我可以使用类型声明函数吗

Question

I have a program which can use any combination of five DLLs to do some work. All the DLLs have the same interface, same exported functions and procedures, and only differ in what they actually do.

In order to make things easier, sanity-check-wise, I'd like to define the functions only once and then share the definition unit between anything. The problem then would be to define a function of a given type.

Imagine:

type
  TMyFunc = function(inP : Integer): outP : Boolean;

Can I now define a function called, say, doit , and force its definition to be of type TMyFunc , without redefining everything?

I can't just type:

function doit: TMyFunc;

Is there any way to do this?

It would really make defining everything easier, more secure and so on.

Right now, everything is defined several times, and making a necessary interface change at one point means that I need to remember to go everywhere and duplicate the change (and duplication is bad). If I could do the declaration in one place only then if I forgot to make a change somewhere compilation would fail, which would be great.

Answer 1

Is there any way to do this?

The answer is, quite simply, no you cannot do this. When you declare a function, you must include its complete argument list, return type, etc.

Answer 2

THIS is wrong:

type
  TMyFunc = function(inP : Integer): outP : Boolean;

a function cannot have two return types (both outP and Boolean) !

INSTEAD:

type
    outP = Longint; // or whatever you want outP to be.

    TMyFunc1 = function (inP : Integer): outP;
    TMyFunc2 = function (inP : Integer): Boolean;

you can now define a function variable called, say, doit1, and force it's definition to be of type TMyFunc1. likewise: doit2 is type TMyFunc2.

var
    doit1 : TMyFunc1;
    doit2 : TMyFunc2;

if you have functions:

const
    DllName1 = 'MyFirstDLL.DLL';
    DllName2 = 'MySecondDLL.DLL';

function TestFunc1(inP : Integer): outP; external DllName1;

function TestFunc2(inP : Integer): Boolean; external DllName2;

then you can assign these functions to the placeholders doit1, doit2:

procedure AssignPlaceholders;
begin
    doit1 := TestFunc1;
    doit2 := TestFunc2;
end;

NOTE: You must declare the placeholder types (TMyFunc1, TMyFunc2) EXACTLY the same as the real functions, including specifiers such as stdcall, cdecl etc.

After you call AssignPlaceholders, you can call doit1 and/or doit2 just the same as any other function.