C Struct inheritance?

C Struct inheritance?

[Kai Meyer]

I'm basically straight out of acadamia with a 4 year CS degree, so be 
gentle :)

I've been working adding a Linux port to a current Windows-only project, 
and I've run into what appears to be inheritance with C structs. I've 
distilled the problem down to the following code:

// Test for struct inheritance
#ifdef __cplusplus
struct base
#else
typedef struct _base
#endif
{
int b;
#ifdef __cplusplus
};
#else
} base;
#endif
#ifdef __cplusplus
struct derived : public base
{
#else
typedef struct _derived
{
base;
#endif
int d;
#ifdef __cplusplus
};
#else
} derived;
#endif
int main()
{
base b;
derived d;
b.b = 5;
d.b = b.b;
d.d = 10;
return 0;
}

It is apparent that the struct is to behave the same whether compiled in 
C or C++. I admit to being a bit confused to see Polymorphism in C. This 
code builds and runs in my WinDDK environment for windows, as well as 
for Linux g++, but Linux gcc gives me the following error:
struct_test.c:20: warning: declaration does not declare anything
struct_test.c: In function ‘main’:
struct_test.c:33: error: ‘derived’ has no member named ‘b’

That totally makes sense. The C I know doesn't have polymorphism.

So, I need to derive a solution that will be cross-platform, and 
hopefully not require any changes to existing code that uses these 
structs. I'm sort of holding my breath for a gcc option like 
"-magic_structs". Any help or general advice is welcome.

Re: C Struct inheritance?

[Kai Meyer]

After mailing the list, I showed a co-worker my post, and he spent all 
of 15 minutes tracking it down. MS extensions call it "Anonymous Structs":
http://msdn.microsoft.com/en-us/library/z2cx9y4f.aspx

GCC has a -fms-extensions option, which does then allow this behavior. 
Hurray! It appears to give me what I need for now.

-Kai Meyer

On 06/17/2011 10:33 AM, Kai Meyer wrote:
> I'm basically straight out of acadamia with a 4 year CS degree, so be 
> gentle :)
>
> I've been working adding a Linux port to a current Windows-only 
> project, and I've run into what appears to be inheritance with C 
> structs. I've distilled the problem down to the following code:
>
> // Test for struct inheritance
> #ifdef __cplusplus
> struct base
> #else
> typedef struct _base
> #endif
> {
> int b;
> #ifdef __cplusplus
> };
> #else
> } base;
> #endif
> #ifdef __cplusplus
> struct derived : public base
> {
> #else
> typedef struct _derived
> {
> base;
> #endif
> int d;
> #ifdef __cplusplus
> };
> #else
> } derived;
> #endif
> int main()
> {
> base b;
> derived d;
> b.b = 5;
> d.b = b.b;
> d.d = 10;
> return 0;
> }
>
> It is apparent that the struct is to behave the same whether compiled 
> in C or C++. I admit to being a bit confused to see Polymorphism in C. 
> This code builds and runs in my WinDDK environment for windows, as 
> well as for Linux g++, but Linux gcc gives me the following error:
> struct_test.c:20: warning: declaration does not declare anything
> struct_test.c: In function ‘main’:
> struct_test.c:33: error: ‘derived’ has no member named ‘b’
>
> That totally makes sense. The C I know doesn't have polymorphism.
>
> So, I need to derive a solution that will be cross-platform, and 
> hopefully not require any changes to existing code that uses these 
> structs. I'm sort of holding my breath for a gcc option like 
> "-magic_structs". Any help or general advice is welcome.