Calling a callback function in Delphi from a C++ DLL

Question

I have a C++ DLL that I wrote that has a single exposed function, that takes a function pointer (callback function) as a parameter.

#define DllExport   exte         


        

Answer 1

Declare a function pointer in Delphi by declaring a function type. For example, the function type for your callback could be defined like this:

type
  TGetProperty = function(object_type, object_instnace, property_identifier, device_identifier: UInt; value: PSingle): Boolean; cdecl;

Note the calling convention is cdecl because your C++ code specified no calling convention, and cdecl is the usual default calling convention for C++ compilers.

Then you can use that type to define the DLL function:

function RegisterCallbackGetProperty(GetProperty: TGetProperty): Boolean; cdecl; external 'dllname';

Replace 'dllname' with the name of your DLL.

To call the DLL function, you should first have a Delphi function with a signature that matches the callback type. For example:

function Callback(object_type, object_instnace, property_identifier, device_identifier: UInt; value: PSingle): Boolean cdecl;
begin
  Result := False;
end;

Then you can call the DLL function and pass the callback just as you would any other variable:

RegisterCallbackGetProperty(Callback);