Solidity Minimalist Tutorial: 8. Initial Value

Recently, I have been relearning Solidity, consolidating the finer details, and also writing a "Solidity Minimalist Tutorial" for newbies to learn. Lectures are updated 1~3 times weekly.

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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 variables.

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 equation
  • external: blank equation

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 equation
    function fe() external{} // external blank equation

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 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. Variables with different types have different initial values. The delete operator can reset the value of the variable to the initial value.