Built-in Objects Reference

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The UTXO_Compiler contract language provides two built-in objects: BVM and self. This document explains what they represent and how to use them in contracts.

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1. self Object

• Object Name: self
• Description: Built-in object representing the current contract instance. self.X is replaced at compile time with a constant baked into the bytecode, so it is not subject to ownership rules and may be read repeatedly from any public or private function.
• Example:

  # Use self to access contract member variables
  Contract P2PKH:
      def verify(sig: hex, pubKey: hex):
          pubKey_copy = pubKey.Clone()
          pubKeyHash = Hash160(pubKey_copy)
          EqualVerify(pubKeyHash, self.pubKeyHash)   # check the stored hash for this instance
          Return CheckSig(sig, pubKey)

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2. BVM Object

• Object Name: BVM (Bytecode Virtual Machine)
• Description: A built-in object representing the bytecode virtual machine, providing access to VM metadata such as version, input/output counts.
• Predefined Member Mapping Table:

  Member Name	Corresponding Value	Purpose
  "version"	1	VM version number
  "locktime"	2	Lock time
  "inputCount"	3	Number of inputs
  "outputCount"	4	Number of outputs
  "inputsHash"	5	Hash of input data
  "unlockingInput"	6	Unlocking input data
  "outputsHash"	7	Hash of output data

• Example:

  # Process transaction data and verify
  tx_data = Cat(pretx.VLIO, tx_data)  # concatenate transaction data
  txid = Hash256(tx_data)  # compute transaction ID
  
  # Access BVM object metadata
  meta_data = BVM.unlockingInput  # get unlocking input metadata
  {meta_data_txid, meta_data_remain} = Split(meta_data, 32)  # extract transaction ID from metadata
  
  EqualVerify(txid, meta_data_txid)  # verify transaction ID consistency

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Beginner Pitfalls

• Treating self as runtime storage: self.X is written into bytecode at compile time, not mutable on-chain storage.
• Forgetting transaction context during local debugging: contracts that read BVM.* need simulated data from settxfile.
• Checking only one output field: when current outputs matter, use BVM.outputsHash for the aggregate commitment.

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Quick Review

• self represents the current contract instance and is commonly used for deployment-time instance data.
• self.X is replaced into bytecode at compile time and can be read repeatedly.
• BVM exposes transaction-context data available during script execution.
• BVM.outputsHash is commonly used to verify that current transaction outputs match contract rules.
• Contracts that read BVM.* need simulated transaction data when debugged locally.

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Next Steps

• Language Specification — Complete formal grammar definition

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🇨🇳 中文版