c++ call object method using pointer - c++

Question

I created a shared lib, in which I defined a class Foo. In class Foo, there is a function named sayHi(). My question is once I created a Foo object foo, how can I call sayHi() of it. I put my code here.
mylib.h
#ifndef FUNTEST_MYLIB_H
#define FUNTEST_MYLIB_H
class Foo {
public:
int id = 0;
void sayHi();
};
#endif //FUNTEST_MYLIB_H
mylib.cpp
#include <iostream>
#include "mylib.h"
using namespace std;
void Foo::sayHi()
{
cout << "Implemented by lib" << endl;
cout << "id is: " << id << endl;
}
Foo* create()
{
return new Foo();
}
then I use the following commands compile the shared lib:
g++ -c -std=gnu++11 -fPIC mylib.cpp
g++ -shared -fPIC -o mylib.so mylib.o
At the client side, I write two files:
mylib2.cpp
#include <iostream>
#include "mylib.h"
using namespace std;
void Foo::sayHi() {
cout << "Implemented by caller" << endl;
cout << "id is: " << id << endl;
}
main.cpp
#include <iostream>
#include <dlfcn.h>
#include "mylib.h"
using namespace std;
Foo* (*create)();
void (*sayHi)();
int main() {
void* lib = dlopen("./mylib.so", RTLD_LAZY);
create = (Foo* (*)())dlsym(lib, "_Z6createv");
sayHi = (void (*)())dlsym(lib, "_ZN3Foo5sayHiEv");
Foo* foo = create();
sayHi();
foo->sayHi();
foo->id = 100;
cout << "Set id to " << foo->id << endl;
/*
*
* how can I make the follow statement 'sayHi();'
* output the following content:
* Implemented by lib
* id is: 100
*/
sayHi(); // line 29, FIXME
foo->sayHi();
return 0;
}
compile the client with following command:
g++ -std=gnu++11 main.cpp mylib.h mylib2.cpp -ldl
finally run the client:
./a.out
the output is:
Implemented by lib
id is: 0
Implemented by caller
id is: 0
Set id to 100
Implemented by lib
id is: -1407102376 // FIXME
Implemented by caller
id is: 100
how can I make the statement 'sayHi();' at line 29 output the following content:
Implemented by lib
id is: 100
I know if i call the sayHi() function directly, without the foo object, the output will be wrong. But how can I fix it?

Answer 1

The declaration of the sayHi global variable in main.cpp is wrong. You're declaring it as a function pointer. However, since it points to a member function, it needs to be a member function pointer.
void (Foo::*sayHi)();
Then when you call it you need a Foo object:
(foo->*sayHi)();
Alternatively, if you make the sayHi method virtual, you shouldn't need to load the symbol for it at all, since that will be effectively handled by the vtable.