What's the Solidity statement to print data to the console?

Question

Sometimes it would be really helpful to be able to see string representations of variable values in the Geth console, like of argument values in functions. Can I somehow print them to the Geth console?

Answer 1

You are most likely looking for Events. Not only do they help with debugging, but they are useful in normal production code.

Events are declared like functions, like so:

event VoteCast(address voter, uint votes, bool inFavor);

Then somewhere (for example, in a vote counting function):

function vote(bool inFavor) {
    var votes = shares[msg.sender];
    // ...
    emit VoteCast(msg.sender, votes, inFavor);
}

In javascript, a contract object has an event method, which can be used to read events when they happen. In fact, it can read events that happened in the past.

var voteCast = someContract.voteCast();
voteCast.watch(function(err, result) {/* some callback */});
// Alternately, to get the events all at once.
voteCast.get(function(err, result) /* some other callback* /)

Events have a number of quirks, which are too many to get into detail here. Nonetheless, they're an integral part of high-level dapp work.

Answer 2

EDIT: logX is deprecated and doesn't appear in Solidity 0.8+ documentation. The bellow example is only working if using Solidity up to version 0.7.6 and should now be replaced by Events or Solidity libraries enabling the use of console.log() inside your contract code.

Since my first answer, many projects improved the debug experience:

• https://github.com/trufflesuite/truffle-logger-example

  import "truffle/Console.sol";
  

• https://hardhat.org/tutorial/debugging-with-hardhat-network.html
• https://remix-ide.readthedocs.io/en/latest/hardhat_console.html

  import "hardhat/console.sol";
  

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Deprecated answer

Print doesn't exist in Solidity. Use the logX statement as indicated in the manual instead.

Answer 3

Since you mentioned geth console, you can try console.log() for debugging purposes:

console.log ("Your debug message" + debug.object);

This is possible because geth supports web3.js