The things you should know about I/O of function in Solidity

In this article, we will discuss the public function with I/O restrictions and returning multiple outputs in Solidity.

Input:

Invalid inputs for a public function in Solidity:

• Map, eg: mapping(uint256 => address).
• Multi-dimensional and unfixed size arrays, eg: uint[][]. This is only true for the version below 0.7. The new version 0.8 of Solidity is supported for multi-dimensional and unfixed size arrays for the input of the public function.

Example:

pragma solidity ^0.8.0;
contract FunctionExample {
    function mapFunction(mapping(uint256 => address) memory input) public {
        
    }
}

When you try to compile the above smart contract, you will get a compile error:

TypeError: Types containing (nested) mappings can only be parameters or return variables of internal or library functions.

A similar example for multi-dimensional and unfixed size arrays (Solidity below 0.7):

pragma solidity ^0.7.0;
contract FunctionExample {
    function arrayFunction(uint[][] memory input) public {}
}

Error:

TypeError: This type is only supported in ABI coder v2. Use "pragma abicoder v2;" to enable the feature.

To fix that, you can change to use Solidity ^0.8.0 or use a multi-dimensional array with fixed size:

1/

pragma solidity ^0.8.0;
contract FunctionExample {
    function arrayFunction(uint[][] memory input) public {}
}

2/

pragma solidity ^0.7.0;
contract FunctionExample {
    function arrayFunction(uint[3][3] memory input) public {}
}

The reason for this restriction: Gas. A bigger array will need more used gas. With a map or unfixed size array, the gas can be infinity. We also cannot predict the amount of will be used of gas. The smart contract should be: simple, reliable, and predictable.

Output:

Invalid output for a public function in Solidity (same as Input rule):

• Map, eg: mapping(uint256 => address).
• Multi-dimensional and unfixed size arrays, eg: unit[][]. ]. This is only true for the version below 0.7.

Example with compile error:

pragma solidity ^0.7.0;
contract FunctionExample {
    
    mapping(uint256 => address) mapData;
    uint[][] unfixedSizeArray;
    
    function getMapdata() public  returns(mapping(uint256 => address) memory){
        return mapData;
    }
    
    function getArrayData() public returns ( uint[][] memory ) {
        return unfixedSizeArray;
    }
}

Restructuring assignments:

pragma solidity ^0.8.0;
contract FunctionExample {
    
    function returnMultipleValues() public view returns  ( uint, bool, address) {
       return (12 , true , msg.sender);
    }
function getData() public view returns ( uint, bool, address, string memory){
        (uint a, bool b, address c ) = returnMultipleValues();
        return (a, b, c, "Hello World");
    }
}