Solidity – Understanding Bit Summation and Its Impact on Gas Costs

The problem is that totalSupply() implementation is missing. Seems like autogenerated getters do not count. So you need to explicitly create the function.

contract StandardToken is Token {
    uint256 _totalSupply;

    function totalSupply() constant returns (uint256 totalSupply) {
        totalSupply = _totalSupply;
    }
}      

In Solidity a bool is stored in 1 byte (8 bits). However, only a single bit is actually used. Under the hood, The hex value of true is 0x01, false is 0x00. Converted to binary these are 00000001 and 00000000 - 7 of the 8 bits are not used.

All reads and writes to storage are handled in 32 byte increments. To reduce gas costs, Solidity tightly packs variables where possible so that they are stored within the same 32 bytes. In the case of your struct you have an address (20 bytes) followed by 2 uint8s (1 byte each) and 2 bools (1 byte each). This is a total of 24 bytes, so Solidity is able to pack the entire struct into a single 32 byte slot. There is no need to optimize further on your end.

The concept you are considering (packing more than one bool within a single byte) is known as a bit field. Bit fields are more efficient in cases where you wish to store a sequence of greater than 32 booleans. Mudit Gupta has written an excellent blog post that explains how to accomplish this via bitshifting:

The easiest way is to take a uint256 variable and use all 256 bits of it to represent individual booleans. To get an individual boolean from a uint256 , use this function:

function getBoolean(
    uint256 _packedBools,
    uint256 _boolNumber
)
    public
    view
    returns (bool)
{
    uint256 flag = (_packedBools >> _boolNumber) & uint256(1);
    return (flag == 1 ? true : false);
}

To set or clear a bool, use:

function setBoolean(
    uint256 _packedBools,
    uint256 _boolNumber,
    bool _value
)
    public
    view
    returns (uint256)
{
    if (_value)
        return _packedBools | uint256(1) << _boolNumber;
    else
        return _packedBools & ~(uint256(1) << _boolNumber);
}

With this technique, you can store 256 booleans in one storage slot. If you try to pack bool normally (like in a struct) then you will only be able to fit 32 bools in one slot. Use this only when you want to store more than 32 booleans.