Callback2<P1, P2, P2, P3, P4> when_callback;

OnCallback1(P1 p1, P2 p2, P3 p3, P4 p4, T1 t1, T2 t2, T3 t3){
    ...
}

Attach(){when_callback = THISBACK3(OnCallback1, val1, val2, val3)};

#include <Core/Core.h>

using namespace Upp;

struct Foo {
	int x;

	void Action()                { Cout() << "Action: " << x << '\n'; }
	void ActionWithParam(int y)  { Cout() << "ActionWithParam: " << x + y << '\n'; }

	Callback WhenDo;
	Callback1<int> WhenDo1;
	Callback4<int, String, String, String> WhenDo4;
	void Do()                    { WhenDo(); WhenDo1(2);WhenDo4(4, "2 par", "3 par", "4 par");}

	Foo(int x = 0) : x(x) {}
};

void Fn()
{
	Cout() << "Fn!" << '\n';
}

struct Bar {
	Foo foo;

	void Action() { Cout() << "foo's Do called\n"; }
	void ActionMaxPar(int p1, String p2, String p3, String p4) { Cout() << "foo's Do called with max par par 4 = "<<p4<<"\n"; }
	void ActionMaxParAndArg(int p1, String p2, String p3, String p4, int a1, String a2, String a3, String a4) { Cout() << "foo's Do called with max par and arg arg4="<<a4<<"\n"; }

	typedef Bar CLASSNAME;

	Bar() { foo.WhenDo = THISBACK(Action);
		String v_arg1("1 arg");
		String v_arg2("2 arg");
		String v_arg3("3 arg");
		String v_arg4("4 arg");
		int v_arg1_i = 1;
		
		foo.WhenDo1 = THISBACK3(ActionMaxPar, v_arg1, v_arg2, v_arg3);
		foo.WhenDo4 = THISBACK(ActionMaxPar);
		foo.WhenDo4 << THISBACK4(ActionMaxParAndArg, v_arg1_i, v_arg2, v_arg3, v_arg4);
	}
};

struct Safe : Pte<Safe> {
	void Action() { Cout() << "safe action!\n"; }
};

CONSOLE_APP_MAIN
{
	Foo a(10);
	Callback cb1 = callback(&a, &Foo::Action);
	Callback cb2 = callback(Fn);
	Callback1<int> cb3 = callback(&a, &Foo::ActionWithParam);
	Callback cb4 = callback1(&a, &Foo::ActionWithParam, 30);

	cb1();
	cb2();
	cb3(10);
	cb4();

	Cout() << "---------\n";
	cb4 << cb2;
	cb4();

	Cout() << "---------\n";
	Bar b;
	b.foo.Do();

	Cout() << "---------\n";
	{
		Safe f;
		cb4 = pteback(&f, &Safe::Action);
		Cout() << "callback valid: " << (bool)cb4 << '\n';
		cb4();
	}
	Cout() << "callback valid: " << (bool)cb4 << '\n';
	cb4();
}