Why do Solidity examples use bytes32 type instead of string?

Question

In many Solidity examples I read that use strings for parameters or return values, I see they are typed as bytes32 although there's a string type. What is the real reason for that? Thanks.

Answer 1

2 main reasons:

1. Contracts currently cannot read a string that's returned by another contract.
2. The EVM has a word-size of 32 bytes, so it is "optimized" for dealing with data in chunks of 32 bytes. (Compilers, such as Solidity, have to do more work and generate more bytecode when data isn't in chunks of 32 bytes, which effectively leads to higher gas cost.)

Answer 2

I have test in this site https://ethfiddle.com/zLxE5Y-8B4

contract TestGas {
    string constant statictext = "Hello World";
    bytes11 constant byteText11 = "Hello World";
    bytes32 constant byteText32 = "Hello World";

    function  getString() payable public  returns(string){
        return statictext;
    }

    function  getByte11() payable public returns(bytes11){
        return byteText11;
    }

    function  getByte32() payable public returns(bytes32){
        return byteText32;
    }
}

And function getString spent 21875 gas,

function getByte11 spent 21509 gas,

function getByte32 spent 21487 gas.

So if your string length is fixed, just use bytes32.