Variable Types

Data types

Variables have a data type that determines the kind of value that can be stored in the variable. Solidity supports a variety of data types, including:

• Boolean: bool

• Integer: int and uint of various sizes

• Address: address

• Bytes: bytes and byte

• String: string

• Arrays: array

• Structs: struct

• Enumerations: enum

Default values

The concept of “undefined” or “null” values does not exist in Solidity. Newly declared variables (without assignment) always have a default value dependent on its type.

• (u)int = 0

• bool = False

• string = ""

Data Location

Solidity variables can be classified according to their data location.

Value type

A value type stores its data directly in the memory it owns. Variables of this type are duplicated whenever they appear in functions or assignments. A value type will maintain an independent copy of any duplicated variables.

• uint, int, address, bool, enum, bytes

Therefore, a change in the value of a duplicated variable will not affect the original variable.

Reference type

A reference type stores the address of the data’s location; acts as a pointer to a value stored elsewhere.

If you examine the location of where the reference type was created, it will contain the pointer directing us to the actual location of the value; not the value itself.

• mapping, struct, array

  • string arrays, byte arrays, array members, fixed/dynamic arrays

• When a reference type variable is assigned to another variable or when a reference type variable is sent as an argument to a function, EVM creates a new variable instance and copies the pointer from the original variable into the target variable.

• This is known as passing by reference.

• Both the variables are pointing to the same address location. Both the variables will share the same values and change committed by one is reflected in the other variable.