Solidity return function why the constant?

Question

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?

Answer 1

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.

Answer 2

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.