Estimate Solidity function gas cost

Question

When i deploy a smartcontract with remix, i can call functions in order to test it.

I do not really know if this is remix or metamask which is doing this job: Before validating the transaction, i have a popup which tells me if the execution of the function will cause an error. And i know how much gas the function will consume.

So i suppose there is a kind of "pre-execution" of the smartcontract's code, before sending the transaction.

What i want to know if it is possible to do the same thing inside a solidity smartcontract.

Is it possible to know how much gas is necessary before calling an external contract inside a solidity function ? And is it possible to pre-execute it in order to see if it will fails ?

Thanks

Answer 1

Is it possible to pre-execute it in order to see if it fails?

Not really:

• Yes: in a contract function, you can call another contract function without reverting, get a status of whether or not it has failed, and handle that as desired
• No: if it hasn't failed, then it means that it had already executed

For example:

pragma solidity 0.6.12;

contract A {
    uint256 public x;

    function func(uint256 _x) public {
        require(_x < 42);
        x = _x;
    }
}

contract B {
    uint256 public successCount;

    function run(address a, uint256 x) public {
        bytes memory input = abi.encodeWithSelector(bytes4(keccak256("func(uint256)")), x);
        (bool success, bytes memory output) = a.call(input);
        if (success)
            successCount++;
    }
}