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a Consensys factory class has this method:

//verifies if a contract that has been deployed is a Human Standard Token. //NOTE: This is a very expensive function, and should only be used in an eth_call. ~800k gas

function verifyHumanStandardToken(address _tokenContract) constant returns (bool) ...

Do certain constant methods use gas? Don't they just query the blockchain on the local node?

Can somebody clarifies this? Thanks.

marked as duplicate by Ismael, Achala Dissanayake, Richard Horrocks, Rob Hitchens - B9lab, benjaminion Sep 26 '17 at 4:41

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This is not what constant means. A constant function means that your node does not need a transaction to run it - because it does not alter any state. However, if your constant function is called within a transaction - it will cost gas as part of a transaction. What you could do with constant code is running it outside (e.g. before) a transaction on your node. Then it will not cost any gas.

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You should read Edmund's answer on this topic.

Constant functions can be run by your node for free.

If however you want them to be run as part of a transaction, they need to be run on chain (which costs gas). This is because Ethereum is a trustless ledger. If it were not run on chain, the result could be tampered with.

  • Check the contract: github.com/ConsenSys/Tokens/blob/master/contracts/… line 19, as you can see, the function is intended to be called independently. In such case, should cost anything right? I guess my real question is, under which case you would call this method in chain transaction? why? – John Smith Aug 19 '17 at 20:03
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A constant function is expected to keep the state of the contract constant. There should be no point (although permitted) in sending a transaction to the contract to execute a constant function.

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