The Contract module provides functionality for the runtime to deploy and execute WebAssembly smart-contracts.
This module extends accounts based on the
Currency trait to have smart-contract
functionality. It can be used with other modules that implement accounts based on
These “smart-contract accounts” have the ability to instantiate smart-contracts and make calls
to other contract and non-contract accounts.
The smart-contract code is stored once in a code cache, and later retrievable via its hash. This means that multiple smart-contracts can be instantiated from the same hash, without replicating the code each time.
When a smart-contract is called, its associated code is retrieved via the code hash and gets executed. This call can alter the storage entries of the smart-contract account, instantiate new smart-contracts, or call other smart-contracts.
Finally, when an account is reaped, its associated code and storage of the smart-contract account will also be deleted.
Senders must specify a gas limit with every call, as all instructions invoked by the smart-contract require gas. Unused gas is refunded after the call, regardless of the execution outcome.
If the gas limit is reached, then all calls and state changes (including balance transfers) are only reverted at the current call’s contract level. For example, if contract A calls B and B runs out of gas mid-call, then all of B’s calls are reverted. Assuming correct error handling by contract A, A’s other calls and state changes still persist.
Contract call failures are not always cascading. When failures occur in a sub-call, they do not “bubble up”, and the call will only revert at the specific contract level. For example, if contract A calls contract B, and B fails, A can decide how to handle that failure, either proceeding or reverting A’s changes.
Pallet::instantiate_with_code- Deploys a new contract from the supplied wasm binary, optionally transferring some balance. This instantiates a new smart contract account with the supplied code and calls its constructor to initialize the contract.
Pallet::instantiate- The same as
instantiate_with_codebut instead of uploading new code an existing
Pallet::call- Makes a call to an account, optionally transferring some balance.
The Contract module is a work in progress. The following examples show how this Contract module can be used to instantiate and call contracts.