delphi

Delphi / Object Pascal

Delphi (RAD Studio) and Free Pascal (Lazarus) keep Object Pascal alive. It is famous for Single EXE deployment and instant compilation.

When to Use

Rapid Application Development (RAD) for Windows Desktop: Building native Windows desktop applications with rich VCL graphical controls.
Legacy Enterprise Maintenance: Modernizing and maintaining established corporate desktop software suites.
Single-Binary Zero-Dependency Native Compilation: Producing fast, standalone native binaries that run without external runtime frameworks.
High-Performance Database Clients: Interfacing with enterprise SQL databases via native FireDAC data access components.

Quick Start

program HelloWorld;
{$APPTYPE CONSOLE}
uses
  System.SysUtils;

begin try Writeln('Hello from Delphi Object Pascal!'); except on E: Exception do Writeln(E.ClassName, ': ', E.Message); end; end.


## Core Concepts

### Object Pascal Strong Typing & Class Architecture

Clean, readable object-oriented architecture with explicit interfaces:

```pascal
unit OrderProcessor;

interface

type
  IOrderService = interface
    ['{8F3D1E1C-3A62-47CF-A98B-7D12A9E32049}']
    function ProcessOrder(const OrderId: Integer; const Amount: Double): Boolean;
  end;

  TOrderService = class(TInterfacedObject, IOrderService)
  public
    function ProcessOrder(const OrderId: Integer; const Amount: Double): Boolean;
  end;

implementation

function TOrderService.ProcessOrder(const OrderId: Integer; const Amount: Double): Boolean;
begin
  // Business logic execution
  Result := Amount > 0.0;
end;

end.

Visual Component Library (VCL) & FireMonkey (FMX)

VCL: Native Windows-only components wrapping direct Win32/Win64 APIs.
FireMonkey (FMX): Cross-platform GPU-accelerated UI framework targeting Windows, macOS, iOS, and Android.

FireDAC Unified Database Connectivity

High-performance data access layer:

var
  Query: TFDQuery;
begin
  Query := TFDQuery.Create(nil);
  try
    Query.Connection := FDConnection1;
    Query.SQL.Text := 'SELECT * FROM Customers WHERE Balance > :MinBalance';
    Query.ParamByName('MinBalance').AsCurrency := 1000.0;
    Query.Open;
    // Process records
  finally
    Query.Free;
  end;
end;

Common Patterns

Safe Memory Management with Try..Finally

Problem: Object memory leaks in non-ARC desktop platforms when exceptions occur.

Solution: Use explicit try..finally destruction blocks:

procedure ProcessCustomerList;
var
  List: TStringList;
begin
  List := TStringList.Create;
  try
    List.Add('Customer 1');
    List.Add('Customer 2');
    List.SaveToFile('customers.txt');
  finally
    List.Free;
  end;
end;

Best Practices

Do:

Always Wrap Object Allocations in try...finally: Guarantee that .Free executes to prevent memory leaks in non-ARC Windows runtimes.
Use FireDAC for All Database Operations: Standardize on FireDAC; deprecate legacy BDE and dbExpress components.
Target 64-Bit Windows (Win64): Ensure new applications compile for 64-bit to utilize modern memory spaces and system libraries.
Use Parameterized Queries: Always use ParamByName() to prevent SQL injection vulnerabilities.

Don't:

Ignore compiler hints and warnings: Treat Delphi compiler warnings with priority; they frequently identify uninitialized variables.
Use global variables in units: Keep state encapsulated inside classes and records.
Block the UI thread with long database queries: Execute heavy queries asynchronously using TTask.Run from the Parallel Programming Library.

Troubleshooting

Error	Cause	Solution
`Access Violation at address ... (EAccessViolation)`	Attempting to access an uninstantiated or freed object pointer.	Verify object was created with `.Create` before calling methods.
`[dcc32 Fatal Error] F2084 Internal Error`	Compiler cache corruption or circular unit dependency.	Clean build directory and resolve circular references in `uses` clauses.
`Memory Leak Detected in ReportMemoryLeaksOnShutdown`	Object created without matching `.Free` call.	Ensure every object creation is guarded with a `try..finally Free; end;` block.

References

Free Pascal