Absence of null in Solidity

Aug 31, 2022

One of the weirdest Solidity programming language's quirks is the absence of null. Coming from Swift, where nullability is one of the core building blocks, it felt foreign. At first, I didn't understand how to code without such a useful feature. This post elaborates on how to find a way around that in Solidity when building smart contracts.

Concept of undefined

The concept of undefined, null, nil, None, etc. exists in languages like JavaScript, Java, Python, Swift, .etc. but it does not exist in Solidity.

In Solidity, we can call it zero or default value concept instead. That is because each value gets a slot in the memory once it is created, and should contain something.

Default values

To talk about default values, we should split the Solidity types into two blocks:

dynamically sized types like string, bytes, and arrays;
non dynamically sized types like int, bool and address.

Non dynamically sized types

With non-dynamically sized types, the game is pretty straightforward. Here are the default values for those:

int or uint256 default value is zero 0;
for bool it is false;
address type default value is zero address 0x0000000000000000000000000000000000000000.

If we want to create a struct, the default value is a tuple of all its member's default values.

For an enum default value is the first case. This can be a very foreign approach, but that is because all the enum cases behind the scenes are an array of uint8 integers.

Dynamically sized types

With dynamically sized types, it is a different story:

default value for string is an empty string;
array's default value is an empty array;
bytes default value is empty or no bytes.

Code along

To illustrate default values in the code, let's create an Employee struct with an enum EmployeeType.

enum EmployeeType {
  Employee,
  Contractor,
  PartTime
}

struct Person {
  EmployeeType employeeType;
  bool deleted;
  string name;
  uint256 yearOfBirth;
  address walletAddress;
  uint256[] doorAccess;
}

Now we can initialize a new instance of the Employee struct. Remember, we don't need to provide any member values but allocate them in the memory.

Person person;

If we print out the person value, we will get a tuple of default values for each Person struct member.

tuple(uint8,bool,string,uint256,address,uint256[]): 0,false,,0,0x0000000000000000000000000000000000000000,

Checking nullability

Now that we know the default values, we can check if something is "null" or, to precisely say, it has the default value.

function check() external view {
  console.log(person.deleted == false);
  console.log(bytes(person.name).length == 0);
  console.log(person.yearOfBirth == 0);
  console.log(person.walletAddress == address(0));
  console.log(person.employeeType == EmployeeType.Employee);
  console.log(person.doorAccess.length == 0);
}

Once we execute this function, we get true for all the checks. This is how we can understand that there is no value defined.

  Deleted true
 Name true
 Year of birth true
 Wallet address true
 Employee type true
 Door access true

Worth mentioning is that we converted it to bytes for the' string' type and checked its length. There is another way to achieve that, but that we are going to talk about in one of the future posts.

TL;DR

Solidity programming language doesn't have the nullability feature, which is common in many languages like Swift and JavaScript. Instead, types have the default values, like zero 0 for uint. It is vital to know the default values because it comes in handy once we want to check if something is or isn't defined.