Accessing the members of a struct referenced by a void pointer

Question

I've got a function that takes a void pointer as a parameter. I want to pass this function a pointer to a struct, and then access the values of that struct within the function.

//the struct
struct s{
    int val;
};

//the function tries to access the object
int callback(void* p_obj)
{    
    //try creating a new struct based on p_obj 
    s2 = *(struct s*)p_obj;
    std::cout << "VALUE IN CALLBACK: ";
    std::cout << s2.val << std::endl; //prints a big-ass int
    return 0;
}

//main calls the function
int main()
{
    s s1;
    s1.val = 42;
    void* p1 = &s;

    //show some output
    std::cout << "s1.val: ";
    std:cout << s1.val << std::endl; //prints 42

    //std::cout << "p1->val: "; 
    //std:cout << *(struct s*)p1->val << std::endl; //does not compile

    s p2 = *(struct s*)p1;
    std::cout << "p2.val: ";
    std:cout << p2.val << std::endl; //prints 42

    //call the function
    callback(&p1);
    return 0;
}

I would expect the output in the callback function to be

VALUE IN CALLBACK: 42
VALUE IN CALLBACK: 42

but, instead, I think it's printing a memory address

VALUE IN CALLBACK:1989685088
VALUE IN CALLBACK:1989685088 

Trying to access the members of a void pointer directly results in an error.

int callback(void* p_obj)
{
    std::cout << "VALUE IN CALLBACK: ";
    std::cout << (struct s*)p_obj->val << std::endl;
}

error: 'void*' is not a pointer-to-object type

Why is this? How can I access the members of a struct that a void* is pointing to?

EDIT: Fixed some typos in the writeup

Answer 1

You have two errors:

1. *(struct s)p_obj needs to be *(struct s*)p_obj , as p_obj is not a structure object.

2. Because of operator precedence the expression (struct s*)p_obj->val is actually equal to (struct s*)(p_obj->val) . Which means you try to dereference the void* pointer and cast the member val to struct s* .

   You should do ((struct s*)p_obj)->val to cast the pointer p_obj .

And more typos: *void p_obj is very much wrong, it should be void* p_obj . Please take care to copy-paste your minimal, complete, and reproducible example , not retype is as that might add extra errors not in your real code, which distracts from the actual errors and problems.