Delphi Programming

Covers Delphi's Object Pascal — an object-oriented descendant of Pascal Programming historically known for rapid application development on Windows (via the VCL) and, later, cross-platform UI development (via FireMonkey/FMX).

Classes and interfaces

type
  IShape = interface
    function Area: Double;
  end;

  TCircle = class(TInterfacedObject, IShape)
  private
    FRadius: Double;
  public
    constructor Create(ARadius: Double);
    function Area: Double;
  end;

constructor TCircle.Create(ARadius: Double);
begin
  FRadius := ARadius;
end;

function TCircle.Area: Double;
begin
  Result := Pi * FRadius * FRadius;
end;

Object Pascal classes support single inheritance plus interface implementation (similar in shape to Java or C#'s model) — interface declares a contract with no implementation, and a class implements it alongside its own inheritance chain. Result is the implicit variable a function assigns its return value to, rather than an explicit return statement.

Properties

type
  TPerson = class
  private
    FName: string;
    procedure SetName(const Value: string);
  public
    property Name: string read FName write SetName;
  end;

procedure TPerson.SetName(const Value: string);
begin
  FName := Trim(Value);   // validation/transformation on write
end;

A property exposes a field-like syntax (person.Name := 'Ada') backed by getter/setter methods — letting validation, computation, or change notification run transparently on read/write without the caller needing to know it isn't a plain field.

The VCL/FireMonkey component model

Delphi's defining feature is its visual, drag-and-drop component model: UI elements (buttons, forms, data-bound controls) are components with published properties and events, editable in a visual designer and persisted to a .dfm/.fmx form file alongside the Object Pascal source — VCL targets Windows specifically, FireMonkey (FMX) targets multiple platforms (Windows, macOS, iOS, Android, Linux) from one codebase.

Common pitfalls

• Manual memory management for non-reference-counted classes — plain TObject-descended classes (unlike TInterfacedObject/interfaces, which are reference-counted) must be explicitly Freed; forgetting to leaks memory, the same fundamental discipline as C Programming's malloc/free.
• Mixing VCL and FireMonkey assumptions — code and components written for one aren't directly portable to the other despite superficially similar names; check which framework a given unit targets.
• Confusing =/:= — the same Object-Pascal-family distinction as in Pascal Programming: := for assignment, = for comparison.
• Ignoring try/finally for resource cleanup — without wrapping manual allocation in try ... finally ... Free; end, an exception between allocation and cleanup leaks the resource.

Learn more

• Embarcadero Delphi documentation
• Pascal Programming for the foundational language Object Pascal extends.