Storage vs. memory in Solidity

Overview

memory is a keyword used to store data for the execution of a contract. It holds functions argument data and is wiped after execution.

storage can be seen as the default solidity data storage. It holds data persistently and consumes more gas.

Storage vs memory

storage

memory

Stores data in between function calls

Stores data temporarily

The data previously placed on the storage area is accessible to each execution of the smart contract

Memory is wiped completely once code is executed

Consumes more gas

Has less gas consumption, and better for intermediate calculations

Holds array, state and local variables of struct

Holds Functions argument

Example of storage

pragma solidity ^0.5.12;
 
// Creating a contract
contract fellowCoders
{
  // Initialising array coders
  int[] public coders;
 
  // Function to insert values
  // in the array coders
  function Numbers() public
  {
    coders.push(1);
    coders.push(2);
 
    //Creating a new instance
    int[] storage myArray = coders;
     
    // Adding value to the
    // first index of the new Instance
    myArray[0] = 0;
  } 
}

Explanation

  • Line 4: We create a contract, fellowcoders, to illustrate the storage keyword.

  • Line 7: We Initialize an array, coders.

  • Line 11: We create a Numbers function to insert values in the array coders.

  • Line 17: We create a new instance myArray.

  • Line 21: We add a value to the first index of the new instance.

Example of memory

pragma solidity ^0.5.12;
 
// Creating a contract
contract fellowCoders
{ 
  // Initialising array coders
  int[] public coders;
   
  // Function to insert
  // values in the array
  // coders
  function Numbers() public
  {
    coders.push(1);
    coders.push(2);
     
    //creating a new instance
    int[] memory myArray = coders;
     
    // Adding value to the first
    // index of the array myArray
    myArray[0] = 0;
  } 
}

Explanation

  • Line 4: We create a contract, fellowCoders, to illustrate the memory keyword.

  • Line 7: We initialize a coders array.

  • Line 12: We create a function to insert values in the coders array.

  • Line 18: We use the memory keyword to create a new instance.

  • Line 22: We add value to the first index of the array.

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