There are 3 functions for validation in Solidity: require, revert, and assert.

Require

• Used at the beginning of a function
• Validates against illegal input
• Verifies state conditions prior to execution
• Refunds leftover gas

Example:

require(msg.sender == owner, "Only. owner can execute this action");

Revert

• Identical to require
• Useful for more complex logic flow gates (i.e., complicated if-then blocks)
• Refunds leftover gas

Example:

revert("Something funky has occured");

Assert

• Used at the end of a function
• Validates something that is impossible
• Critical for static code analysis tools
• Does not refund leftover gas

Example:

assert(num >= 0);

Note that require and revert refund only the remaining gas. Any gas used before that statement will be consumed. Both these functions use the same opcode REVERT under the hood.

assert is different because it does not refund any gas and uses all of the gas available for the transaction. It is expected to be used less frequently and should be used for checking conditions that are abnormal and not expected to happen.