How to use dynamic loading of shared libraries in C + + __c++

First, take a look at the test procedures

1. Library section

Hello.h

#ifndef android_hello_h
#define ANDROID_HELLO_H
class hello_base
{public
  :
    virtual ~hello_base () {};
    virtual int setctl (int ctl) = 0;
    virtual int getctl (int ctl) = 0;
};

Class Hello:public Virtual hello_base
{public
  :
    virtual ~hello () {};
    Static hello* Instance ();
    int setctl (int ctl);
    int getctl (int ctl);

  Private:
    static hello* _instance;
    int myctl;
};

extern "C" hello_base* Creathello ();
typedef hello_base* Creathelloclass ();

#endif

Hello.cpp

#include "hello.h"
#include <stdio.h>
extern "C" hello_base* Creathello () {return
  hello::instance ( );
}



hello* hello::_instance = 0;

hello* hello::instance ()
{
  printf ("TK------>>>>>>hello::instance---------\ n");
  if (_instance = = 0) {
    _instance = new Hello ();
  }
  return _instance;
}

int hello::setctl (int ctl)
{
  printf ("TK------->>>>>>setctl-----\ n");
  This->myctl = CTL;
  return 1;
}

int hello::getctl (int ctl)
{
  printf ("TK------->>>>>>getctl-----\ n");
  Return this->myctl;
}

Compiled into a dynamic link library, general requirements for PIC location are irrelevant:

g++-shared-fpic-o libhello.so Hello.cpp

Build: libhello.so

2. Test Cases

Test.cpp

#include "so/hello.h"
#include <stdio.h>
#include <dlfcn.h>

int main ()
{
  void* Hello = Dlopen ("/home/lianxi/c++/ku/so/libhello.so", rtld_lazy);
  if (!hello) {
    printf ("dlopen/home/lianxi/c++/ku/so/libhello.so error!\n");
    return-1;
  }
  Dlerror ();
  creathelloclass* Creat_hello = (creathelloclass*) dlsym (Hello, "Creathello");
  Const char* Dlsym_error = Dlerror ();
  if (dlsym_error) {
    printf ("Error%s\n", dlsym_error);
    return-1;
  }
  hello_base* Mhello = Creat_hello ();
  Mhello->setctl (5);
  int result = MHELLO->GETCTL (0);
  printf ("TK------>>>>>result is%d\n", result);
  Dlclose (hello);
  return 1;
}

Compiling: g++-o test test.cpp-l.-ldl

Execution./test results:

TK------>>>>>>hello::instance---------
tk------->>>>>>setctl-----
TK------->>>>>>getctl-----
tk------>>>>>result is 5

Second, analysis

1.LIBDL Library for C language development, when you need to dlsym a symbol in C + +, you need to declare that the symbol in the library is extern "C", such as the example of extern "C" hello_base* Creathello ();

2. Because it's not possible to dlsym as many methods in C + + classes This use of C + + for pure virtual class instantiation is a run-time link characteristics, that is, in C + + for the class to use the dynamic link of the shared library, must define a pure virtual subclass.

For information about shared libraries in C, see the following links:

"C Compiler principle" dynamic loading and static loading of shared libraries