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DynObj - C++ Cross Platform Plugin Objects

, 27 Sep 2007
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DynObj is an open source library that provides a C++ application with run-time class loading facilities

Introduction

DynObj is an open source library that provides a C++ application with run-time class loading facilities (AKA plugins). It's written in standard C++, and can be used with any C++ compiler with good template support, so it is cross-platform from the outset. It uses a minimal platform specific layer to handle OS specifics (Win32, Linux + various Unix supported now).

The project started out with me needing a way to support plugins in a cross platform application. The approaches I found were either too heavy weight (Modzilla XPCOM) or were platform/compiler specific. An article by Chad Austin provided a good starting point for the DynObj library.

This article will cover some ground, so it is split in a number of sections:

  • Introduction - This page. General description of the problem area.
  • Background - Explores C++ classes and the linking process.
  • Solution - Describes the DynObj library.
  • Sample - A sample plugin together with a main application.
  • Library documentation - Documentation for the DynObj library.
  • Building - Documentation for building the DynObj library.

Background - Problem Area

C++ is a very feature rich language and within the same link time module (all sources and libraries) functionality can be exposed and shared without much difficulty. You may need to get some link flags right, but it can be done, with templates and the whole C++ machinery operating.

This can be extended to run-time with shared libraries (DLLs on Windows, shared dynamic object files (SO) on various Unices).

However, it quickly becomes rather difficult, since there is an intricate linking process going on between the application and the loaded modules. With C++ name mangling and automatically generated templates and a rich set of compiler/linker options, dependencies become complicated. With a code base under heavy development, linking with a a DLL compiled on a different system a month ago is not likely to work.

This approach usually assumes that the same compiler is used for host and library. To link the binaries, often the compiler version must be similar. You could not expect to link together a template or class library compiled with G++ with an application compiled with a Microsoft based compiler. You'd have problems doing it with a version of the same compiler from a year ago.

The approach described above can be termed a 'tight' or a 'full' linking scheme. C++ has never had it easy to make its internal features available to the outside world in a standard way. Old style:

extern "C" int FunctionToExport(...)

has often been the way (and yes, this works reliably but can only expose global functions and variables).

A Plugin Approach

A plugin is a more decoupled run-time library, supposedly not dependent on the main application being a certain version, and preferably the requirements on the plugin should be a bit more loose (they shouldn't need to know about every header file and #define in the application, and they need not mutually resolve massive amounts of global symbols).

C++ doesn't provide any language or standard library support for this, but there are some good starting points in the language.

What can be investigated is what parts of the language can be used without creating link-time complexity, and what parts to be avoided for a plugin. Maybe one is not confined to only extern "C" ....

On the Windows platform, COM is a way forward. To be language neutral, it limits (severely) what features of C++ can be exposed and then re-used inside another run-time module (including expressing inheritence relationships). It is based on a particular way of sharing the VTables for run-time binary objects. However, it works, but has not evolved in a cross-platform direction.

Middle Ground

What we're looking at is if there is some larger middle ground that can be defined, between (A) the compile time process when the full C++ feature set (and lots of header files) are be shared, and (B) the run-time DLL loading where only type-less symbols can be looked up in a loaded module (extern "C"...")

The next section will define this middle ground and later introduce an object framework (DynObj) around it.

A C++ Class

We have this class definition:

class AnExample {
public:
    AnExmaple( int i ) : m_i(i) { }
    virtual int Add( int i );
    int Sub( int i );
    int Get( ){ return m_i; }
    bool operator < (int i){ return m_i
    static int StaticFunc( );
protected:
    int m_i;
};

For linking (which is the key issue in dynamic loading), this applies to the members of AnExample:

  • The constructor AnExample(int i) is inline so it does not generate any linking
  • The virtual function int Add(int i) is an entry in the VTable of the class. To the compiler and linker, it is an index in this table.
  • The function Sub(int i) does generates linking. It refers to a function implemented somewhere else.
  • Get() is an inline function and does not generate linking.
  • operator < is also an inline function (no linking).
  • The StaticFunc() member requires linking. This is true for any static member.
  • The member variable m_i is a type and offset into the binary object. It does not generate linking.

So, it seems it's only static members and functions that are both non-virtual and non-inline member that generate any linking!

VTables

To clarify things here a bit we should remember what virtual functions are:

  • Each class (that has one or more virtual functions) has a unique virtual function table (VTable).
  • The VTable is per type, not per instance.
  • A pointer to the VTable (if the object has one) is always stored first in the object. It is often referred to as the VPTR.
  • A virtual member function is at run-time an index into this VTable where the address of the function to use is stored.
  • A derived class has a copy of the base class VTable first in its own VTable. The new functions it introduces are stored at the end of that copy.

The VTable in itself is in itself a symbol located inside a DLL. However, if an object is instantiated from inside the DLL/SO that owns the VTable, this is not part of the link process either.

So it seems we have found some middle ground.

C++ Inheritance

We have a class that uses inheritance:

class SideBaseClass {
public:
    virtual const char *GetPath( );
    virtual bool SetPath( const char *path );
};

class MultiBaseClass : public AnExample, public SideBaseClass {
public:
    virtual int Add( int i );
    virtual bool SetPath( const char *path );
protected:
    int m_j;
};

MultiBaseClass has two base classes and overrides a function from each base class. From a run-time perspective, the inheritance boils down to the following object layout for an instance of MultiBaseClass:

word offset 0 VPTR for MultiBaseClass
word offset 1 m_i (base class data)
word offset 2 m_j (derived class data)
word offset 3 VPTR for SideBaseClass

The VTable for MultiBaseClass will be:

slot 0 MultiBaseClass::Add(int i)
slot 1 SideBaseClass::GetPath( )
slot 2 SideBaseClass::SetPath( const char *path)

This object data layout can vary with compiler and data type (compilers have settings for padding and alignment). However, this is constant also between compilers:

  1. The VPTR (if any) is always stored first in the object.
  2. The first virtual function in a base class always occupies slot 0 in the VTable.
  3. Subsequently declared virtual functions stored in order of declaration.

For derived classes, the following applies:

  1. A derived class inherits its default VTable from its first base class that has a VTable (call it main base class).
  2. New virtual functions are appended (in order of declaration) at the end of the default VTable.
  3. For other base classes with VTables (side bases), a full copy of the object is stored at an offset into the binary object, including a copy of side base class VTable.
  4. The VTable of side bases can be modified (functions are overridden) but it cannot be extended.

This is the main picture, there are a couple of exceptions to above rules. They are:

  1. The ordering of overloaded functions in the VTable does not correspond with declaration order for some compilers (MSVC among others, for historical reasons).
  2. Virtual destructors are handled in different ways by different compilers. They cannot be directly invoked across a compiler and DLL/SO boundary.

A central part of the DynObj framework is to account for offsets between multiple bases generated by potentially different compilers.

What is important to recognize here is that: "Inheritance (neither single nor multiple) does not generate any linking."

Common Ground Defined

A class definition:

  • with one or more (non-template, non-virtual) base classes
  • with any number of virtual functions
  • with operators that are either virtual or inline
  • and any inline function
  • with any non-static data members

can be reused and implemented by both a host application and a plugin. Furthermore, they can use each other's implementations of these classes. Code made by one compiler may use such a class compiled from another.

For function members, this works all the time, also in the cross-compiler case. For data members, it works as long as the compilers on each side have used the same sizes and padding for the data members.

When moving (casting) between base classes, the address offset must at all times be calculated based on offsets generated by the source (plugin) compiler.

This common ground is of course in addition to the old extern "C" MyFunction(...) style. That is, however, an important part that we will make use of below.

The Run-Time Link

We know now that instances of a class fulfilling above spec can be used by both the host application and the plugin, without requiring a linking process. But how do we handle creation and destruction of object instances?

Since a call across the plugin boundary is essentially typeless, we cannot communicate the type directly to the plugin as a C++ type.

Plugin Object Creation

Say that from the host, we want the plugin to create an instance of AnExample:

AnExample *pe = new AnExample; // This doesn't work 

This would just make the host instantiate it.

Since we don't have the compile time type machinery here, (remember, we're communicating with a binary module possibly from another compiler), we have to communicate the type to the plugin in some other way.

To solve this problem, we use a factory function in the plugin that takes the type encoded as a string and an integer:

extern "C" void* CreatePluginObject( const char *type_name, int type_id )
    if( !strcmp(type_name,"AnExample") &&
        type_id == ANEXAMPLE_TYPE_ID )
        return new AnExample;
    else return NULL;
}

Then on the host side we can do:

typedef void* (*PluginFactoryFn)(const char *type, int id);

// First locate CreatePluginObject inside a loaded DLL
PluginFactoryFn create_fn = /* Locate it */;

// Now create object
AnExample *pae = (AnExample*)create_fn( "AnExample",
        ANEXAMPLE_TYPE_ID );

The DynObj framework automates this conversion step so that we can use the expression:

AnExample *pe = do_new<AnExample>; // This works

to instantiate objects from inside plugins. The conversion:

  • C++ type => (type string,type ID)

is taken care of by templates classes available in the host application.

Plugin Object Destruction

There are some points to consider here to keep cross-compiler compatibility:

  • We cannot be sure that the host and the plugin share the same memory allocator (on Windows this is often not the case). So using C++ delete on plugin objects is not a good idea.
  • Virtual destructors are used in different ways by different compilers.

Essentially, the host must make sure a plugin object is 'recycled' by the same plugin that created it. To handle this, the DynObj framework has used a solution where each object that is created has a virtual member function doDestroy():

DynObj *pdo = /* Create object and use it */;
pdo->doDestroy(); // End of object

We see here that we have used DynObj as a base class for objects that are created by a plugin.

The DynObj framework works with any classes, but objects that can be created and destroyed by plugins must derive from DynObj.

Linking Revisited

The solution with factory functions gives the responsability of setting up the VTable to the plugin, and so, all the functions we need from the plugin are contained in these pre-linked VTables. Each instantiated object comes back with a VPTR as its first binary member.

The only run-time linking we have to do is to lookup these factory functions inside the plugin DLL (and possibly some other init/exit functions). This keeps the host and the plugin in a loosely coupled relationship, defined by the plugin interface. Next comes the description of the DynObj solution using this approach.

Solution

This describes the properties of the DynObj library solution to the plugin/linking problem.

Cross-Platform

The library is written in C++, and a decent C++ compiler should build it (tested with MSVC 8 and G++ [4.1.2 and 3.4.5]). It relies on a minimalistic cross-platform layer for dynamic linking and a spartan threading interface.

Cross Compiler

The library/plugin compiler can be a different one than the main application compiler. All casting between types is allways done based on offsets from the source (library) compiler.

C++ Classes used Across DLL/SO Boundary

DynObj supports ordinary C++ classes across the plugin boundary. Any class that consists of:

  • Zero, one or more base classes/interfaces
  • Virtual functions (argument overloading not supported)
  • Inline functions
  • Operators
  • Data members (keep track of member alignment!)

So a fairly large subset of the C++ class concept can be used over the boundary. This is what cannot be used:

  • Non-virtual member functions implemented in a separate source file
  • Static members (functions,data)

Object Model

The object from a plugin represents a full C++ object, including the possibility of having multiple nested base classes. At the source code level, a tagging scheme is used to decide which bases to expose. The whole (exposed part) of the inheritance tree is communicated to users of the object.

The object is usually accessed using a single inheritance interface/class. Using a cast operation (query type) one can move between the different interfaces/sub-objects that are implemented.

C++ Type Query

An object can implement a number of interfaces and/or classes. To query an object for another type, the C++ template:

template<class U, class T> U do_cast(T t)

is used. It operates the same way as C++ dynamic_cast<> and provides typed safe casts across the plugin boundary. do_cast (and related functions) provides similar functionality to QueryInterface in COM.

An example:

DynI pdi = /* Basic DynI pointer from somewhere */;
DynSharedI pdsi = do_cast<DynSharedI*>(pdi)

Arbitrary Types and DynI Derived Types

The library introduces a small interface and class collection, based on DynI (a class which knows its own type and can be queried for other types). Both classes based on DynI and arbitrary classes with virtual methods may be used across the plugin boundary.

When using classes derived from DynI, a separate registration step may be skipped, since a DynI object always knows its own type.

The provided classes derived from DynI also provides for a certain way of instantiating and destroying objects (DynObj), for handling objects with shared ownership (DynSharedI), and also for weak references.

When using arbitrary classes, they must have at least one virtual member function. The library provides templates that safely detect if an object has a vtable or not. To use such objects across a plugin boundary, one instance of the type must be registered first.

Simple Type Identifiers

Types are identified based on the pair:

  • Type string
  • Type identifier (32-bit integer)

This is a simple scheme that does not guarantee global (world-wide) type uniqueness. It can, however, guarantee that the types used inside the application are unique. It is always simple to find the string name for types. In cast operations, usually only the type integer is carried around (no 128 bit ID structures).

Most times we don't need to know these, we just use the C++ types (which in their turn use the type strings/IDs when needed).

Plugin Role

Plugins can use types from the main application (as long as it has headers for it) and also from other loaded plugins. It can also instantiate plugin objects (from itself, other plugins, or the main app).

Light-Weight

The library is self-contained and relatively small, including the cross-platform layer. A compressed archive of the source is around 200 KB. It does not rely on STL, Boost or any other big component library. It is not tied to a single platform API.

Facilities

The library includes a collection of practical classes, to handle libraries, object instantiation/destruction, smart pointers and more.

Optionally (and recommended) one can use the class DoRunTimeI, which provides shared resources to the application and the plugins. Among other things it makes sure that the various libraries access the same type information, it provides for a pool of named 'published' objects, per-object and per-thread error handling.

A run-time string class, DynStr (in itself a plugin object) is provided, giving plugins a way to deal with Unicode strings.

Source Code Preprocessor

To setup a C++ class as a plugin type, some registration needs to be done and a library file must be created. To help with this, a tool pdoh (Parse DynObj Header) is used. It reads C++ source file and triggers on // %%DYNOBJ tags in the source code.

The pdoh tool outputs most of the glue code that is needed, including generating type ID:s.

With other Languages

The library relies on the default way of using vtables in C++ together with a binary type description structure. This is a simple binary scheme. So, plugin classes could be used from any language that can use these. A C implementation is straight forward (an object would be a structure with the first member being a pointer to an array of functions). Also, a plugin class could be implemented in another language and used from C++.

Inline functions cannot be shared with another language (they are really compiled on both host and plugin side).

Requirements

The library relies on these features from the C++ compiler:

  • It uses vtables in the default way (one pointer per function, first function at index 0, new functions are stored in declaration order)
  • Support for extern "C" style of exposing non-mangled function names
  • Support for __cdecl function calling convention

When a library is compiled, this information is stored and made available at load time, so an incompatible library can be detected. Virtual destructors are not used across plugin boundaries, since compilers implement them in slightly different ways. Some earlier versions of g++ (prior to version 2.8) used two slots per function in the VTable, that would not have been compatible.

When exposing data members in a class across a plugin boundary, the best is to make each member fill up one word (32/64-bit) in the structure. That avoids any possibility of unaligned data access. The size of an exposed type (using sizeof from the plugin compiler) is stored in the type information. The user of a plugin class could detect if data members are aligned differently.

The calling convention can be configured when the library is compiled, some other convention could be used as long as the main and plugin compiler agree on it. On Linux, the default (implicit) calling convention is __cedcl. Next: A sample using the DynObj library.

A Sample: Plugin + Application

Here we will create a couple of plugin libraries and use them from a simple main application. It wil demonstrate how to instantiate plugin objects, how to use plugin objects as ordinary C++ classes, how to query for supported types.

Creating an Interface File

We start out with defining a simple interface file that manages data about a person (PersonI.h):

#include <span class="code-keyword"><string.h> // We use strcmp below 
</span>
class DynStr; 
// %%DYNOBJ class(DynI) <---Directive to pdoh preprocessor 
class PersonI : public DynObj { 
public: 
    // DynI methods <---Implement GetType and Destroy - for all DynObj:s 
    virtual DynObjType* docall doGetType( ) const; 
    virtual void docall doDestroy( ) { delete this; } 

    // PersonI methods <---Add our new methods 
    virtual const char* docall GetName( ) const = 0; 
    virtual int docall GetAge() const = 0; 

    virtual bool docall SetName( const char *name ) = 0; 
    virtual bool docall SetAge(int age) = 0; 

    // ---Simple default inline implementation of operator
    virtual bool docall operator<( const PersonI& other ) const { 
        return strcmp(GetName(),other.GetName()) < 0; 
    } 

    // ---Non-virtual, inline convenience function
    // Derived cannot override. 
    PersonI& operator=( const PersonI& other ) { 
        SetAge( other.GetAge() ); 
        SetName( other.GetName() ); 
        return *this; 
    } };

Then, from a command prompt/shell, we run the pdoh preprocessor on this file (the -o option tells the parser to write generated code directly into the file instead of to stdout):

$ ./pdoh PersonI.h -o
$

Looking at the header file, we see that a section at the beginning of the file has been added:

// %%DYNOBJ section general
// This section is auto-generated and manual changes will
// be lost when regenerated!!


#ifdef DO_IMPLEMENT_PERSONI
#define DO_IMPLEMENTING // If app has not defined it already
#endif
#include <span class="code-string">"DynObj/DynObj.h"
</span>

// These are general definitions & declarations used 
// on both the user side [main program]
// and the implementor [library] side. 

// --- Integer type ids for defined interfaces/classes ---
#define PERSONI_TYPE_ID 0x519C8A00

// --- Forward class declarations ---
class PersonI;

// --- For each declared class, doTypeInfo template specializations ---
// This allows creating objects from a C++ types and in run-time casts
DO_DECL_TYPE_INFO(PersonI,PERSONI_TYPE_ID);

// %%DYNOBJ section end

This section provides the glue needed to convert from a C++ PersonI type to the type strings and type ID:s that are used across the plugin boundary.

If we would like to move this section we're free to do that. The next time the preprocessor is run on the same file, it will keep the section where we put it.

We also see that code has been inserted at the end of the file:

// %%DYNOBJ section implement 
// This section is auto-generated and manual changes 
// will be lost when regenerated!! 
// ... comments 

// Define the symbol below from -only one place- in the project implementing 
// these interfaces/classes [the library/module].
#ifdef DO_IMPLEMENT_PERSONI 

// Generate type information that auto-registers on module load 
DynObjType 
g_do_vtype_PersonI("PersonI:DynObj",PERSONI_TYPE_ID,1,sizeof(PersonI)); 
// DynI::doGetType implementation for: PersonI 
DynObjType* PersonI::doGetType() const { 
    return &g_do_vtype_PersonI; 
}
#endif // DO_IMPLEMENT_...

The preprocessor has inserted code to do two things:

  • Declare a DynObjType structure for our type.
  • It provides a default implementation of doGetType() for our class

When we define the symbol DO_IMPLEMENT_PERSONI from a C++ source file, the code above ends up in that file.

Creating an Implementation File

Next we create a source file that implements the interface (PersonImpl1.cpp):

// This will cause PersonI class registration info to come in our file. 
#define DO_IMPLEMENT_PERSONI 
#include <span class="code-string">"PersonI.h" 
</span>

// We're also implementing our class 
#define DO_IMPLEMENT_PERSONIMPL1

The defines above puts the class registration code into this source file. Each interface/type that is handled must be declared as a global registration structure once. The defines DO_IMPLEMENT_... correspond to a class we're implementing in this file.

// Declare the class to the pre-processor. 
// %%DYNOBJ class(dyni,usertype) 
class PersonImpl1 : public PersonI { 
public:

Here we tell the preprocessor that a plugin class is being defined. The flag usertype informs it that this class can be instantiated by the host. Therefore it must generate a factory function for this type in the library section.

// DynObj methods 
virtual DynObjType* docall doGetType( ) const; 
virtual void docall doDestroy( ) { delete this; } 

// PersonI methods 
virtual const char* docall GetName( ) const { 
    return m_name; 
} 
...

The above implementats functions in DynObj and PersonI. Since we are inside a class PersonImpl1 which definintion is never is exposed, we can generate the function bodies inside the class definition.

// Constructor, Init from a string: "Bart,45" 
PersonImpl1( const DynI* pdi_init ) : m_age(0) { 
    // We will do this setup slightly awkward now, and improve in 
    // the following examples. 
    *m_name = 0; // NUL terminated 
    ...

The constructor for a DynObj always take a simple argument of type const DynI*. Since DynI can implement any interface, we can pass pretty much any type of data to the constructor. To pass simple data like int/double/const char* and friends, one can use an instance of template class DynData<T>.

protected:
   // Need not really be protected since user of PersonI cannot look here anyway.
   char m_name[NAME_MAX_LENGTH];
   int m_age;
};
// %%DYNOBJ library

The last comment tells the preprocessor that we want library code inserted at this location. In this library section it will put the factory functions and any glue needed to instantiate plugin objects to the host.

Next we run the parser on this source file (the -p option tells pdoh where it can find template code):

$ ./pdoh PersonImpl1.cpp -o -p../
Found library insertion point
$

The parser has now inserted code that generates glue for library functions. The glue code can be included/excluded using #define DO_MODULE:

// %%DYNOBJ section library 
...
// Only include below when compiling as a separate library
#ifdef DO_MODULE
...
// The object creation function for this module 
extern "C" SYM_EXPORT DynObj* CreateDynObj(
    const char *type, int type_id, 
    const DynI *pdi_init, int object_size )
{
    ...
        if( ((!strcmp(type,"PersonImpl1") || type_id==PERSONIMPL1_TYPE_ID)) || 
            ((!strcmp(type,"PersonI") && type_id==PERSONI_TYPE_ID)) ){ 
                return new PersonImpl1(pdi_init); 
            }
            DO_LOG_ERR1( DOERR_UNKNOWN_TYPE, ... );
            return 0; 
}

After compiling, we can connect this as a plugin to a host application, the preprocessor has generated the bits and pieces that are required, both for the host and the plugin side.

A Main Application

Finally we create the main application (main1.cpp) that uses the plugin:

#include <span class="code-keyword"><stdio.h> 
</span>
#include <span class="code-keyword"><stdlib.h> 
</span>

// DynObj support 
#include <span class="code-string">"DynObj/DynObj.h"
</span>
#include <span class="code-string">"DynObj/DynObjLib.h"
</span>
#include <span class="code-string">"DynObj/DoBase.hpp"
</span>

// Interfaces we're using
#include <span class="code-string">"PersonI.h"
</span>
#include <span class="code-string">"ProfessionI.h"
</span>

int main( ) { 
    // Check that things have started OK 
    if( !doVerifyInit() ){ 
        printf("Failed DoVerifyInit\n"); 
        exit(-1); 
    }

Now we want to start using the plugin. For this, we use DynObjLib which wraps a cross-platform run-time (DLL/SO) library loader (initial code for this came from Boby Thomas).

It is worth noting that the main application and the library are loosely linked, making it easy to implement on any platform that supports explicit run-time loading of binary libraries.

// Load library 
DynObjLib lpimpl1( "pimpl1", true ); 
if( lpimpl1.GetState()!=DOLIB_INIT ){ 
    printf( "Could not load library (pimpl1): status (%d)\n", 
        lpimpl1.GetState() ); 
    exit( -1 ); 
}
// Create object 
PersonI *pp = (PersonI*)lpimpl1.Create("PersonI",PERSONI_TYPE_ID); 
if( pp ) { 
    pp->SetName( "George" ); 
    pp->SetAge( 34 ); 
    ...

We have instantiated the object the 'raw' way here, giving type name and ID to DynObjLib. After that, it is just to start using the object as any standard C++ object.

We next query the object for an interface using do_cast:

ProfessionI *pi = do_cast<ProfessionI*>(pp); 
if( pi ) 
    ;// Use the interface 

pp->doDestroy();
return 0;
}

The template do_cast<T> takes care of the details of checking if ProfessionI is supported. It transforms the C++ type to type name and ID. Using type information from the plugin, it can walk the class layout and return an adjusted interface pointer.

It is important that any address offsets applied inside objects are always based on information from the plugin compiler. When using an interface pointer returned in this way, we can only assume it is valid for the duration of the current function. We do not need to release it in any way. Finally, we delete the object using DynObj::doDestroy (which will recycle it in the memory manager of the plugin that created it).

Further documentation here.

The DynObj Library

DynObj is an open-source cross-platform library that uses the run-time plugin approach just decribed. Although the mechanisms used are generic and fairly simple, the library fills many gaps and make it straight-forward to use plugins inside a C++ application.

The library provides:

  • A small class hierarchy (VObj, DynI, DynObj, DynSharedI) establishing some common ground between a host and a plugin. The DynObj library also works with classes that are not rooted in this hierachy.
  • A type description facility that allows types to be defined and shared by both host and plugins (DynObjType).
  • A way to convert C++ types to a plugin library (doTypeInfo, DO_DECL_TYPE_INFO).
  • Cast functions to query an object about the types it implements. This is similar to dynamic_cast<T> in C++ or QueryInterface in COM (do_cast<T>, doGetObj,...).
  • Instantiating C++ objects from plugins (do_new<T>).
  • A plugin library loading/unloading mechanism(DynObjLib).
  • C-level functions to handle objects.
  • Other practical C++ classes and templates for objects.

In addition to these library facilities, it includes a tool (pdoh) that parses C++ header files and generates source code for type registration.

VObj, DynI, DynObj and Friends

All classes discussed below are defined in DynObj.h. The library is based on some properties of objects with VTables:

  • The VPTR is always stored first in a binary object
  • VTables are shared by all instances of a class, but not with instances of any other class (so it provides a type identifier).

In C++ there is not a built-in way to denote these classes. However, we define the class VObj to represent an object with a VTable with unknown size and methods. VObj in that sense becomes the 'stipulated' root class for all classes that contain one or more virtual functions.

The VObj Class

Base: (no base class)
Methods: (no methods)

We see that VObj does not introduce any methods (it cannot since that would interfere with derived classes which use the first VTable slot).

However, VObj has a number of convenient inline functions to query for types (VObj::IsA, VObj::CanBeA, VObj::GetObj,...), asking about errors (VObj::Get/Set/ClearError) and more.

To determine if a class is a VObj or not, these templates can be used:

bool has_vtable = IsVObj<SomeType>::v;

template<class T>
VObj* to_vobj( T* pt );

This provides type safety so that we cannot try to convert say a char* to an interface pointer (the compiler would give an error).

The DynI Class

Base: VObj
Returns Methods: Arguments
DynObjType* doGetType
void* doGetObj const char* type_name
const char* doGetError int *perr_code
void doClearError

The DynI class provides a way to know its type (doGetType) and for asking about other types it supports (doGetObj). To ask if a DynI supports the DynStr interface:

DynI *pdi = /* Wherever pointer comes from */;
DynStr *pds = pdi->doGetObj("DynStr");

This is equivalent to:

DynStr *pds = do_cast<DynStr*>(pdi);

The DynI class has an advantage over VObj:

  • It knows its own derived type

In contrast, to find the type of a VObj, a lookup into a global table, using the VPTR, has to be made (and works only after the types have been registered).

Since DynI is used across DLL (and possibly compiler) boundaries, we cannot use C++ exceptions. To provide error handling, the methods doGetError and doClearError are introduced. They allow for an object specific error state, without burdening the class with member variables for this. SetError is not a member, since object errors are usually are not set from 'outside'.

We see also that the DynI interface has no support for creation or destrcution. The same applies to VObj. The lifespan that can be assumed is that of the current method.

If a reference to the object is to be kept, these are different ways to go about it:

  • Ask for a DynSharedI interface (ref counted ownership)
  • Create a weak reference (if object supports NotifierI))
  • The object may be a known global or singleton which explicitely allows for references to be stored

The DynObj Class

Base: DynI
Returns Methods: Arguments
void doDestroy
void doDtor

The DynObj interface represents an object that can be created and destroyed. It represents an object owned from a single point. Usually the functions doDestroy and doDtor would be implemented like:

void MyClass::doDestroy(){ ::delete this; }
void MyClass::doDtor(){ this->~MyClass(); }

An object is destroyed through doDestroy. doDtor provides access to the destructor of the class (library internal use).

Objects can be created in some different ways:

  • Using do_new<T>
  • Using DynObjLib::Create(...)
  • Temporary objects can be created and released using DynObjHolder<T>

The DynSharedI Class

Base: DynObj
Returns Methods: Arguments
int doAddRef
int doRelease

The DynSharedI interface represents an object with shared ownership. doAddRef and doRelease increases and decreases the ownership counter.

DynSharedI derives from DynObj since it depends on a way of destroying itself (DynObj::doDestroy) when the lifetime counter reaches 0.

To protect the object from being deleted before its actual end-of-life, a doDestroy method can check that the counter is actually zero:

virtual void docall doDestroy( ) { 
    if( !m_ref_cnt ) 
        ::delete this; 
    else 
        SetError(DOERR_DESTROY_ON_NON_ZERO_REF,
        "DynSharedI - Destroy on non-zero ref"); 
}

Documenation on building the DynObj library.

Building and Using the Library

Directory layout of the library:

  • doc
    • doxygen - docs generated from doxygen
  • src
    • DynObj - main source of DynObj library
      • samples - samples of using library here
      • tools - source for the pdoh tool
      • msdev - Visual C++ project for DynObj and samples here
    • pi - sources for platform independence layer
    • utils - utility like C++ classes

A description of the pdoh tool is available here.

Build Model

Plugins are usually opened while the application is running, so there is no build-time link between the main application and the plugins. When a plugin module is loaded, by default, the linker is instructed not to backlink into the application (UNIX). On Windows, backlinking is not possible.

An application can expose functionality to plugins through interfaces. DoRunTimeI provides a way to make named instances of objects known globally.

Compiler Defines

A number of compilers define controls how libraries and main applications are built. The defines are described in detail in src/DynObj/DoSetup.h (and under DynObj defines in doxygen doc).

When compiling a library DO_MODULE should be defined. Also, the name of the library should be stored in DO_LIB_NAME (i.e. #define DO_LIB_NAME "DynStr").

The main application should define DO_MAIN.

The default settings in DoSetup.h are OK when compiling the samples. In general the defines are used like this (example DO_USE_DYNSHARED):

  • #define DO_USE_DYNSHARED - The option is not used
  • #define DO_USE_DYNSHARED 0 - The option is not used
  • #define DO_USE_DYNSHARED 1 - The option is activated

This allows having sensible defaults and for overriding them reliably from outside in a build environment.

Building the DynObj library

There are two build methods provided with the library:

  • Cross-platform GNU makefile - This works for the g++ compiler on Unices and Windows (mingw).
  • Visual C++ - Solution and project files for for Visual C++ on Windows.

Building a Plugin (Module)

The compiler define DO_MODULE should be set.

A plugin module requires compiling with:

  • DynObj.cpp
  • vt_test.cpp

Building a Main Application (Using Plugins/Modules)

The compiler-defined DO_MAIN should be set. The main application links against one of two static libraries:

  • dolibdort - Enables using DynObj:s and DoRunTime
  • doliblt - A minimalistic library without support for DoRunTime

The libraries are projects in the Visual C++ solution file. To build the libraries using the makefile:

$ cd src/DynObj
$ make dolibdort
$ make doliblt

The DynStr Library

A run-time plugin class for strings is provided: DynStr. This enables plugins to use a C++ string class in a safe way, internally and in function calls. The DynStr library is built with:

$ cd src/DynObj
$ make ds

Building the Samples

The samples defines a PersonI and ProfessionI interfaces. Then three slightly different implementations are provided in three plugins: pimpl1, pimpl2, pimpl3. Three different main applications are provided as well: main1, main2, main3. There are sub-projects for each of them in the VC++ solution file.

From the command prompt:

$ cd samples
$ make pimpl1
$ make pimpl2
$ make pimpl3
$ make bmain1
$ make bmain2
$ make bmain3

The pdoh Tool

This tool takes a C++ header or source file and outputs a modified version of the file, provided it finds // %%DYNOBJ tags in it. It basically scans for class and struct declarations and collects inheritance information.

The pdoh tool can generate these sections:

  • A general section (in a header file). This part is used by both the plugin and the main application. It contains class declarations, type IDs and the bridge from C++ types to type IDs.
  • An implement section (by default in a header file). This part is used only by the plugin. To keep things simple, the code is generated inside the header file (that keeps things in one place) and the plugin must trigger inclusion of this section with a #define DO_IMPLEMENT_NAMEOFLIB.
  • A library section. This goes into a source file and makes up the part that the user of the plugin communicates with directly. The most important function is the one that receives a type ID/ type string and instantiates this object to the caller.

By default pdoh sends its output to stdout. Use option -o to overwrite the input file, or -oNewFile.h to write to another file. To generate less comments in the output -b can be used.

pdoh can be integrated into a build environment, it is a simple file scanner so it is fast. If it does not find any //%%DYNOBJ tags it will not generate any output.

If the tags have not been modified since the previous run (on the same file), it will also not generate any output (so it does not affect file time stamps when there is no need for it).

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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Arne Steinarson

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Comments and Discussions

 
QuestionWon't compile with Visual Studio 2010 on Win7 (64-bit) PinmemberMichael B Pliam24-Jan-14 10:09 
GeneralVersion Problem? [modified] Pinmemberhujunxi11-Jan-11 6:28 
GeneralLicense PinmemberI niXman18-Nov-10 1:38 
GeneralReally good Pinmemberblur30-Nov-07 6:19 
GeneralGood work Pinmemberwtwhite2-Oct-07 3:03 
QuestionMFC? PinmemberTomM1-Oct-07 5:39 
AnswerRe: MFC? PinmemberArne Steinarson1-Oct-07 6:00 
GeneralExcellent Article PinmemberNeville Franks29-Sep-07 13:48 
GeneralRe: Excellent Article PinmemberArne Steinarson29-Sep-07 14:49 

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