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I think I have a use case for a library, but I've yet to use one.

I want to deploy a Manager contract as a token, and another for a crowdfund, that will each hold all their respective storage variables, and simply delegate every function call down to libraries that perform the actual logic. In this way I can update bugs if they are found by re-uploading the Library and changing the Library address, and all of my storage will persist.

If I have a balances mapping in my ERC20 Manager contract, can the Library access that mapping? Do I need to pass it in as a parameter, or can I just pass in the parameters that I received in the Contract call? Do I even need to do that, since it is run in the context of the calling contract? In the library can I simply go balances[address] += whatever in my transfer function and because I will have a balances mapping on my contract, it will work and find the address in that mapping, and if I didn't have that mapping on my contract, the library function would just throw?

I guess the root of what I want is, to store no variables/struct/anything in the Library, just a list of functions, so I can simply replace the business logic of my app, kind of creating a server/database sort of thing. But is the library aware of the variables on my calling contract automatically?

EDIT: After more research, it seems you can not update a reference to a library once compiled. So, I would instead create a Contract that does what my Library does, no storage, just functions, and run delegatecalls to it. My question stands - does the contract have access to all of the storage of the calling contract automatically?

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Short Answer

The calling contract will not have access to the base contract automatically but you can use contract inheritance to gain access to all the functionality and variables of the base contract.

When a contract inherits from multiple contracts, only a single contract is created on the blockchain, and the code from all the base contracts is copied into the created contract.

Core Solution

I believe the core problem that you're trying to solve is of making your contracts upgradable by not losing your storage data when you redeploy your contracts. The basic thing to do is use an Eternal Storage and encapsulate your logic by using libraries. You should take a look at this excellent blog post by Elena Dimitrova, a developer at Colony, to understand this concept.

This post talks about making your libraries upgradable too.

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