问题
struct buyer{
uint amount;
Status status;
}
mapping(address=>buyer) public buyers;
mapping(uint=>address) buyerIndex;
uint public buyerNum;
//Order a product.
function(){
uint doubleValue=value*2;
uint amount=msg.value/doubleValue;
if(buyers[msg.sender]==null){ //Error in this line
buyer abuyer=buyer({amount:amount,status:Status.Created}); //Error in this line
buyerNum++;
buyerIndex[buyerNum]=msg.sender;
buyers[msg.sender]=abuyer;
}else{
buyers[msg.sender].amount+=amount;
}
Order(msg.sender,amount*doubleValue,amount);
}
If a buyer is not recorded in the buyer mapping, then buyerNum++; but I don't know how to tell whether a buyer is in the mapping
回答1:
In solidity every variable is set to 0
by default.
You should think of mappings
as all possible combinations are set to 0
by default.
In your specific case I would use the following:
if (buyers[msg.sender].amount == 0)
回答2:
You could create none
variable to use it as a NULL
:
uint80 constant NULL = uint80(0);
回答3:
There is nothing like null
in solidity.
Just check for the length of the address:
if(buyers[msg.sender].length == 0){
// do your thing
}
See also this answer on ethereum stack exchange.
回答4:
As Viktor said, default value for all possible values in mapping is zero. So if a buyer
has not already inserted in mapping, the amount
value for that address
will be zero. But this approach has a flaw, if a buyer
does exists but its balance became zero after some operations, you will treat it as it does not exist.
I think the best approach is to add a exists
member to the buyer
struct with bool
type. Default value for this member is false
and when the buyer get created, you initialize it with true
value. So you can check exactly if a buyer exist or not via this member.
Buyer struct:
struct buyer{
uint amount;
Status status;
bool exists;
}
Initialize buyer:
buyer memory b = buyer(0, status, true);
Check if buyer exists:
if(buyers[msg.sender].exists) {
//so can buy
}
回答5:
Instead of using one of the values or creating an extra boolean, you can check for the byte size of the struct.
if( bytes( buyers[msg.sender] ).length > 0 ) {
// buyer exists
}
来源:https://stackoverflow.com/questions/37852682/are-there-null-like-thing-in-solidity