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How to Handle Delphi Anonymous Methods in C++

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This topic describes how to handle Delphi anonymous methods in C++ code.

Implementation of Delphi Anonymous Method Types

Under the cover, Delphi implements anonymous methods types (also known as method references) via an interface that implements an Invoke(...) method.

So a method that takes a method reference parameter in Delphi is exposed to C++ as a method that takes an interface. Here's an example:

interface
 
type
    TFilterPredicate = reference to function(const Path: string;
        const SearchRec: TSearchRec): Boolean;
// …
 
class function GetFiles(const Path: string; const Predicate: TFilterPredicate):
    TStringDynArray; overload; inline; static;

The following is the C++ header file that RAD Studio provides for the above:

typedef System::DelphiInterface<TFilterPredicate> _di_TFilterPredicate;
__interface TFilterPredicate  : public System::IInterface 
{
    virtual bool __fastcall Invoke(const System::UnicodeString Path,
        const System::Sysutils::TSearchRec &SearchRec) = 0 ;
};
// …
static System::TStringDynArray __fastcall GetFiles(const System::UnicodeString
    Path, const _di_TFilterPredicate Predicate)/* overload */;

As shown above, TFilterPredicate is exposed as an interface on the C++ side. C++ code that seeks to specify a function or member function as a method reference parameter has to wrap the latter behind an interface that exposes the Invoke() method.

Handling Anonymous Method Types in C++

Using a Lambda Expression

If you use a Clang-enhanced C++ compiler, you can use a lambda expression wherever a Delphi API expects a Delphi anonymous method. The DelphiInterface class converts automatically into a lambda expression.

For example:

#include <System.hpp>
#include <System.IOUtils.hpp>
#include <iostream>
 
int main()
{
  String ext(".cpp");                   
  TStringDynArray files = TDirectory::GetFiles(TDirectory::GetCurrentDirectory(),
                          [ext](const String Path, const System::Sysutils::TSearchRec &SearchRec) -> bool
                          {
                            return ExtractFileExt(SearchRec.Name) == ext;
                          });
  std::cout << "Found " << files.Length 
            << " files with ext: '" << AnsiString(ext).c_str() << "'\n";
  for (int i=0; i<files.Length; ++i)
    std::cout << AnsiString(files[i]).c_str() << std::endl;
}

Using a Functor (Function Object)

A C++ template can be used to encapsulate an interface that exposes an Invoke() method.

The following C++ code shows an example of a template that can be used to pass C++ methods or member functions as method references to Delphi:

#include <System.hpp>
 
enum _DummyType{};      // Parameter used as default
 
template <typename INTF, // Interface with Invoke
          typename F,    // Functor type
          typename R,    // Return type
          typename P1 = _DummyType,  // Param #1
          typename P2 = _DummyType,  // Param #2
          typename P3 = _DummyType,  // Param #3
          typename P4 = _DummyType,  // Param #4
          typename P5 = _DummyType>  // Param #5
class TMethodRef : public TCppInterfacedObject<INTF>
{
private:
  F callback;
public:
  TMethodRef(F _callback) : callback(_callback) {}
  INTFOBJECT_IMPL_IUNKNOWN(TInterfacedObject);
 
  R __fastcall Invoke(P1 p1) {
    return callback(p1);
  }
  R __fastcall Invoke(P1 p1, P2 p2) {
    return callback(p1, p2);
  }
  R __fastcall Invoke(P1 p1, P2 p2, P3 p3) {
    return callback(p1, p2, p3);
  }
  R __fastcall Invoke(P1 p1, P2 p2, P3 p3, P4 p4) {
    return callback(p1, p2, p3, p4);
  }
  R __fastcall Invoke(P1 p1, P2 p2, P3 p3, P4 p4, P5 p5) {
    return callback(p1, p2, p3, p4, p5);
  }
};

The following code shows how to use the template shown above to call GetFiles:

#include <System.IOUtils.hpp>
#include <iostream>
 
struct Filter {
  Filter(String _ext) : ext(_ext)
  {}
  bool operator()(const System::UnicodeString, const System::Sysutils::TSearchRec &SearchRec) {
    return ExtractFileExt(SearchRec.Name) == ext;
  }
  String ext;
};
 
int main()
{
  typedef TMethodRef<TDirectory::TFilterPredicate,
                     Filter,
                     bool,
                     const System::UnicodeString,
                     const System::Sysutils::TSearchRec&> MyMethRef;
  String ext(".cpp");                   
  TStringDynArray files = TDirectory::GetFiles(TDirectory::GetCurrentDirectory(),
                          TDirectory::_di_TFilterPredicate(new MyMethRef(Filter(ext))));
  std::cout << "Found " << files.Length 
            << " files with ext: '" << AnsiString(ext).c_str() << "'\n";
  for (int i=0; i<files.Length; ++i)
    std::cout << AnsiString(files[i]).c_str() << std::endl;
}

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