Where are class methods in memory?

[BrownB]

Hello, I'm new to C++ object programming, I've been working for one year with C.

I googled and searched through this forum, but found nothing.

I'm now trying to use some classes in my C application, so I'm having an "adaptation" work: I'm trying to compile the c++ part in an obj, and then using the classes through C functions and structures.

I need to know where are placed in memory the methods of Cpp structures. To be clear:

struct A_cpp_base
{
    int buf[100];

    void A_cpp_base( void ){ memset( this , 0 , sizeof( *this ) ) };
    void method( void ){......};
    ........
};

struct A_c_base
{
    int buf[100];
};

struct A_cpp : public A_cpp_base
{
    int   a;
    int   b;
    int   d[100];

    void A_cpp( void ){ memset( this , 0 , sizeof( *this ) ) };
    void method( void ){......};
    ........
} A_cpp_object;

struct A_c{
    A_c_base publicFromACppBaseAdapted;
    int   a;
    int   b;
    int   d[100];
}A_c_object;

How can I copy the members of a A_cpp_object in a A_c_object as fast as possible?

I was thinking about something as:

    memcpy(  &A_c_object , &A_cpp_object , sizeof( A_c_object ) )

But I need to know where are placed the public components of the base class of the object that I need to copy.

Thank you for help,
BrownB

[Salem]

> I need to know where are placed in memory the methods of Cpp structures.
They're not stored in the structure at all, you only get one copy of the code no matter how many instances of the data you have (a good thing).

> memcpy( &A_c_object , &A_cpp_object , sizeof( A_c_object ) )
I think you're on thin ice here, though you're probably OK for this example. Push hard enough on the C++ side (say inheritance in your class) and the association will break.

Better to go with something like a specific class member function which does the appropriate memberwise copying to your C structure.

struct A_cpp{
    int   a;
    int   b;
    int   d[100];

    void A_cpp( void ){ memset( this , 0 , sizeof( *this ) ) };
    void method( void ){......};
    void copy_to_c ( struct A_c *c_copy );
    ........
} A_cpp_object;

void A_cpp_object::copy_to_c ( struct A_c *c_copy ) {
  c_copy->a = a;
  // etc etc
}

[BrownB]

> I need to know where are placed in memory the methods of Cpp structures.
They're not stored in the structure at all, you only get one copy of the code no matter how many instances of the data you have (a good thing).

I wrote in the wrong way, I was meaning "members", not "methods", my fault.

A list of questions, I hope "smart" questions...
What kind of assumptions can I do on a structure members location in a instance of that structure? I know that for "normal" structures is the same as in C: they follow the declariation sequence.

But what about DERIVED structures? Where will the base class members be placed? Is this a compiler issue?

Thank you for you help!
BrownB

PS: can somebody give me a site about "C++ adaptation to C"? I heard about "wrappers" but I'm not shure it's what I need..

[Prelude]

>What kind of assumptions can I do on a structure members location in a instance of that structure?
Very few. Like C structures, C++ classes also introduce padding between and after members for alignment purposes. Working with the individual bytes of an object (such as with memset) is flirting with danger. Remember that accessing uninitialized memory (like...padding, for instance ) is undefined.

>Is this a compiler issue?
That would be a good assumption.

>can somebody give me a site about "C++ adaptation to C"?
Can you be more specific?

[Salem]

> PS: can somebody give me a site about "C++ adaptation to C"?
http://www.parashift.com/c++-faq-lit...c-and-cpp.html

[CornedBee]

In fact, C++ only guarantees source-order -> memory-order for members of the same visibility.
That is, for this:

class A
{
  int a;
public:
  int b;
private:
  int c;
public:
  int d;
};

the order might be abcd, but it also might be acbd, or any other variation where a comes before c and b before d.

[BrownB]

> PS: can somebody give me a site about "C++ adaptation to C"?
http://www.parashift.com/c++-faq-lit...c-and-cpp.html

Thank you, I read it, and I now understand your hints. The only robust way is to do a member-to-member copy.

Regarding a class conversion, I'm doing this:

let's have a class A and a derived class Aderived:

calss A
{
    public:
       int buf[100];
       long count;

       void A( void ){};
       int Amethod1( params1 );
       long Amethod2( params2 );
}

calss Aderived : public A
{
    public:
       char string[10];
       long length;
}

In C code I'll use this, declared in the "interface" header:

struct A_c
{
       int buf[100];
       long count;
}

struct Aderived_c
{
        A_c  inherited;
       char string[10];
       long length;
}

while for the methods, I'll write them in an extended version, adding also a parameter as pointer to the structure which will be affected. Something like this:

       void A_create( A **pClass );    //void A( void ){};
       int A_Amethod1( A *pClass , params1 );        // Amethod1();
       long A_Amethod2( A *pClass , params2 );        // Amethod2();

Will there be something wrong?

Thank you very much, it's always a pleasure!
BrownB

[Thantos]

In C++ its call inhertience not conversion. When you public inhertient another class that class' public part is included in your public part, their protected part is included in your private part, and their private part means private.

What you showed with your C example is private inhertience / composition (technically composition). In this type of inhertience the class becomes a member of your class. You will only have access to the public part of that class. If you wish to allow outside access to that class you must either put it in your public part or put accessor functions in your public part.

[anonytmouse]

If you want code that can be called from C and also called from C++ in a C++ friendly class, your best bet would be to write your code in C and create a C++ wrapper class that calls into the C code. Just include the appropriate C struct as a member of your C++ class and delegate the class methods to C functions.

#if defined(__cplusplus)

class A
{
  private:
    struct A_c;

  public:
       A( )
       {
           A_create(&A_c); /* Delegate to C create function. */
       }

       ~A( )
       {
           A_destory(&A_c);
       }

       int Amethod1( int params1 )
       {
           return A_Amethod1(&A_c, params1);
       }

       long Amethod2( int params2 )
       {
           return A_Amethod2(&A_c, params2);
       }
};

#endif /* defined (__cplusplus)

The other alternative is to write your code in C++ and create a wrapper so it can be accessed from C. The disadvantage to this approach is that your code will need a C++ compiler to be compiled. With the C code/C++ wrapper solution, you can use defines so that the C++ wrapper is not compiled on a C compiler. The advantage is that you can leverage the benefits of C++ in your code.

Here is an example of a wrapper that enables a C++ class to be called from C code.

/* Create a handle type for your class. */
typedef void* CLS_HANDLE;

extern "C" void A_create(CLS_HANDLE* ppHandle)
{
   A* temp = new A;
   *pHandle = (CLS_HANDLE) temp;
}

extern "C" int A_Amethod1(CLS_HANDLE pHandle, params1 )
{
   A* temp = (A*) pHandle;
   return temp->Amethod1(params1);
}

extern "C" int A_Amethod2(CLS_HANDLE pHandle, params2 )
{
   A* temp = (A*) pHandle;
   return temp->Amethod2(params2);
}

extern "C" void A_destroy(CLS_HANDLE pHandle)
{
   A* temp = (A*) pHandle;
   delete temp;
}

[Thantos]

anonytmouse might I suggest the #ifdef preprocessor command. It seems to be the more preferred preproccessor command for seeing if a symbol has been defined yet. It also has the avantage of not requireing ()

[anonytmouse]

I used to use ifdef but changed to if defined because it is more consistent and as far as I can tell more common in new code. More discussion on this issue can be found at the following pages.

MSDN: The #ifdef and #ifndef Directives

Usenet Thread: #ifdef depreciated?

[CornedBee]

I use #ifndef for inclusion guards and #if defined for everything else.