how to judge if a struct var exists in solidity?

Question

code like this: struct Voter { bool voted; uint vote; uint votetime; bytes32 remark; } mapping(address => Voter) public voters;

I want to judge like this: if(voters[msg.sender]) { do something… }

Answer 1

The void key values in a mapping are set virtualy to 0

for example if there is no voter with the adddress 0X01 if we request the key value corresponding to this address we will get :

bool voted: false
uint256 vote: 0
uint256 votetime: 0
bytes32 remark: 0x0000000000000000000000000000000000000000000000000000000000000000

so depending on how the voting function is implemented you could check using one of this structure's parameter if it =0 e.g (voters[msg.sender].voted==0) or adding an address or an ID parameter to your structure. the idea is to have a parameter which become non-null(!=0) if user is registered or if he votes.