Passing parameters to function pointer

Question

I am trying to pass parameters to a function pointer being passed as a parameter.

Code:

void Test(wchar_t* a, wchar_t* b)
{
    // ...
}

void Test2(void(*Func)(wchar_t*, wchar_t*))
{
    // ...
}

int main()
{
    Test2(Test(L"Hello", L"Testing"));
    return 0;
}

I am getting this error:

argument of type "void" is incompatible with parameter of type "void (*)(wchar_t *, wchar_t *)"

How do I fix this to accomplish what I'm trying to achieve?

Edit: Sorry for not being clear. What I'm actually trying to accomplish is inject a function into a child process and pass two parameters (wchar_t*, wchar_t*) so I can use them. But the main function can either be void or int argc, char** argv. So I accomplished what I'm trying to achieve by simply using global variables

What exactly is it that you're trying to achieve? Your code is, as you've noticed, not valid C so it's not a useful demonstration of your intent. — Greg Hewgill
– Greg Hewgill, Commented Jul 5, 2016 at 21:27
πάντα ῥεῖ's answer is what I'm trying to achieve but I want a different way to do it like in C++ with templates — user1304765
– user1304765, Commented Jul 5, 2016 at 21:32

πάντα ῥεῖ · Accepted Answer · 2016-07-05 21:34:11Z

7

You probably want to have something like

void Test2(void(*Func)(wchar_t*, wchar_t*),wchar_t* x, wchar_t* y)
{
    (*Func)(x,y);
}

int main()
{
    Test2(Test,L"Hello", L"Testing");
    return 0;
}

instead.

As for your comment

How do i do this in C++ with templates?

I could think of

template<typename Param>
void Test2(void(*Func)(Param, Param), Param x, Param y) {
    (*Func)(x,y);
}

void Test(wchar_t* a, wchar_t* b);

int main() {
    Test2(Test,L"Hello", L"Testing");
    return 0;
}

This should just work fine.

edited Jul 5, 2016 at 21:34

answered Jul 5, 2016 at 21:25

πάντα ῥεῖ

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12 Comments

user1304765 Over a year ago

your answer is what I'm trying to achieve. How do i do this in C++ with templates?

πάντα ῥεῖ Over a year ago

@user1304765 Updated my answer. Still not sure what you actually want.

user1304765 Over a year ago

Yea its actually a bit more complicated. I have one function that injects code (a function) into a child process so it has this parameter: int(Main)(wchar_t, wchar_t*). The Main function that I'm passing to it calls a function that takes two wchar_t*.

πάντα ῥεῖ Over a year ago

@user1304765 Well, I don't see so much need for a template function then. I hope your child process is a thread actually though. You cannot change the signature of main() and pass arbitrary parameter types.

user4581301 Over a year ago

@user1304765 Calling main inside your program is not a good idea. In fact it's forbidden by the standard (See section 3.6.1). If this is a mis-communication, groovy, but if not I recommend a re-think.

|

sps · Accepted Answer · 2016-07-05 22:00:44Z

There are more than one way to fix tihs issue, however, let me just try to show why this error is occuring.

Every function has a type of value associated with it. This means, that every function evaluates to a value of some type. This is indicated by its return value.

For example:

int foo(/*whatever*/);

evaluates to an int. So foo(/*whatever*/) can be used anywhere an int is expected. For example like int a = b + foo(/*whatever*/).

Simlarly float bar(/*whatever*/); evaluates to a float, hence bar(/*whatever*/) can be used anywhere a float is expected. For example like float a = b + bar(/*whatever*/).

A function that returns void like void foobar(/*whatever*/) however, evaluates to void and cannot be used where a value of some type (say int, float, etc) is expected.

Now coming to code. This line in your main function has the issue:

int main()
{
    Test2(Test(L"Hello", L"Testing")); /* Issue here */
    return 0;
}

Here you are passing Test(L"Hello", L"Testing") as the argument to Test2. Now remember, that Test(/*whatever*/), evaluates to a void because Test returns a void.

So what you are doing in that line is something like

Test2(/*something that evaluates to a void*/);

However, Test2 expectes a void (*)(wchar_t*, wchar_t*), which is a pointer to a function that returns void, which is different from void.

So what is happening, is that the compiler is seeing that you are passing a void in a place where a void (*) (wchar_t*, wchar_t*) is expected, so it is correctly indicating that error.

There can be different ways to solve this issue which are mentioned in other answers.

skypjack · Accepted Answer · 2016-07-05 22:05:43Z

0

Do I need to use C++ templates?

Of course, you can do that using C++ templates as it follows:

#include<utility>

// ...

template<typename F, typename... A>
void Test2(F &&f, A&&... a)
{
    std::forward<F>(f)(std::forward<A>(a)...);
    // ...
}

// ...

Test2(Test, L"Hello", L"Testing");

But you don't need them to do what you are trying to do.
@πάνταῥεῖ has already explained why in its answer.

answered Jul 5, 2016 at 22:05

skypjack

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