How can I forward declare a struct and have more than two implementation for that

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hi all
I have a struct that is used in its following class
and my class is base of two other class
I want to know if I can forward declare the struct and have different definition of that in my two child header

What I have tried:

smilar to this

//base header
struct forw;
class Base
{
    
}
//header child 1
struct forw
{
   int a;
   char b;
}
class child1::Base
{
}
//header child2

//header child 1
struct forw
{
   int a;
   int c;
}
class child2::Base
{
}

Posted 29-Dec-18 19:23pm

saide_a

Updated 29-Dec-18 19:52pm

v2

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KarstenK 30-Dec-18 4:19am

Work with different struct and with pointers to them. It is better to be precise, as Griff told. Or you will learn it "the hard way" (by having strange bugs). ;-)

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OriginalGriff · Answer 1 · 2018-12-29T19:53:00

Solution 1

Not a good idea, not at all.
Yes you can do it, provided the two child classes are in separate files. But ... it's a very bad idea as the two structs may have different "physical" sizes (in this case, they don't, since the first one will be "rounded up" to a multiple of 32 bits anyway). And when you have different sizes, you will start messing with data that is outside the struct when you use it. That can lead to very unpredictable results, from random data corruption to application crashes.

Don't do it. Just because you can do something, doesn't mean you should...

Posted 29-Dec-18 19:53pm

OriginalGriff

Comments

saide_a 30-Dec-18 1:58am

actually I want that different physical size

OriginalGriff 30-Dec-18 2:03am

No, you don't - it'a bad idea. If you want different sizes, then call them different things. Otherwise you will get unpredictable effects going on, and they are very hard to work out and fix later on.

Why do you think you want that?

saide_a 30-Dec-18 2:26am

I have a packet and my packet implementation is different in my child classes
and, in my base class I used this packet too,
then I am going to define it in this way
#define FRAME_PACKET_TYPE(buff) (*((uint8_t*)&buff[0]))
thanks

OriginalGriff 30-Dec-18 2:37am

Don't - use a union in your struct instead:
https://www.tutorialspoint.com/cprogramming/c_unions.htm
That way, the memory allocation required for the struct will always be correct: it will be the largest allocation it needs regardless of where you create it.

It's also a whole load less confusing when you read it back for maintenance in six months time...