deploying an instance, use the library_call system call. Analogous to Ethereum’s delegatecall, it enables you to use code in an existing class without deploying a contract instance.