// polygon.hpp:#ifndef POLYGON_HPP#define POLYGON_HPPclass polygon {protected:    double side_length_;public:    polygon()        : side_length_(0) {}    void set_side_length(double side_length) {        side_length_ = side_length;    }    virtual double area() const { }};// the types of the class factoriestypedef polygon* create_t();typedef void destroy_t(polygon*);#endif

//main.cc:#include "polygon.hpp"#include <iostream>#include <dlfcn.h>int main() {    using std::cout;    using std::cerr;    // load the triangle library    void* triangle = dlopen("./triangle.so", RTLD_LAZY);    if (!triangle) {        cerr << "Cannot load library: " << dlerror() << '\n';        return 1;    }    // load the symbols    create_t* create_triangle = (create_t*) dlsym(triangle, "create");    destroy_t* destroy_triangle = (destroy_t*) dlsym(triangle, "destroy");    if (!create_triangle || !destroy_triangle) {        cerr << "Cannot load symbols: " << dlerror() << '\n';        return 1;    }    // create an instance of the class    polygon* poly = create_triangle();    // use the class    poly->set_side_length(7);        cout << "The area is: " << poly->area() << '\n';    // destroy the class    destroy_triangle(poly);    // unload the triangle library    dlclose(triangle);}

// triangle.cc:#include "polygon.hpp"#include <cmath>class triangle : public polygon {public:    virtual double area() const {        return side_length_ * side_length_ * sqrt(3) / 2;    }};// the class factoriesextern "C" polygon* create() {    return new triangle;}extern "C" void destroy(polygon* p) {    delete p;}

# Compiler/Linker/dynamic linkerCC = g++LD = g++# flags to compile object files that can be used in a dynamic libraryCFLAGS = -fPIC# on some platforms, use '-fpic' instead.# Flags to create a dynamic library.DYNLINKFLAGS = -fPIC -shared -nostdlib -rdynamic# on some platforms, use '-G' instead.# files removalRM = /bin/rm -f# program's object filesPROG_OBJS = main.o# program's executablePROG = prog# libraries to link with the program during compile time.LIBS = -ldl# shared library filesLIB_FILES = triangle.so# shared libraries object filesLIB_OBJS = triangle.oprog: prog.o	g++ $(CFLAGS) $(LIBS) -o prog main.o	prog.o: main.cc	g++ $(CFLAGS) $(LIBS) -c main.cc# top-level ruleall: $(LIB_FILES) $(PROG)	$(PROG): $(PROG_OBJS)	$(LD) $(LDFLAGS) $(PROG_OBJS) $(LIBS) -o $(PROG)# compile C source files into object files.%.o: %.cc	$(CC) $(CFLAGS) -c $<# create our librarystriangle.so: triangle.o	$(LD) $(DYNLINKFLAGS) $< -o $@# clean everythingclean:	$(RM) $(PROG_OBJS) $(PROG) $(LIB_OBJS) $(LIB_FILES)# clean the program and its object files, don't touch the library.cleanprog:	$(RM) $(PROG_OBJS) $(PROG)# clean the library and its object files onlycleanlib:	$(RM) $(LIB_OBJS) $(LIB_FILES)# clean the library's object files onlycleanlibobjs:	$(RM) $(LIB_OBJS)