C++ access callback data

Question

I am attempting to create a wrapper around class functions. The purpose of my wrapper is to test input, output, and enforce order of operations with various calls throughout my program. I am trying to not make any changes to the callee class. Attached is an example of what I am trying to achieve, but unable to figure out.

Main.cpp

#include "func_warpper.h"
#include "func.h"

int main()
{
    func_wrapper fw
    fun func;
    int origValue = 5;
    fw.caller([&](int origValue) { func.f(origValue); }, origValue);
    int output = func.getResult().number;
    std::cout << " value outputed by function 2 : " << output << std::endl;
    // output
    // note that above line does give me the result I am looking for
    // however, I want to be able to get this inside the function of caller
    return 0;
}

func.h .... I want this to be unmodified

#ifndef FUN_H
#define FUN_H

class fun 
{
    public:
        struct result
        {
            int number;
        };
        fun();
        ~fun();
        void f(int value);
        struct result getResult(){return this->testResult;};
        private:
            struct result testResult;
};
#endif

func.cpp .... I want this to be unmodified

#include "func.h"


fun::fun(){
    this->testResult.number = 0;
    return;
}
fun::~fun(){
    return;
}
void fun::f(int value){
    int updateValue = value * 5;
    this->testResult.number = updateValue;
}

func_wrapper.h .... I can modify this until the cows come home, please go ham with recommended changes :)

class func_wrapper
{
public:
    struct new_result
    {
        int new_number;
    };
    func_wrapper();
    ~func_wrapper();

    void caller(std::function<void(int)> clb, int val);
    struct new_result getNewResult() { return this->new_testResult; };
    private:
        struct new_result new_testResult;
};

#endif

func_wrapper.cpp .... same as above, I can modify this until the cows come home, please go ham with recommended changes :)

#include "func_wrapper.h"


func_wrapper::func_wrapper()
{
    //ctor
    this->new_testResult.new_number = 0;
    return;
}

func_wrapper::~func_wrapper()
{
    //dtor
}

void func_wrapper::caller(std::function<void(int)> clb, int val)
{
    std::cout << " value entered into function: " << val << std::endl;
    // clb(val); seems to call the function but does not store locally anything
    clb(val);
    clb;
    // clb; seems to store all the information locally however I seem unable to 
    // to reach the infromation: clb -> [functor] -> func -> testResult -> number
    // would like ...
    int output = clb ???  // the result of what gets filled from number struct
    // if I attempt to #include func.h
    // func func;
    // func.getResult().number; locally the answer is zero with or without delay
}

Through several days of searching, I have not found anything that can help with this problem, to include similar enough questions on stack overflow. Any help would be greatly appreciated, thank you.

Answer 1

@JohnFilleau had mentioned to pass the class object instead of the function from within the class. The following is the solution based on example code that he provided, and I modified to work with the example. I realize the question is confusing but would like to thank both JohnFilleau and Taekahn for the discussion.

In main.cpp

int main()
{
    func_wrapper fw;
    fun func;
    int origValue = 5;
    fw.caller2(func, origValue);
return 0:
}

func_wrapper::caller2

    void func_wrapper::caller2(fun& fun, int val)
{
    std::cout << " value entered into function: " << val << std::endl;
    fun.f(val);
    int output = fun.getResult().number;
    std::cout << " did this work: " << output << std::endl;

}

In the header I had to add

#include "func.h"

with the change to the header as follows

void caller2(fun& fun, int val);