Variable

Solidity supports three types of variables.

• State Variables − Variables whose values are permanently stored in a contract storage.

• Local Variables − Variables whose values are present till function is executing.

• Global Variables − Special variables exists in the global namespace used to get information about the blockchain.

Solidity is a statically typed language, which means that the state or local variable type needs to be specified during declaration. Each declared variable always have a default value based on its type. There is no concept of “undefined” or “null”.

State variable

Example:-

pragma solidity ^0.5.0;
contract SolidityTest {
   uint storedData;      // State variable
   constructor() public {
      storedData = 10;   // Using State variable
   }
}

Local variable

Example:-

pragma solidity ^0.5.0;
contract SolidityTest {
   uint storedData; // State variable
   constructor() public {
      storedData = 10;   
   }
   function getResult() public view returns(uint){
      uint a = 1; // local variable
      uint b = 2;
      uint result = a + b;
      return result; //access the local variable
   }
}

Global variable

These are special variables which exist in global workspace and provide information about the blockchain and transaction properties.

Rules for naming Variables

While naming your variables in Solidity, keep the following rules in mind.

• You should not use any of the Solidity reserved keywords as a variable name. These keywords are mentioned in the next section. For example, break or boolean variable names are not valid.

• Solidity variable names should not start with a numeral (0-9). They must begin with a letter or an underscore character. For example, 123test is an invalid variable name but _123test is a valid one.

• Solidity variable names are case-sensitive. For example, Name and name are two different variables.

Variable scope

Scope of local variables is limited to function in which they are defined but State variables can have three types of scopes.

• Public − Public state variables can be accessed internally as well as via messages. For a public state variable, an automatic getter function is generated.

• Internal − Internal state variables can be accessed only internally from the current contract or contract deriving from it without using this.

• Private − Private state variables can be accessed only internally from the current contract they are defined not in the derived contract from it.

Example:-

pragma solidity ^0.5.0;
contract C {
   uint public data = 30;
   uint internal iData= 10;
   
   function x() public returns (uint) {
      data = 3; // internal access
      return data;
   }
}
contract Caller {
   C c = new C();
   function f() public view returns (uint) {
      return c.data(); //external access
   }
}
contract D is C {
   function y() public returns (uint) {
      iData = 3; // internal access
      return iData;
   }
   function getResult() public view returns(uint){
      uint a = 1; // local variable
      uint b = 2;
      uint result = a + b;
      return storedData; //access the state variable
   }
}