How to Handle Delphi Anonymous Methods in C++
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This topic describes how to handle Delphi anonymous methods in C++ code.
Contents
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;
}