C++: Inheritance, linking (and dynamic libraries)

Question

I have the following MWE:

// classes.hpp
#pragma once

struct Base {
    virtual ~Base() = default;
    virtual void tell() = 0;
};

struct Derived : Base {
    Derived() = default;
    ~Derived() = default;
    void tell() override;
};

then,

// classes.cpp
#include <iostream>
#include "classes.hpp"

extern "C" {
Derived* instantiate()
{
    return new Derived{};
}

void deinstantiate(Derived* ptr)
{
    delete ptr;
}
}

void Derived::tell()
{
    std::cout << "Hello!" << std::endl;
}

which I compile on a linux machine as a shared library g++ -fPIC -shared -o classes.so classes.cpp and the main program:

// main.cpp
#include <iostream>
#include <dlfcn.h>

#include "classes.hpp"


int main()
{
    using inst_t = Derived*(*)(void);
    using deinst_t = void(*)(Derived*);

    void* handle = dlopen("./classes.so", RTLD_NOW);
    inst_t inst = reinterpret_cast<inst_t>(dlsym(handle, "instantiate"));
    deinst_t deinst = reinterpret_cast<deinst_t>(dlsym(handle, "deinstantiate"));
    // some tests here...

    Derived* instance = inst();
    instance->tell();
    deinst(instance);
    dlclose(handle);

    return EXIT_SUCCESS;
}

which I compile with g++ -ldl -o main main.cpp

It works well. However If I remove the inheritance of Derived from Base, i.e. if I substitute above calsses.hpp file with

// new classes.hpp
#pragma once

struct Derived {
    Derived() = default;
    ~Derived() = default;
    void tell();
};

I got a link error

/usr/bin/ld: /tmp/ccLuFAZr.o: in function `main':
main.cpp:(.text+0x66): undefined reference to `Derived::tell()'
collect2: error: ld returned 1 exit status

which I is sensible since it cannot find the symbol from classes.cpp. However it is not completely clear to me how subclassing Derived from Base can solve this problem since Base is not used at all. (I think it should be a matter o vtable creation). So

1. How is it possible?
2. Is there any other way to solve the problem without using (dummy) inheritance?

EDIT

Of course the cause of a linking error can be found in a question whose answers list all the possible cause of linking errors. However, in my opinion this question is by far more generic than mine which deals with a particular situation (the use of dynamic shared library, ecc).

EDIT 2

Non-question: Why, without inheritance, do I get link error? (I know)

Question: Why using inheritance do I not get link error?

Answer 1

void Derived::tell() is defined in libclasses.so.

With #include "classes.hpp", the class is defined in main.cc, instance->tell(); is an attempt to call void Derived::tell() defined in main, and the member function is not defined there for sure.

You might load the symbol void Derived::tell() from libclasses.so and call it like below:

// Derived::tell() is the _ZN7Derived4tellEv mangled name.
void (Derived::*tell)() = reinterpret_cast<void(Derived::*)()>(dlsym(handle, "_ZN7Derived4tellEv"));
(instance->*tell)();

Unfortunately the cast a pointer to a member function pointer is not allowed. You should wrap a member call into a regular function in libclasses.so:

extern "C" void call_tell(Derived* d) {
  d->tell();
}

auto* tell = reinterpret_cast<void(*)(Derived* d)>(dlsym(handle, "call_tell"));
tell(instance);

Answer 2

You need to put the "void tell();" from Derived as "virtual void tell();". The derived should be:

struct Derived {
  Derived() = default;
  ~Derived() = default;
  virtual void tell();

};