Type casting msg.value to uint248

Question

I am new to solidity programming and sorry if my question is very basic.

I am wondering if we can cast msg.value to uint248

Example below ; uint value; value = (uint248) (msg.value);

Answer 1

Here's a simple example to demonstrate casting msg.value to a uint248 value using Browser Solidity with the following code:

pragma solidity ^0.4.8;

contract Test {
    uint248 public value;

    function Test() {
        value = 123;
    }

    function () payable {
        value = uint248(msg.value);
    }
}

The screen below shows the deployment of the code to the JavaScript VM:

I've set the Value field to 456.789 and clicked on the (fallback) function, simulating the sending of 456.789 ETH to the contract:

I've set the Value field back to 0 and clicked on the value button to show that the msg.value of 456.789 ETH was casted to a uint248 field:

Answer 2

For casting, use value = uint248(msg.value);

Casting to save 8 bits in this case is not worth it and will probably cost more gas due to unpacking: see Why does uint8 cost more gas than uint256?

Even when using a struct of uint248 and uint8, it is best to actually test whether you are getting some gas savings.

Answer 3

Try this: value = uint248(msg.value);