I'm still playing around with my first bigger smart contract, and I'm creating something like public sale, ICO, etc.

Now I'm at the point where I want to specify some conditions like:

  • one address can participate only once
  • maximum allocations, and etc.

My question is: what is the correct approach to implement such conditions, how to do some checks in my function before proceeding if I can't do logical negation on structs, mappings?

1.) I have declared struct with some data and mapping where I want to store my structs:

struct Investor {
        address account;
        uint invested;

mapping(address => Investor) public investors;

2.) Here is my invest function where I want to deal with some checks:

    function invest() external payable {

        // Here I want to check if msg.sender already exists as an investor in my mapping.


3.) My approach was just to check if msg.sender is in my mapping and just negate it:


But it doesn't work as I would expect from JS experience as structs are not convertable to type boolean. How to deal with conditions like this? Should I just change the way I store my data?

compile error


4.) Looks like it's not working even with out negation.


Is not working: "! cannot be aplied to type address".




Doesn't work and code cannot compile with error:

enter image description here

  • This might work require(!investors[msg.sender].account)
    – pbsh
    Jan 26, 2022 at 13:08
  • It doesn't work for me, as I'm getting same kind of error but saying that negation cannot be applied to type address
    – BlackH3art
    Jan 26, 2022 at 17:18
  • This should work require(investors[msg.sender].account!=address(0))
    – pbsh
    Jan 26, 2022 at 17:26
  • Yes. This working. Thanks. But what if I would have mapping like: mapping(address => uint) and wanted to check if some address is already in my mapping?
    – BlackH3art
    Jan 26, 2022 at 17:34
  • In that case, require(investors[msg.sender] != 0) should work. This is because, in a mapping, all the uninitiated indexes default to zero.
    – pbsh
    Jan 26, 2022 at 17:38

1 Answer 1


Adding the answer so it's easier for others to find.

In a mapping, by default all uninitiated indexes default to zero. So does structs. So a mapping, to see if it's initialised, we can just check value. In this case, we can check the address and make sure it's not zero.


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