Isn't it possible to define modifiers in libraries? I'm really asking myself why, because thats some functionality you would need over and over again.

If I would like to hypothetically implement some ownable modifier and let that be used in many of my contracts, I would have to implement it in every of my contracts or use another contract (no library), that provides the modifier.

I've seen https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/ownership/Ownable.sol and they implement the functionality also as a contract rather than a function as, I guess, modifiers cannot be accessed from Solidity libraries?

My question: should I now implement my modifiers in my abstract contracts or define modifiers in a contract on its own like the Zeppelin guys did? Whats the up/downsides?


Library may define and use modifiers, but it cannot export them. Modifiers are compile-time feature, kind of macros. Modifier code is substituted by compiler to every method that uses this modifier. For complex modifiers this may be a problem, but you may always move modifier's logic into a function (library function, probably), and then refer to this function from within modifier. You may also implement common modifiers in interface to share them among several contracts:

library Foo {
    modifier nonZero (uint x) {
        require (x != 0);

    function isPrime (uint x) public nonZero (x) returns (bool) {
        // Complicated logic here

interface Bar {
    modifier onlyPrime (uint x) {
        require (Foo.isPrime (x));

contract Zoo is Bar {
    function fooBar (uint x) public onlyPrime (x) {
        // Here we know that x is prime!
  • Thanks for the concise answer
    – AirUp
    Mar 19 '19 at 21:12

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