[Ethereum] how to use multiple modofier to form combinations of boolean conditions

modifierssolidity

I have read this article and this SO. And I understand the functionality of modifier. When modifiers are placed on a function with a spacing they will be evaluated in the order of placing thus creating an And condition. However, I would to understand how multiple modifiers can be placed so that combination of other Boolean conditions such as OR be evaluated?

For Instance,

modifier modA {
  // verify something
  _;
  // verify something
  _;
}
modifier modB {
  // verify something
  _;
  // verify something
  _;
}
function Fun() modA modB {
  // Do something
}

In this case, modA AND modB will be the execution. But is it possible to use them to form OR condition?

Best Answer

There is no direct way in solidity. I'll explain you why. Modifier is like a macro function while compiling it will replace your modifier with its actuval implementation. Here _; replace with your function implementation.

contract ModiferTesting {
   uint public a;
   uint public b;
   uint public c;

   modifier modA() {
    a = a + 1;
    _;
   }

   modifier modB() {
    b = b + 1;
    _;
    b = b + 1;
    _;
    b = b + 1;
    _;
  }

  function test() public modA modB {
    c = c + 1;
  }
}

If you debug, above code you can find a=1, b=3, c=3

Try to write an other modifier that check's or condition.

Related Solutions

Using Multiple Call Arguments in Solidity – What Happens?

The low-level .call function allows you to call a function by its function signature calculated as bytes4(sha3("functionName()")). Also take a look at https://ethereum.stackexchange.com/a/9722/16 and Call contract and send value from Solidity. If your function has parameters, the signature looks as follows bytes4(sha3("functionName(uint256,string,address)")) - keep in mind that instead of uint you have to use uint256. The following parameters are the actual values of the parameters. Thus, the whole call would look like

x.call(bytes4(sha3("functionName(uint256,string,address)")), myUIntVal, myStringVal, myAddressVal);

You can also specify a specific gas or value (ether (but in Wei!) to send along) like this:

x.call.value(1234)(bytes4(sha3("functionName(uint256,string,address)")), myUIntVal, myStringVal, myAddressVal);

Solidity Modifiers – How to Check Boolean Variable is True in Modifier

It is not exactly clear what you want to do since there is input parameter in your modifier.

If you want to check if address calling function is valid then modify your modifier like this:

modifier onlyValidSender()
{
    require (Accounts[msg.sender].active);
    _;
}

If you want to make sure that you are calling your function on Account which is valid then modify like this:

modifier onlyValid(address _id)
{
    require (Accounts[_id].active);
    _;
}

Here you can see full code which implements two types of modifiers.

pragma solidity 0.4.24;

contract Test
{

mapping (address => Account) public Accounts;

struct Account {
    address id;
    string company;
    string country;
    string industry;
    bool active;
}


modifier validAccount(address _accountId) {
    require (Accounts[msg.sender].active);
    _;
}

function Add(address _id, string _company, string _country, string _industry,bool _active)
{
    Account memory  acc = Account(_id,_company,_country,_industry,_active);
    Accounts[_id] = acc;
}

function GetCompany(address _id) onlyValid(_id) public constant returns(string)
{
    return Accounts[_id].company;
}

modifier onlyValid(address _id)
{
    require (Accounts[_id].active);
    _;
}

modifier onlyValidSender()
{
    require(Accounts[msg.sender].active);
    _;
}

function GetCountry(address _id) onlyValidSender public view returns(string)
{
        return Accounts[_id].company;

}

}

GetCompany will only return value when _id Account is active.

GetCountry will return value only when account asking for it is valid.

Best Answer

Related Solutions

Using Multiple Call Arguments in Solidity – What Happens?

Solidity Modifiers – How to Check Boolean Variable is True in Modifier

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