Solidity 0.6.x – Overriding a Function

solidity-0.6.x

I am trying the function override capability in solidity 0.6.0. However, when I am trying to override a function in the same contract it is giving me an error that the function has been defined twice. Here is the code:

contract derivedContractv2 {

    function add(uint newa, uint newb) virtual public pure returns (uint _newValue) {
        return newa+newb;
    }

    function add(uint newesta, uint newestb) override public pure returns (uint _newestValue) {
        return newesta+newestb;
    }

}

The error generated is as follows:

browser/integratedContract.sol:13:5: DeclarationError: Function with same name and arguments defined twice.
function add(uint newa, uint newb) virtual public pure returns (uint _newValue) {
^ (Relevant source part starts here and spans across multiple lines).
browser/integratedContract.sol:17:5: Other declaration is here:
function add(uint newesta, uint newestb) override public pure returns (uint _newestValue) {
^ (Relevant source part starts here and spans across multiple lines).

and

browser/integratedContract.sol:17:46: TypeError: Function has override specified but does not override anything.
function add(uint newesta, uint newestb) override public pure returns (uint _newestValue) {
^------^

Is there a way in which I can override the function defined within the same contract?

Best Answer

The virtual function must be in a parent contract. The override function must be in a child contract. Now you have both the functions in the same contract.

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Solidity Inheritance – How to Call a Function in Parent Contract from Child Contract?

I got confused in the explanation, but I believe what you're trying to achieve is reading and updating the state of the parent contract from child contract. Here is how you can achieve it:

pragma solidity ^0.6.0;

contract parent {  
    uint public num;
    
    function setValue(uint _value) public {  
        num = _value;  
    }
    
    function getValue() public view returns(uint) {  
        return num;  
    }  
}


contract child {
    parent parentInstance;
    constructor(address _parent_address) public {
        parentInstance = parent(_parent_address);
    }
    
    function getParentValue() public view returns(uint){  
        return parentInstance.getValue();
    }  
    
    function setParentValue(uint _child_value) public {
        parentInstance.setValue(_child_value);
    }  
}

The difference in my example is that in the child contract I've created instance of the parent contract and now you can refer to parents contract state. This is achieve by passing the address of the parent contract inside the constructor of child contract.

Additional logic, how to pass the address automatically to child contract:

pragma solidity ^0.6.0;

contract parent {
    child public childContract;
    uint public num;
    
    constructor() public {
        childContract = new child(address(this));
    }
    
    function setValue(uint _value) public {  
        num = _value;  
    }
    
    function getValue() public view returns(uint) {  
        return num;  
    }  
}


contract child {
    parent parentInstance;
    constructor(address _parent_address) public {
        parentInstance = parent(_parent_address);
    }
    
    function getParentValue() public view returns(uint){  
        return parentInstance.getValue();
    }  
    
    function setParentValue(uint _child_value) public {
        parentInstance.setValue(_child_value);
    }  
}

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