Question

Is there any way to force a compiler (well GCC specifically) to make a class compile to object oriented C? Specifically what I want to achieve is to write this:

class Foo {
public:
  float x, y, z;
  float bar();
  int other();
  ...etc
};

Foo f;
float result = f.bar()
int obSize = sizeof(Foo);

Yet compile to exactly the same as:

Struct Foo { float x, y, z; };
float Foo_bar(Foo *this);

Foo f;
float result = Foo_bar(&f);
int obSize = sizeof(Foo);

My motivation is to increase readability, yet not suffer a memory penalty for each object of Foo. I’d imagine the class implementation normally obSize would be

obSize = sizeof(float)*3 + sizeof(void*)*number_of_class_methods

Mostly to use c++ classes in memory constrained microcontrollers. However I’d imagine if I got it to work I’d use it for network serialization as well (on same endian machines of course).

Answer 1

Your compiler actually does exactly that for you. It might even be able to optimize optimistically by putting the this pointer in a register instead of pushing it onto the stack (this is at least what MSVC does on Windows), which you wouldn’t be able to do with standard C calling convention.

As for:

obSize = sizeof(float)*3 + sizeof(void*)*number_of_class_methods

1. It is plain false. Did you even try it ?
2. Even if you had virtual functions, only one pointer to a table of functions would be added to each object (one table per class). With no virtual functions, nothing is added to an object beyond its members (and no function table is generated).
3. void* represents a pointer to data, not a pointer to code (they need not have the same size)
4. There is no guarantee that the size of the equivalent C struct is 3 * sizeof(float).