Enum defines the multiple values and creates the object with one of the values. Enum is an enumerable constant in Solidity.

The user creates a custom type to select one constant from multiple predefined constants.

Enums are useful to hold the status of transactions status and the flow of small contracts.

How to Create and declare an enum in solidity?

Enum types can be declared using the enum keyword. It assigns the index to each constants in enumeration list, For example, constants=0,constant=1 … constants=N-1. Syntax:

enum enumName {constants1,constant2 ... constantsN}

Once the Enum type is declared, you can create a variable of a new type

enumName [visibile] variableName;

enumName is a new type declared with enum visible are valid visible modifiers in solidity variableName is a valid variable name

Here is an example of below things

  • How to create an enum custom type
  • How to declare enum variable and access value.
// Solidity program example for Enum Type declaration
pragma solidity ^0.5.0; 
   
// Defining contract 
contract EnumTest {
    enum Number { One,Two,Three}
    Number public number;
    constructor(){
        
    }
}

Let’s see some examples

How to access Enum values in solidity?

// Defining contract 
contract EnumTest {
    enum Number { One,Two,Three}
    Number public number=Number.Three;
    constructor() public {

    }
    function getNumber() public view returns (Number) {
    return number;
}

Output: { “0”: “uint8: 2” }

How to Convert Enum to number type in solidity

Enum constants are defined with index=0 to order the enum constants.

By default, It does the implicit conversion of enum constants to the number, and explicit conversion is not allowed.

use uint(enumvariable) to get uint or integer value.

Here is an example code

// Solidity program example for Enum Type declaration
pragma solidity ^0.5.0; 
   

// Defining contract 
contract EnumTest {
    enum Number { One,Two,Three}
    Number public number=Number.Three;
    constructor() public {
    }
    function getNumber() public view returns (uint) {
     return uint(number);
}
}

Output:

{
    "0": "uint256: 2"
}

How to Convert Enum to String in solidity?

Enum types are converted to strings using a custom function. This function returns a string for each enum constant.

// Solidity program example for Enum Type declaration
pragma solidity ^0.5.0; 
   

// Defining contract 
contract EnumTest {
    enum Number { One,Two,Three}
    Number public number=Number.Three;
    constructor() public {

    }
    function getNumber() public view returns (uint) {
     return uint(number);

}
function getNumberName() external view returns (string memory) {
        Number strNumber = number;
        if (strNumber == Number.One) return "One";
        if (strNumber == Number.Two) return "Two";
        if (strNumber == Number.Three) return "Three";
        return "";
    }
   
}

Output:

{
    "0": "string: Three"
}

How to find Invalid enums syntax errors

Below are the following errors for invalid enum values

Enum values do not contain Boolean values(true, false), But strings True and False are accepted.

enum Number { true,Two,Three}
  • Numbers are not allowed, for example, negative and positive numbers
    enum Number { 1,Two,Three}
  • Semicolons are not ended for enum declaration The below declaration gives an error ParserError: Function, variable, struct or modifier declaration expected.
    enum Number { One,Two,Three};
Removing semicolon(;) from line end works and compiling the file.

How to declare a variable of a string?

Variables declared of a string with below syntax.

string [scope] name;

scope is an optional such as public, private, and valid solidity scopes. string is a keyword and data type defined in the language. name is an identifier declared and valid names are alphanumeric which starting must be a letter. Reserved words not used with name value.

Another way using `string literal syntax allows you to declare and assign the values with a single statement.

string public name = "john";

String literals are enclosed in double quotes(") or single quotes(')

If it is declared in a global contract, called a global scope variable

pragma solidity ^0.5.0; 
   
// Create contract for String type testing
contract StringTest {
    string name;
    string role="admin";

}

There is another way to declare string types with byte32. Byte32 works with string declaration as well as string literal syntax.

pragma solidity ^0.5.0; 
   
// Create contract for String type testing
contract StringTest {
    byte32 name;
    byte32 role="admin";

}

Both string and byte32 do store the string data.

String takes more gas compared to byte32. It is always preferable to use byte32 in place string.

Explicit conversion is not required to convert byte32 to/from a string.