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# Smart Contract

A "smart contract" is an application program that runs on the TRON network. It's a collection of code (its functions) and data (its state) that resides at a specific account address on the TRON network. Smart contracts are a type of TRON account. This means they have a balance and they can send transactions over the network. However they're not controlled by a user, instead they are deployed to the network and run as programmed. User accounts can then interact with a smart contract by submitting transactions that execute a function defined on the smart contract, but interactions with them are irreversible.

Perhaps the best metaphor for a smart contract is a vending machine. With the right inputs, a certain output is guaranteed. This logic is programmed into the vending machine: `money + snack selection = snack dispensed`, then user can get a snack from a vending machine.

A smart contract, like a vending machine, has logic programmed into it. Here's a simple example of smart contract - vending machine:


Like how a vending machine removes the need for a vendor employee, smart contracts can replace intermediaries in many industries.
# Properties of Smart Contracts
Smart contracts in TRON network have the following properties:
  • Premissionless
Anyone can write a smart contract and deploy it to the TRON network. You just need to learn how to code in a smart contract language, and have enough TRX to deploy your contract. Deploying a smart contract is technically a transaction, so you need to pay your resource fee in the same way that you need to pay for a simple TRX transfer. However resource consumed for contract deployment are far higher.
    TRON has developer-friendly language for writing smart contracts:Solidity. However, they must be compiled before they can be deployed so that TRON virtual machine can interpret and store the contract.
  • Composability
Smart contracts are public on TRON network and can be thought of as open APIs. That means you can call other smart contracts in your own smart contract to greatly extend what's possible. Contracts can even deploy other contracts. You don't need to write your own smart contract to become a dapp developer, you just need to know how to interact with them. For example, you can use the existing smart contracts of SunSwap, a decentralized exchange, to handle all the token swap logic in your app – you don't need to start from scratch.
# Limitations of Smart Contracts
The TRON network smart contracts have the following limitations:
  • Unable to communicate with external system
Smart contracts cannot communicate directly with external systems, so smart contracts themselves cannot get information about "real-world" events, and this bottleneck limits smart contract application scenarios, but this is by design. Relying on external information could jeopardise consensus, which is important for security and decentralization. But the oracle can be used to solve this problem.
  • Maximum execution time of smart contracts
In order to ensure high throughput and stable operation of the network, TRON sets the maximum execution time of TVM to 80ms to ensure that the TRON network can generate a new block every 3s, so the maximum execution time allowed by smart contracts is 80ms . The maximum execution time of TVM is the `#13` dynamic parameter of the TRON network, and the super representative committee can modify this parameter by initiating proposals.
     For complex smart contracts, the execution may timeout and trigger an `OUT_OF_TIME` error, and the caller will be deducted from the full fee_limit fee. So, to avoid smart contracts executing overtime, try to split large contracts into smaller contracts and reference each other as needed, and be aware of common pitfalls and recursive calls to avoid infinite loops.