Function modifiers allow you to efficiently change the behavior of functions. In some ways, it’s similar to inheritance, but there are restrictions, particularly in variable scope.

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Objectives

By the end of this lesson you should be able to:
  • Use modifiers to efficiently add functionality to multiple functions

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Adding a Simple OnlyOwner Modifier

By default, public functions can be called by anyone, without restriction. Often this is desirable. You want any user to be able to see what NFTs are for sale on your platform, sign up for a service, or read various items stored in state. However, there will be many functions you don’t want any user to be able to do, such as setting the fee for using the app, or withdrawing all funds in the contract! A common pattern to protect these functions is to use modifiers to make sure that only the owner can call these functions.
For a production app, you’ll want to use a more robust implementation of onlyOwner, such as the one provided by OpenZeppelin.

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Adding an Owner

The address of the deployer of a contract is not included as an accessible property. To make it available, add it as a state variable and assign msg.sender in the constructor.

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Creating an onlyOwner Modifier

[Modifiers] are very similar to functions and are declared with the modifier keyword. The modifier can run any Solidity code, including functions, and is allowed to modify state. Modifiers must have a special _ character, which serves as a placeholder for where the code contained within the modified function will run. Create a simple onlyOwner modifier, which returns an error of NotOwner with the sending address if the sender is not the owner.
Test your modifier by adding a function that uses it:
To test, deploy your contract and call the iOwnThis function. You should see the message “You own this!”. Next, switch the Account, and try the function again. You should see an error in the console: 
call to Modifiers.iOwnThis errored: VM error: revert.

revert
	The transaction has been reverted to the initial state.
Error provided by the contract:
NotOwner
Parameters:
{
 "_msgSender": {
  "value": "0x4B20993Bc481177ec7E8f571ceCaE8A9e22C02db"
 }
}
Debug the transaction to get more information.
Always verify the output of a function call in the console. The result that appears under the button for the function is convenient, but it does not clear or change if a subsequent call reverts.

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Modifiers and Variables

Modifiers can have parameters, which essentially work the same as in functions. These parameters can be independent values, or they can overlap with the arguments provided to a function call.

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Modifiers with Parameters

Modifier parameters can be the arguments provided to the functions they modify. You can perform calculations and trigger errors based on these values. 
error NotEven(uint number);

modifier onlyEven(uint _number) {
    if(_number % 2 != 0) {
        revert NotEven(_number);
    }
    _;
}

function halver(uint _number) public pure onlyEven(_number) returns (uint) {
    return _number / 2;
}

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Independent Scope

While modifiers are used to modify functions and can share inputs, they have separate scopes. The following example will not work: 
// Bad code example, does not work
modifier doubler(uint _number) {
    _number *= 2;
    _;
}

function modifierDoubler(uint _number) public pure doubler(_number) returns (uint) {
    return _number; // Returns the original number, NOT number * 2
}

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Conclusion

Function modifiers are an efficient and reusable way to add checks, trigger errors, and control function execution. In this lesson, you’ve seen examples of how they can be used to abort execution under certain conditions. You’ve also learned that they have separate scopes and cannot be used to modify variables within the function they modify.