Coalesce Operator (??)

The ?? operator provides a concise way to handle default values. It returns the left-hand operand if it is not the default value for its type; otherwise, it returns the right-hand operand.

Basic Usage

The syntax is value ?? default.

var name: String := '';
PrintLn(name ?? 'Anonymous');

name := 'Alice';
PrintLn(name ?? 'Anonymous');

Anonymous
Alice

Default Values by Type

The operator checks against the language-defined "empty" or "zero" state for each type.

Example: Integer and Floats

Note that 0 is considered the default value for numbers.

var count := 0;
PrintLn(count ?? 10);

var price := 0.0;
PrintLn(price ?? 19.99);

10
19.99

Example: Objects and Arrays

Highly useful for ensuring an object or array is initialized.

var list: array of Integer;
// If list is empty/nil, returns a new array
var activeList := list ?? [1, 2, 3]; 
PrintLn(activeList.Length);

var obj: TObject := nil;
var instance := obj ?? new TObject;
PrintLn(instance.ClassName);

3
TObject

Chaining

You can chain multiple ?? operators to find the first non-default value.

var input1 := '';
var input2 := '';
var input3 := 'Hello';

var result := input1 ?? input2 ?? input3 ?? 'Default';
PrintLn(result);

Hello

Short-circuiting

The right-hand side of a ?? expression is only evaluated if the left-hand side is the default value.

function GetExpensiveDefault: String;
begin
  PrintLn('Computing...');
  Result := 'Heavy Value';
end;

var s := 'Quick Value';
// GetExpensiveDefault is NEVER called here
PrintLn(s ?? GetExpensiveDefault());

Quick Value

Use with JSON

The coalesce operator is particularly powerful when navigating JSON structures or dynamic data where fields might be missing or null.

var data := JSON.Parse('{"id": 123}');
var title := data.title ?? 'Untitled'; // 'title' field is missing
PrintLn(title);

Untitled