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I am just starting with solidity. I have a function like so:

function get() constant returns (uint) {
    return storedData;
  }

What is the use of the constant keyword here? I understand that after this keyboard we are defining the return type but why does it need constant in front of it? Are there alternatives to this such as var?

Kex
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2 Answers2

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The "constant" keyword means that function will not alter the state of the contract, that means it won't alter any data, and so contract state and data remain... constant.

Such a function does not consume gas when executed by itself in your node (it may add to gas consumption if ran inside a function that alters contract state/data, because such a function call will need to be executed by miners and included in a block.

Fatima Maldonado
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  • Excellent! Thanks for your answer. – Kex Aug 25 '17 at 03:47
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    What about function `set(uint x) { storedData = x; }` what would happen if I add the keyword constant to this? Why does the `get` need the word constant? Would it cost gas if omitted even though it doesn't modify data? – Kex Aug 25 '17 at 03:50
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    "constant on functions used to be an alias to view, but this was dropped in version 0.5.0." https://docs.soliditylang.org/en/v0.8.10/contracts.html#function-modifiers – elpddev Mar 08 '22 at 15:22
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To give a little more context a constant declaration indicates that the function will not change the state of the contract (although currently this is not enforced by the compiler).

When generating the compiled binaries, declaring a function constant is reflected on the ABI. The ABI is then interpreted by web3 to figure out whether it should send a sendTransaction() or a call() message to the Ethereum node. Since calls are only executed locally, they're effectively free.

See this snippet from the web3.js library:

/**
 * Should be called to execute function
 *
 * @method execute
 */
SolidityFunction.prototype.execute = function () {
    var transaction = !this._constant;

    // send transaction
    if (transaction) {
        return this.sendTransaction.apply(this, Array.prototype.slice.call(arguments));
    }

    // call
    return this.call.apply(this, Array.prototype.slice.call(arguments));
};

Calling a constant function from another contract incurs the same cost as any other regular function.

Yao Sun
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  • If it incurs the same cost, why do I need it then? Why is it used in the `ERC20 balanceOf()` function? the function only returns `return balances[_owner];` and changes no state. Where is the sense of it? – sunwarr10r Nov 10 '17 at 16:41
  • just to be more clear, is it not better a developer uses the view modifier for a function instead of constant as view function ensures that the state of the contract is not altered & it also enforces that – Nature Jul 03 '22 at 23:42