guys, I am developing ERC-20 token and I am wondering why SafeMath is used as as library in the OpenZeppelin implementation? Will not it be more optimal if we supply it as a base contract on the inheritance chain?
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SafeMath's functions are all declared as internal. That means that the library doesn't actually have to be deployed, as the functions are 'inlined' by the compiler: their code is just copied from the library code to the calling function.
An inheritance relationship like contract A is B, A is a B. In case of SafeMath, that does not hold if I just want to use SafeMath's functionality in another contract.
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Actually, as far as gas cost is concerned, it is more optimal when used as a library, since you do not make external function calls (the library functions reside within your contract).
In addition, SafeMath is a "library by nature", i.e., it supplies a specific yet general kind of functionality, so it makes a lot more sense to declare it as a library.
The only down side of it being a library, is that if you ever want to make changes in it, then you will have to redeploy the entire contract, since it does not contain a reference to a SafeMath contract, for which you can add a setter function in advance.
