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WTF Solidity: 8. Initial Value

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Recently, I have been revisiting Solidity, consolidating the finer details, and writing "WTF Solidity" tutorials for newbies.

Twitter: @0xAA_Science | @WTFAcademy_

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Codes and tutorials are open source on GitHub: github.com/AmazingAng/WTFSolidity

Initial values of variables​

In Solidity, variables declared but not assigned have their initial/default values. In this tutorial, we will introduce the initial values of common variable types.

Initial values of value types​

  • boolean: false
  • string: ""
  • int: 0
  • uint: 0
  • enum: first element in enumeration
  • address: 0x0000000000000000000000000000000000000000 (or address(0))
  • function
    • internal: blank function
    • external: blank function

You can use getter function of public variables to confirm the above initial values:

    bool public _bool; // false
    string public _string; // ""
    int public _int; // 0
    uint public _uint; // 0
    address public _address; // 0x0000000000000000000000000000000000000000

    enum ActionSet {Buy, Hold, Sell}
    ActionSet public _enum; // first element 0

    function fi() internal{} // internal blank function
    function fe() external{} // external blank function

Initial values of reference types​

  • mapping: a mapping which all members set to their default values

  • struct: a struct which all members set to their default values

  • array

    • dynamic array: []
    • static array（fixed-length): a static array where all members set to their default values.

You can use getter function of public variables to confirm initial values:

    // reference types
    uint[8] public _staticArray; // a static array which all members set to their default values[0,0,0,0,0,0,0,0]
    uint[] public _dynamicArray; // `[]`
    mapping(uint => address) public _mapping; // a mapping which all members set to their default values
    // a struct which all members set to their default values 0, 0
    struct Student{
        uint256 id;
        uint256 score; 
    }
    Student public student;

delete operator​

delete a will change the value of variable a to its initial value.

    // delete operator
    bool public _bool2 = true; 
    function d() external {
        delete _bool2; // delete will make _bool2 change to default(false)
    }

Verify on Remix​

  • Deploy InitialValue.sol and check the initial values of the different types.

  • After using the delete operator, the value of the variables are reset to their initial values.

Summary​

In this section, we introduced the initial values of variables in Solidity. When a variable is declared but not assigned, its value defaults to the initial value, which is equivalent as 0 represented in its type. The delete operator can reset the value of the variable to the initial value.