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Bytes32 vs String

In Solidity, the type of data storage you choose can affect the gas cost of your contract operations, especially when storing or modifying state variables. The Ethereum Virtual Machine (EVM) charges gas for all operations, and the choice between using bytes32 and string types can impact these costs.

Demo Code

Below, we present two simple contracts, SetBytes32 and SetString, which illustrate the difference in gas usage when setting a bytes32 type versus a string type with the same content.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

contract SetBytes32 {
bytes32 public b32;

// use bytes32: 22222 gas
function setBytes32() public {
b32 = "Hello Conflux!";

contract SetString {
string public str;

// use string: 22682 gas
function setString() public {
str = "Hello Conflux!";

When deployed and executed, the SetBytes32 contract uses less gas (22,222 gas) compared to the SetString contract (22,682 gas). The difference, while not substantial, highlights how bytes32 is more gas-efficient for storing fixed-size data compared to string, which is designed for dynamically sized data.

Detailed Explanation of Gas Savings

The choice between bytes32 and string in Solidity impacts gas consumption due to the differences in how each handles data storage:

  • bytes32: A fixed-size data type occupying exactly 32 bytes, regardless of the content's length. Since bytes32 is fixed, there's no need for additional space to store data length, simplifying the contract's storage requirements. In Ethereum smart contracts, storage operations are expensive, but bytes32 optimizes these by compactly fitting into a single storage slot of the EVM, which is precisely 32 bytes. This configuration allows for efficient read and write operations, minimizing gas costs.

  • string: A dynamically sized data type that requires additional storage for length information every time it is stored. This dynamic nature introduces complexity, as string often occupies multiple storage slots, particularly when exceeding 32 bytes. Managing these dynamic arrays increases computational demands for locating the start and end points of the data, leading to higher gas usage.

Recommendations for Gas Optimization:

Use bytes32 for fixed-length strings up to 32 bytes, and string for variable-length content or when lengths may exceed 32 bytes.

🌟 When possible, use bytes32 for fixed-size data to save on gas costs.