Getting infinite gas estimates for simple functions

Question

I am testing out solidity in remix ide using simple contracts. This is the contract that I wrote:

 contract mortal {

address owner;

function mortal() {
    owner = msg.sender;
}

function kill(){
    if(msg.sender == owner){
        selfdestruct(owner);
    }
   }
}

contract Hello is mortal
{

string public message;

function Hello(){
    message = 'This is the initial Message';
    }

function getMessage() public constant returns(string){
    return message;
    }

function setNewMessage(string newMessage) public payable {
    message = newMessage;   
    }
} 

On looking at details I am getting infinite gas estimates for each of these functions.

{
"Creation": {
    "codeDepositCost": "243200",
    "executionCost": "infinite",
    "totalCost": "infinite"
},
"External": {
    "getMessage()": "infinite",
    "kill()": "30636",
    "message()": "infinite",
    "setNewMessage(string)": "infinite"
}
}

Please elaborate why am I getting infinite gasEstimates for these functions and how can I avoid this mistake ?

Answer 1

The contract compiles just fine:

pragma solidity ^0.4.15;

contract mortal {
    address owner;

    function mortal() public {
        owner = msg.sender;
    }

    function kill() public {
        if (msg.sender == owner) {
            selfdestruct(owner);
        }
    }
}

contract Hello is mortal {
    string public message;

    function Hello() public {
        message = 'This is the initial Message';
    }

    function getMessage() public constant returns(string) {
        return message;
    }

    function setNewMessage(string newMessage) public payable {
        message = newMessage;   
    }
} 

but then the warnings appear...

but it submits/confirms on Ropsten just fine..

and the method calls work fine too..

This may just be a bug in the compiler's static analysis.

Answer 2

I suspect this is because the cost of those functions is indeed unbounded. Strings can be of any length, so setNewMessage() needs to store an unbounded amount of data, and getMessage() needs to read an unbounded amount of data.

If you want to avoid that warning, you'd have to use a data type with a fixed upper bound on its size.

Answer 3

It is because you are using string which does not has a particular size i.e., it is unbound data type. You can use bytes32 in place of string to limit the cost of gas on execution code because solidity considers it a 32 bit literal.