C++ - Issue of using function pointer to member function - c++

Question

I've applied solutions based on some search made, but the problem still there. Thank you so much for the help.
error: must use '.*' or '->*' to call pointer-to-member function ...
source code:
#include <stdio.h>
class A
{
public:
struct data;
typedef int (A::*func_t)(data *);
typedef struct data
{
int i;
func_t func;
}
data;
data d;
void process()
{
d.func(&d);
}
A()
{
d.i = 999;
d.func = &A::print;
}
int print(data *d)
{
printf("%d\n", d->i);
return 0;
}
};
int main()
{
A *a = new A;
a->process();
return 0;
}

Answer 1

d.func(&d);
is not enough. func is a member-function-pointer which is pointing to a non-static member of A. So it can be invoked on an object of A. So you need to write this:
(this->*(d.func))(&d);
That would work as long as you write this inside A.
If you want to execute func from outside, say in main(), then the syntax is this:
A a;
(a.*(a.d.func))(&a.d);
That is an ugly syntax.

Answer 2

Your process function attempts to call d.func but it is a pointer to member function. A pointer to member function must be called on some object. Presumably you want the instance of A to be this, in which case your process function should look like:
void process()
{
(this->*(d.func))(&d);
}
Note the use of the ->* operator to call a member function when you have a pointer to it.

Answer 3

Other answers have already said you need to say (this->*d.func)(&d) to call a pointer-to-member function (because you need to provide the object that it's a member of)
Another option is to make the function a static function, which doesn't need special syntax to call. To do that, change the typedef like so:
typedef int (*func_t)(data *);
Then make the print function static:
static int print(data *d)
{
...
}
Now you can just call d.func(&d)

Answer 4

Unfortunately what you are trying to do will not be possible, the reason being that print is not a static member function. This means it expects an implicit first argument that is the this pointer.
I suggest you try using the std::function and std::bind function, something like this:
class A
{
struct data
{
std::function<void(const data&)> func;
int i;
};
data d;
public:
A()
{
d.func = std::bind(&A::print, *this);
d.i = 999;
}
void process()
{
d.func(d);
}
void print(const data& my_data)
{
std::cout << my_data.i << '\n';
}
};
Of course, since the print function now have a proper this pointer, you no longer need to pass the data structure to it:
class A
{
struct data
{
std::function<void()> func;
int i;
};
data d;
public:
A()
{
d.func = std::bind(&A::print, *this);
d.i = 999;
}
void process()
{
d.func();
}
void print()
{
std::cout << d.i << '\n';
}
};

Answer 5

Calling pointer the members require the class it is a member of to be the this param.
Try:
A a;
a.*(d.func)(&d);