Modifiers are functions you can apply to other functions. They are really useful to make sure some prerequisite are met before calling a function. Let’s take a simple example. In the contracts we already wrote we often have to check if the caller of a function if the owner of the contract:
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function increment() public { if (owner == msg.sender) { // We check who calls the function count = count + 1; } } |
Imagine if our contract have multiple functions that can only be called by the owner, wouldn’t be nice to refactor this if (owner == msg.sender) part? This is where modifiers will help us. Let’s right our first modifier:
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modifier isAdmin() { require(msg.sender == owner); _; } function increment() public isAdmin { count = count + 1; } |
Our modifier looks like a function. The require() function evaluate a condition and will throw an exception if the condition is not met which will stop the execution of our smart contract. The _ keyword tells the compiler to replace _ by the body of the function.
As same as a function, a modifier can accept parameters. Let’s say we could have multiple users and want to check the ownership.
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modifier onlyBy(address _account) { require(msg.sender == _account); _; } function increment() public onlyBy(owner) { count = count + 1; } |
As you can see our modifier is more generic and let us do more.
You could also write the most generic modifier that only runs if any condition is true:
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modifier onlyIf(bool _condition) { require(_condition); _; } function increment() public onlyIf(msg.sender == owner) { count = count + 1; } |
Multiple modifiers are applied to a function by specifying them in a whitespace-separated list and are evaluated in the order presented.
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modifier onlyIf(bool _condition) { require(_condition); _; } function increment() public onlyIf(msg.sender == owner) onlyIf(count < 200) { count = count + 1; } |
In the next tutorial, we’ll learn how we can use one contract to create and interact with other contracts.
Thanks for the tutorial, very nice and clear 🙂
Happy to help 😉
Oh, so cool! Modifiers are so similar to Guards in Elixir programming. Thank you, Jules. You make this look easier than most resources I’ve seen so far.