Writing a library with C and C++ interfaces, which way to wrap?

Question

When preparing a library (let's call it libfoo), I find myself presented with the following dilemma: do I write it as a C++ library with a C wrapper:

namespace Foo {
  class Bar {
    ...
  };
}

/* Separate C header. #ifdef __cplusplus omitted for brevity. */
extern "C" {
  typedef void *FooBar;
  FooBar* foo_bar_new() { return new Foo::Bar; }
  void foo_bar_delete(FooBar *bar) { delete bar; }
}

Or is it better to write it as a C library with a C++ wrapper:

/* foo/bar.h. Again, #ifdef __cplusplus stuff omitted. */

typedef struct {
  /* ... */
} FooBar;

void foo_bar_init(FooBar *self) { /* ... */ }
void foo_bar_deinit(FooBar *self) { /* ... */ }

/* foo/bar.hpp */

namespace Foo {
  class Bar {
    /* ... */
    FooBar self;
  }

  Bar::Bar() {
    foo_bar_init(&self);
  }

  Bar::~Bar() {
    foo_bar_deinit(&self);
  }
}

Which do you prefer, and why? I favour the latter because it means I don't have to worry about my C functions accidentally having exceptions bubble up, plus I prefer C as a language as I feel that it's a smaller semantic minefield. What do other people think?

EDIT: So many good answers. Thanks all. It's a shame that I can only accept one.

Answer 1

Small points:

When you write C library it is useful anywhere - in C, in C++ (with wrapper) and many other languages like Python, Java using bindings etc and most important it requires only C runtime.

When you write C++ wrapper you also need to write a C wrapper, but it is not as simple as you think, for example:

c_api.h:

extern "C" {
  typedef void *Foo;
  Foo create_foo();
}

c_api.cpp:

void *create_foo() 
{
    return new foo::Foo();
}

What is wrong? it may throw! and the program will crash as C does not have stack unwinding semantics. So you need something like:

void *create_foo() 
{
    try {
       return new foo::Foo();
    }
    catch(...) { return 0; }
}

And this for every C++ api function.

So I think that writing a C library and providing a separate C++ wrapper is better solution.

Also it would not require linking with C++ runtime library.

Answer 2

Write the library in the language you prefer to write libraries in. It doesn't technically much matter which way you wrap. Although some C projects may aim to exclude libraries that aren't C whereas it'd be odd for a C++ project to exclude libraries written in C, that's mostly a philosophical objection than a practical one.

Wrapping C in a C++ wrapper will likely result in a slightly larger wrapper but be more acceptable to C programmers.

Note that if you are distributing binaries, C's simplicity is advantageous.