How to define some kind of inheritance between structs in C and C++?

Question

Talking about structs, is it possible to just copy a struct's fields to another struct without explicitly calling the type field name?

Let me show you an example:

struct StructA
{
   char Name[20];
   int Age;
};

struct StructB
{
   StructA FieldStructA;

   int SomeOtherDeclarations;
   // ...
};

So, I could access the StructA fields on StructB doing this:

StructB strB;
strB.FieldStructA.Name[0] = 0;

What I want to do is access the StructA fields through StructB without accessing the data field on StructB. Something like inheritance between classes. Like this:

StructB strB;
strB.Name[0] = 0;

I want to inherit StructA fields on StructB. I know that I could do that with classes, but I have to use struct for some reasons (interop, specific use of stack and so on).

Thanks for your time!

In C++ classes and structs are the same thing except for the default visibility of fields. Everything you can do with a class, you can also do with a struct. — Sylence
– Sylence, Commented Aug 15, 2012 at 19:47
If you settle on C++ there is inheritance for structs, just like for classes. struct B : A { }; works perfectly well. — Bo Persson
– Bo Persson, Commented Aug 15, 2012 at 19:51
Your answer is in the question title. If you are talking about C++. You should pick one of the two languages, really. — juanchopanza
– juanchopanza, Commented Aug 15, 2012 at 19:51
@Sylence: Another difference is that the inheritance of structs is public by default (it's private for classes). — Frerich Raabe
– Frerich Raabe, Commented Aug 15, 2012 at 20:03
How about writing C++ instead of C, i.e., std::string name;. Even in C I would probably use a dynamically allocated chunk of memory instead of a fixed size array. How would you possibly account for Mr. Frued Von Bissmark III? — Ed Swangren
– Ed Swangren, Commented Aug 15, 2012 at 20:06

Greg Hewgill · Accepted Answer · 2012-08-15 19:49:33Z

5

You can do this in C++ using inheritance:

struct StructB: public StructA
{
   int SomeOtherDeclarations;
   // ...
};

Given that, you can then do

StructB strB;
strB.Name[0] = 0;

answered Aug 15, 2012 at 19:49

Greg Hewgill

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4 Comments

Frerich Raabe Over a year ago

For what it's worth, the default inheritance for structs is public already - so you could omit that if you like.

Cubic Over a year ago

I'd prefer always being explicit about it. It'll just end up confusing people otherwise.

David Grayson Over a year ago

I think this is the best answer for C++ and mine is the best answer for C. Too bad the OP asked two questions in one.

André Oliveira Over a year ago

The problem of doing this is that with inheritance I can only set the base struct fields starting at offset 0. What if I have to put those fields at offset 200h for example? Thanks btw for the reply.

David Grayson · Accepted Answer · 2012-08-15 20:04:02Z

1

If you are using gcc, try making it an anonymous member:

http://gcc.gnu.org/onlinedocs/gcc/Unnamed-Fields.html

struct StructA
{
  char Name[20];
  int Age;
};

struct StructB
{
   struct StructA;

   int SomeOtherDeclarations;
   // ...
};

answered Aug 15, 2012 at 20:04

David Grayson

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1 Comment

Johannes Schaub - litb Over a year ago

There is no need for GCC. C11 officially has that feature. :)

Joe · Accepted Answer · 2012-08-15 20:13:25Z

In plain C, you have 2 choices as I see it:

1 - define those fields with a macro. Then you can use them in two structs

#define PERSON_FIELDS \
char Name[20]; \
int Age; \

struct StructA
{
STRUCT_A_FIELDS
};

struct StructB
{
   STRUCT_A_FIELDS

   int thing;
   // ...
};

and you can then use

StructB strB;
strB.Name[0] = 0;


StructA strA;
strA.Name[0] = 0;

In this case, StructA and StructB have no connection to each other. But you have the same fields. This gives you at least a bit of modularisation. Macros like this are commonly used to achieve/fake things that newer languages give you (such as inheritance or genericity).

2 - Alternatively you could keep your above code but use accessor functions to get the values:

struct StructA
{
   char Name[20];
   int Age;
};

struct StructB
{
   StructA FieldStructA;

   int thing;
   // ...
};


int StructB_Thing(struct Struct *obj)
{
    return obj->thing;
}

int StructB_Age(struct Struct *obj)
{
    return obj->FieldStructA.Age;
}

Frerich Raabe · Accepted Answer · 2012-08-15 20:00:29Z

In C++, inheritance is used for interface re-use, not implementation reuse. You can easily get the latter C, but the former requires being aware of a little trick:

struct StructA
{
   char Name[20];
   int Age;
};

struct StructB
{
   StructA FieldStructA;

   int SomeOtherDeclarations;
   // ...
};

The trick is to understand that you can do

void setAge( struct StructA *s, int age ) {
    s->Age = age;
}

struct StructB b;
setAge( (struct StructA *)&b, 33 );
/* b.FieldStructA.Age is now 33 */

Note how you don't have to mention FieldStructA anywhere here: the code works because the 'base' fields are actually the first members of the 'derived' struct. So a value of type struct StructB * can be used in places where you access fields via a struct StructA *. This is the interface reuse: the same set of functions used for accessing StructA can be used for accessing the same fields in a StructB.

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