g++ main.cpp -o xmain  -ldl

main.cpp:
#include <dlfcn.h>
#include <iostream>
int
main()
{
    std::cout << "load: " << std::endl;
    void *foo = dlopen("./libpluginTest.so", RTLD_NOW);
    std::cout << "load: library loaded "<< foo << std::endl;
    if (dlclose(foo) < 0) {
        std::cout << "load: library close failed " << std::endl;
        return 1;
    }
    std::cout << "load: library closed " << std::endl;
    return 0;
}

g++ -I/path/to/boost/includes -fPIC -shared -rdynamic test_plugin.cpp -o libpluginTest.so -L/path/to/boost/libs -lrt

test_plugin.cpp:
#include <iostream>
#include <boost/flyweight.hpp>

typedef boost::flyweight< std::string > SymbolName_t;

SymbolName_t getEmptyStringSymbol() {
            SymbolName_t empty("");
            return empty;
        }



void __attribute__ ((constructor)) library_init() 
{ 
  std::cout << "Library constructor: " << std::endl;
} 
void __attribute__ ((destructor)) library_fini() 
{ 
  std::cout << "Library destructor:" << std::endl;
}


tested on:
g++-4.6 (Ubuntu/Linaro 4.6.3-10ubuntu1) 4.6.3 20120918 (prerelease)
g++-4.7 (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2