Nonpayable

Nonpayable is a function mutability specifier in Solidity that marks a function as able to modify state but not accept Ether. A nonpayable function will reject any incoming Ether sent with the call, causing the transaction to revert if value is provided.

In contrast, a payable function is explicitly allowed to receive Ether. The nonpayable modifier is used to enforce that a function cannot be used to transfer Ether to the contract, which helps prevent accidental loss of funds and clarifies the intended use of the function.

Nonpayable functions are commonly used for internal logic, administrative actions, or state-changing operations that should not be invoked with Ether. Declaring a function as nonpayable makes the contract’s interface stricter and can improve security by reducing the surface area for certain kinds of attacks or misuses.

It is important to distinguish nonpayable functions from the contract’s fallback and receive functions. Ether can still be sent to a contract via a payable receive() or payable fallback() function, but such Ether is not delivered to nonpayable functions unless a separate, payable function is called.

Example:

pragma solidity ^0.8.0;

contract Example {

uint public count;

// This function cannot receive Ether

function increment() external nonpayable {

count += 1;

}

}

In summary, nonpayable explicitly disallows sending Ether to a function, while payable permits it; both are part of Solidity’s state mutability system and influence how a contract handles value transfers.