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Contracts can inherit from more than one contract. In this lesson, we’ll explore how multiple inheritance works in Solidity.

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Objectives

By the end of this lesson you should be able to:
  • Write a smart contract that inherits from multiple contracts

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Multiple Inheritance

Continue working with your contracts in Inheritance.sol. Add a new contract called ContractC with another whoAmI function: 
contract ContractC {
    function whoAmI() external pure returns (string memory) {
        return "contract C";
    }
}

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Inheriting from Two Contracts

You can inherit from additional contracts by simply adding a comma and that contract’s name after the first. Add inheritance from ContractC (an error is expected): 
// bad code example, do not use
contract ContractA is ContractB, ContractC {
    function whoAmExternal() external pure returns (string memory) {
        return whoAmIInternal();
    }
}
The error is because both ContractB and ContractC contain a function called whoAmI. As a result, the compiler needs instruction on which to use. 
from solidity:
TypeError: Derived contract must override function "whoAmI". Two or more base classes define function with same name and parameter types.
  --> contracts/Inheritance.sol:21:1:
   |
21 | contract ContractA is ContractB, ContractC {
   | ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "ContractC":
 --> contracts/Inheritance.sol:6:5:
  |
6 |     function whoAmI() external pure returns (string memory) {
  |     ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "ContractB":
  --> contracts/Inheritance.sol:12:5:
   |
12 |     function whoAmI() external pure returns (string memory) {
   |     ^ (Relevant source part starts here and spans across multiple lines).

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Using Virtual and Override

One method to resolve this conflict is to use the virtual and override keywords to enable you to add functionality to choose which to call. Add the virtual keyword to the whoAmI function in both ContractC and ContractB. They must also be made public instead of external, because external functions cannot be called within the contract. 
contract ContractC {
    function whoAmI() public virtual pure returns (string memory) {
        return "contract C";
    }
}

contract ContractB {
    function whoAmI() public virtual pure returns (string memory) {
        return "contract B";
    }

    // ... additional code
}
Add an override function called whoAmI to ContractA: 
// Bad code example, do not use
function whoAmI() public override pure returns (string memory) {
    return ContractB.whoAmI();
}
You’ll get another error, telling you to specify which contracts this function should override. 
from solidity:
TypeError: Function needs to specify overridden contracts "ContractB" and "ContractC".
  --> contracts/Inheritance.sol:22:32:
   |
22 |     function whoAmI() public override pure returns (string memory) {
   |                              ^^^^^^^^
Add them both: 
function whoAmI() external override(ContractB, ContractC) pure returns (string memory) {
    return ContractB.whoAmI();
}
Deploy and test. The call will now be back to reporting “contract B”.

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Changing Types Dynamically

Add an enum at the contract level in ContractA with members for None, ContractBType, and ContractCType, and an instance of it called contractType. 
enum Type { None, ContractBType, ContractCType }

Type contractType;
Add a constructor to ContractA that accepts a Type and sets initialType. 
constructor (Type _initialType) {
    contractType = _initialType;
}
Update whoAmI in ContractA to call the appropriate virtual function based on its currentType. 
// Bad code example, do not use
function whoAmI() public override(ContractB, ContractC) pure returns (string memory) {
    if(contractType == Type.ContractBType) {
        return ContractB.whoAmI();
    }
    if(contractType == Type.ContractCType) {
        return ContractC.whoAmI();
    }
    return "contract A";
}
You’ll get errors because the function now reads from state, so it is no longer pure. Update it to view. You’ll also have to update the whoAmI virtual functions to view to match. 
function whoAmI() public override(ContractB, ContractC) view returns (string memory) {
    if(contractType == Type.ContractBType) {
        return ContractB.whoAmI();
    }
    if(contractType == Type.ContractCType) {
        return ContractC.whoAmI();
    }
    return "contract A";
}
Finally, add a function that allows you to switch currentType: 
function changeType(Type _newType) external {
    contractType = _newType;
}
Deploy and test. You’ll need to use 0, 1, and 2 as values to set contractType, because Remix won’t know about your enum.

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Final Code

After completing this exercise, you should have something similar to: 
// SPDX-License-Identifier: MIT

pragma solidity ^0.8.17;

contract ContractC {
    function whoAmI() public virtual view returns (string memory) {
        return "contract C";
    }
}

contract ContractB {
    function whoAmI() public virtual view returns (string memory) {
        return "contract B";
    }

    function whoAmIInternal() internal pure returns (string memory) {
        return "contract B";
    }
}

contract ContractA is ContractB, ContractC {
    enum Type { None, ContractBType, ContractCType }

    Type contractType;

    constructor (Type _initialType) {
        contractType = _initialType;
    }

    function changeType(Type _newType) external {
        contractType = _newType;
    }

    function whoAmI() public override(ContractB, ContractC) view returns (string memory) {
        if(contractType == Type.ContractBType) {
            return ContractB.whoAmI();
        }
        if(contractType == Type.ContractCType) {
            return ContractC.whoAmI();
        }
        return "contract A";
    }

    function whoAmExternal() external pure returns (string memory) {
        return whoAmIInternal();
    }
}

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Conclusion

In this lesson, you’ve explored how to use multiple inheritance to import additional functionality into a contract. You’ve also implemented one approach to resolving name conflicts between those contracts.
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