C ++中的工厂模式：生成显式createInstance（） - 方法自动 [英] Factory Pattern in C++: generating explicit createInstance()-Method automatically

问题描述

我在写一个C ++框架时遇到问题，用户应该有比使用它更少的开销。用户可以通过创建一个包含一个类的共享库来发布他们的工作，该类由一个框架的BaseClass派生，并实现一个externCcreateInstance（） - 方法，以便返回一个实例的派生类。因此，框架可以通过使用dlsym（）的共享库调用createInstance-Method来访问用户类。

  class BaseClass { } 
 class UserClass：public BaseClass {} 
 
 externC{
 BaseClass * UserXcreateInstance（）{
 return new UserClass（）; 
} 
} 
  

在框架中：

  typedef BaseClass *（* CreateInstance）（）; 
 void * handle; 
 CreateInstance createInstance; 
 handle = dlopen（libUserLibrary.so，RTLD_LAZY | RTLD_GLOBAL）; 
 createInstance = reinterpret_cast< CreateInstance *> dlsym（handle，UserXcreateInstance）; 
 BaseClass * userX = createInstance（）; 
  

我的问题：是否可以生成UserXcreateInstance（） - 方法，每个用户都是冗余的图书馆，所以用户不必考虑它？

我以为可以使用模板+宏，但我还没有找到一种方式要做到这一点...

另一种方法，我在想，直接通过dlsym和适当的名称来调用任何用户类的构造函数。 （我从配置文件中知道任何命名空间+类名）但是我不认为这是一个正确的解决方案，特别是构造函数调用与常规函数调用不一样，但非常有趣...

解决方案

我无法想像有一种方法可以自动创建，而不需要对用户的每个部分进行任何编码。我可以想像如何简化它，也许使用宏：

  #define OBJECT_CREATOR（X）\ 
 extern C{\ 
 BaseClass * UserXCreateInstance（）{\ 
 return new X（）; \ 
} \ 
} 
  

而用户只需要放在他的cpp文件中：

  OBJECT_CREATOR（UserClass）; 
  

i have the problem in writing a C++ framework, that users should have less overhead than possible to use it. Users can publish their work to the frameworks by creating a shared library that contains a class, which is derived by a frameworks' BaseClass and implementing an extern "C" createInstance()-method in order to return an instance its' derived class. So the framework can access the user class by calling the createInstance-Method through the shared library with dlsym().

class BaseClass{}
class UserClass : public BaseClass{}

extern "C"{  
   BaseClass* UserXcreateInstance(){
    return new UserClass();
   }                        
}

In framework:

typedef BaseClass* (*CreateInstance) ();
void* handle;
CreateInstance createInstance;
handle = dlopen( "libUserLibrary.so", RTLD_LAZY | RTLD_GLOBAL );
createInstance = reinterpret_cast <CreateInstance*> dlsym( handle, "UserXcreateInstance" );
BaseClass* userX = createInstance();

My Question: Is it possible to generate the UserXcreateInstance()-method, which is redundant in each user library, so that the user don`t have to think about it?

I thought it would be possible with templates+macros, but I haven't yet found a way to do this...

Another approach, I was thinking is directly call the constructor of any user class via dlsym and appropiate name mangling. (I know any namespace + class name from a config file) But I don`t think this a proper solution, especially a constructor call is not the same as a regular function call... but very interesting...

解决方案

I can't imagine a way to create this automatically without any coding per part of the user. I can imagine ways to simplify it, perhaps using a macro:

#define OBJECT_CREATOR(X) \
    extern "C" {          \
         BaseClass *UserXCreateInstance() {\
             return new X(); \
         }\
    }

And the user just need to put on his cpp file:

OBJECT_CREATOR(UserClass);

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