Types of memory [Solidity]

 Storage:

Storage is a location that stores state variables that exists permanently in the contract. Storage is like database data stored in a blockchain node file system. It is persistent and has access to multiple executions of the same contract.

Storage is a key-value store that maps 256-bit words to 256-bit words. Storage data is costly(consuming much gas) because it is written in blockchain(like HDD/SDD). Therefore,

It should be used when it is indispensable such as a money balance.

Data in storage is written in the blockchain (hence they change the state available between calls and transactions that’s why are expensive.

Memory:

Memory is a byte-array data that exists only during the function call. Every message call starts with a cleared memory.

Memory is volatile data and not recorded in the blockchain, so it is much cheaper than storage.

Memory is like RAM used to store short-lived data that spans around function calls. Memory can be used for every non-permanent data.

Stack:

The stack holds small local variables. It has a maximum size of 1024 elements and contains words of 256 bits.

It means the maximum size of the stack is 1024 * 256 bits(262,144bits). If you run out of the stack, contract execution will fail.

EVM provides many opcodes to change stack directly such as POP, PUSH, DUP(duplicate), and SWAP.

A stack is an internal place where temporary variables are stored in 32 bits slots it is usually used for value types

Now, let us compare storage and memory:

Storage

• Stores data in between function calls
• The data previously placed in the storage area is accessible to each execution of the smart contract
• Consumes more gas
• Holds array, state, and local variables of the struct

Memory

• Stores data temporarily
• Memory is wiped completely once the code is executed
• Has less gas consumption, and is better for intermediate calculations
• Holds Functions argument

Storage:-

contract StorageC {

  struct LotteryNumber{

    uint256 Lnumber;

    string selection;

  } mapping(address => LotteryNumber) LotteryNumber;

}

In this example, the smart contract’s state struct variable LotteryNumber is kept in storage, and data will persist between function calls.

Memory:-

function multiply(uint256 num) external pure returns(uint256) {

  uint256 result = num * num; return result;

}

In the above example, the num variable is stored temporarily and locally in memory and it will be released as soon as the function call is completed