Confused Stuff v1.0

1. What are initial values for different data types?

What are the initial/zero values for different data types in Solidity?

In Solidity docs they are called initial values:

• Value Types
  • boolean: false
  • string: ""
  • int: 0
  • uint: 0
  • fixed: 0.0 (presumably; this type is not fully supported)
  • enum: the first element of the enum
  • address: 0x0
  • function
    • internal: empty function, returning initial values (if return is needed)
    • external: function that throws when called
• Reference Types
  • mapping: empty mapping
  • struct: a struct where all members are set to initial values
  • array
    • dynamically-sized: []
    • fixed-sized: an array of the fixed size where all elements are set to initial values

When you use the delete keyword it will assign the initial value to the variable, except for mappings, where it doesn't have any effect. For structs the delete keyword will recurse into the members, unless they are mappings.

2. When to use payable keyword?

Presence of payable modifier means it can process transactions with non zero Ether value.

type

address payable

You can use transfer and send on address payable
→address payable declared address can receive ether

function payable

function with payable modifier can receive ether (from msg.value or from other contract’s function)

• constructor function

Used when you need to send ether in deployment.

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Sample {
    address owner;
    uint value;
    constructor() payable {
        owner = msg.sender;
        value = msg.value;
    }

    function getInfo() public view returns(address, uint) {
        return (owner, value);
    }
}

• normal function

Used when you use msg.value or send ether in function

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Sample {
    mapping(address => uint256) balance;

    function deposit() public payable {
        balance[address(msg.sender)] += msg.value;
    }

    function getBalance(address _address) public view returns(uint){
        return balance[_address];
    }
}

• recieve & fallback function

If I send some ether to Sample by other contract, receive wll handle and automatically put value into contract’s balance.

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Sample {

    receive() external payable {
    }

    function getBalance() public view returns(uint){
        return address(this).balance;
    }
}

3. What does uint8 actually mean?

uint8 means unsigned integers of size8 which is range from 0 to 255. In addition int8 is range of -128 to 127.

4. Is returned variable can be used inside function?

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Sample {
    function add(uint[] calldata _arr) external pure returns(uint sum){
        uint i;
        while(i < _arr.length) {
            sum += _arr[i];
            i++;
        }
        return sum;
    }
}

Yes, I think returned variable can also be tread same as function parameters.