A value of type cannot be used to initialize an entity of type

Question

I have a function:

long __stdcall call_DLL(long n, byte s0, byte s1, long(__stdcall *CallBack)(long m, byte s0, byte s1)){
//trying to copy the address of CallBack to another pointer
long *x = &CallBack;
}

I am getting an error:

a value of type "long(__stdcall *CallBack)(long m, byte s0, byte 
s1)"cannot be used to initialize an entity of type "long *"

Anyone know how I can do this?

Why are you trying to copy a function pointer into a `long*`? — NathanOliver, Feb 12 '18 at 17:19
You're trying to assign a pointer of type long; with a pointer of a function. I can't do it captain! I don't have the Type! — UKMonkey, Feb 12 '18 at 17:19
You are trying to assign a pointer to a function, to a pointer to a `long`. These types are simply not compatible. Why would you want to do this? — Algirdas Preidžius, Feb 12 '18 at 17:20
this is a [xy problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). I suppose, the question you actually wanted to ask is "How do I store the pointer to the callback?", your current "problem" cannot be solved — 463035818_is_not_an_ai, Feb 12 '18 at 17:25
Anyone knows how you can do WHAT? If you want to assign `a pointer to a function` value to a variable which is `a pointer to long int` then the answer has already been given by your compiler: you _can't_. But if you want to do something else, please specify what you're trying to achieve. — CiaPan, Feb 12 '18 at 17:26

ash108 · Answer 1 · 2018-02-12T22:38:20.953

1

If you really want to save the callback to use it later, you can do:

long (* __stdcall x)(long, byte, byte) = CallBack;

or in C++ you can also use auto for brevity:

auto x = CallBack;

In either case, use it later like

long ret = x(n, s0, s1);

Otherwise, if you want to just call that CallBack, do something like

long x = CallBack(n, s0, s1);

edited Feb 12 '18 at 22:38

answered Feb 12 '18 at 17:26

ash108

1,763
1
17
21

if OP really needs to store the pointer then `auto` wont help, no? Or is something like `static auto x = &Callback;` allowed? – 463035818_is_not_an_ai Feb 12 '18 at 17:27
The question is tagged both C and C++ (it should be one or the other). If the OP is using a C compiler, `auto` won't work. – Keith Thompson Feb 12 '18 at 17:28

chux - Reinstate Monica · Answer 2 · 2018-02-12T18:44:49.217

Rather than initialize a function pointer with an incompatible long *, initialize it with a compatible type. @Algirdas Preidžius

pointer to function (long, byte, byte) returning long.

long __stdcall call_DLL(long n, byte s0, byte s1,
    long (__stdcall *CallBack)(long m, byte s0, byte s1)) {

  // long *x = &CallBack;
  long (__stdcall *x)(long, byte, byte) = CallBack;

  // sample usage of `x`
  return x(n, s0, s1);
}

A value of type cannot be used to initialize an entity of type

2 Answers2