Linux下c++通过动态链接库调用类

我的程序是一个类，在网上找了半天，都是c的例子，c++的类封装成静态库倒容易，可是如何封装成动态库，在其它程序中调用呢？
Linux下的动态链接库叫so，即Shared Object，共享对象。一些函数就不说了，网上多的是。把我遇到的问题写下来吧

提示错误 undefined reference to `dlopen'
编译时增加“-ldl”选项即可解决。

提示错误 cannot open shared object file: No such file or directory
将当前目录的绝对路径添加到LD_LIBRARY_PATH即可
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/home/zhj/test_so/

提示错误 undefined reference to
检查了一下，原来在makefile里少包含了一个文件。
这一类错误一般都是缺少相应的类文件所致。

例子来源：
http://www.linuxsir.org/bbs/printthread.php?t=266890

这个扩展名为hpp的文件，我分解了一下，下面就上代码吧

polygon.h
//----------
//polygon.h:
//----------
#ifndef POLYGON_H
#define POLYGON_H
#include <stdio.h>

class polygon {
protected:
double side_length_;

public:
polygon();
virtual ~polygon();

void set_side_length(double side_length);

virtual double area() const;
};

// the types of the class factories
typedef polygon* create_t();
typedef void destroy_t(polygon*);

#endif

polygon.cpp
//----------
//polygon.cpp:
//----------
#include "polygon.h"

polygon::polygon()
{
//set_side_length(0);
}

polygon::~polygon()
{
}

void polygon::set_side_length(double side_length)
{
printf("polygon side_length: %f\n\n",side_length);
side_length_ = side_length;
printf("polygon side_length_: %f\n\n",side_length_);
}

double polygon::area() const
{
return 0;
}

triangle.cpp
//----------
//triangle.cpp:
//----------
#include "polygon.h"
#include <cmath>

class triangle : public polygon {
public:
virtual double area() const {
printf("triangle side_length_: %f\n\n",side_length_);
return side_length_ * side_length_ * sqrt(3) / 2;
}
};

// the class factories
extern "C" polygon* create() {
return new triangle;
}

extern "C" void destroy(polygon* p) {
delete p;
}

main.cpp
//----------
//main.cpp:
//----------
#include "polygon.h"
#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;
}

// reset errors
dlerror();

// load the symbols
create_t* create_triangle = (create_t*) dlsym(triangle, "create");
const char* dlsym_error = dlerror();
if (dlsym_error) {
cerr << "Cannot load symbol create: " << dlsym_error << '\n';
return 1;
}

destroy_t* destroy_triangle = (destroy_t*) dlsym(triangle, "destroy");
dlsym_error = dlerror();
if (dlsym_error) {
cerr << "Cannot load symbol destroy: " << dlsym_error << '\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);
}

makefile

objects = main.o polygon.o
main: $(objects)
g++ -g -rdynamic -o $@ $(objects) -ldl
main.o: main.cpp
g++ -g -c main.cpp
polygon.o: polygon.cpp polygon.h
g++ -g -c polygon.cpp

.PHONY : clean
clean :
-rm main $(objects)

用下面的命令编译，生成libtriangle.so
g++ -g -fpic -shared -o libtriangle.so triangle.cpp polygon.cpp

然后make一下，运行main试试吧！