So, following up from the last quiztime which was about the importance of explicit linking, another case from the wonderful world of shared libraries.
This time we study the implications of dlopen, its parameters and C++. Consider the program and module below. If you run that, it will crash somewhat obscurely in libstdc++. Why?

#0  0x00007f687b2eb126 in std::ostream::sentry::sentry(std::ostream&) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#1  0x00007f687b2eb889 in std::basic_ostream >& std::__ostream_insert >(std::basic_ostream >&, char const*, long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#2  0x00007f687b2ebd57 in std::basic_ostream >& std::operator< <  >(std::basic_ostream >&, char const*) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3  0x00007f687a6f01a0 in Impl::print (this=0x558a9bc1f2e0, str="Hello") at module.cc:24
#4  0x0000558a99de3d1b in main (argc=1, argv=0x7ffca28cf538) at main.cc:31

#ifndef INTERFACE_H
#define INTERFACE_H
#include 
class IInterface {
    public:
        virtual ~IInterface() {};
        virtual void print(const std::string& str) = 0;
};
#endif // INTERFACE_H

#include "interface.h"
#include 
class Impl : public IInterface {
    public:
        Impl();
        virtual ~Impl();
        void print(const std::string& str);
};
extern "C" {
void *entry_point(void)
{
    return new Impl;
}
};
Impl::Impl() {};
Impl::~Impl() {};
void Impl::print(const std::string& str)
{
    std::cerr < <"Some text to print\n";
    std::cerr << "Got passed this: " << str << "\n";
    std::cerr << "=====\n";
}

#include 
#include 
#include "interface.h"
extern "C" {
typedef void *(*EntryFunction)(void);
};
int main(int argc, char *argv[])
{
    IInterface *iface;
    EntryFunction f;
    void *lib = dlopen("./module.so", RTLD_NOW | RTLD_DEEPBIND);
    if (lib == nullptr) {
        std::cerr < < dlerror () << "\n";
        return 1;
    }
    f = (EntryFunction) dlsym (lib, "entry_point");
    if (f == nullptr) {
        std::cerr << dlerror () << "\n";
        return 1;
    }
    iface = reinterpret_cast(f());
    while (true) {
        iface->print ("Hello");
    }
}

.PHONY: all clean
all: main module.so
clean:
	rm -f main
	rm -f module.so
main: main.cc
	g++ -g -o $@ $< -ldl
module.so: module.cc
	g++ -g -o $@ -shared $< -fPIC

Say you have a shared library with versioned symbols that has the function init(int *param) defined in two versions, where new_init is made the default for init (that’s what the  @@ means):

#include "shared1.h"
void new_init (int *param)
{
    *param = 0;
}
void old_init (int *param)
{
    *param = -1;
}
__asm__(".symver new_init,init@@VERS_2");
__asm__(".symver old_init,init@VERS_1");

Say you have a second shared library that provides a function do_something() uses that init from the first shared library:

#include "shared1.h"
int do_something ()
{
    int var;
    init (&var);
    return var;
}

And you have an app that just prints the result:

#include "shared2.h"
#include 
int main(int argc, char *argv[])
{
    printf ("Result: %d\n", do_something ());
    return 0;
}

And all of this is cobbled together with the following Makefile:

all: app
libshared.so: shared1.c shared1.h
    cc -o $@ $< -shared -fPIC -Wl,--version-script=version.script
libmoreshared.so: libshared.so shared2.c
    cc -o $@ shared2.c -shared -fPIC
app: libmoreshared.so app.c
    cc -o $@ -L. app.c -lmoreshared -lshared

What's the output of

LD_LIBRARY_PATH=. ./app

Update: Try to answer it without compiling and running it at first.
Update2: I forgot the version.script, sorry.

VERS_1 {
    local:
        old_init;
};
VERS_2 {
    local:
        new_init;
} VERS_1;