[cfe-dev] Building the Bullet Physics SDK with Clang: Success!

[Douglas Gregor]

On Oct 1, 2010, at 11:01 PM, Ryan Gerleve wrote:

> Mostly just for fun, I decided to try compiling the Bullet Physics SDK
> (www.bulletphysics.org) with clang. I am happy to report that aside
> from a very small modification to the Bullet code, clang built the entire
> SDK (libraries and demos) with no problem!
> 
> The small refactor was necessary because the soft body library
> tries to use static const variables to zero-initialize certain objects.
> The objects happen to be structs, without a user-defined constructor,
> derived from another struct with a user-defined default constructor.
> The reduced test case is
> 
> struct Base {
>   Base() {}
> };
> 
> struct Derived: Base {
> };
> 
> static const Derived d;
> 
> Clang says, "error: default initialization of an object of const
> type 'const Derived' requires a user-provided default constructor."
> G++ accepts this code.
> 
> Looking at the C++03 standard, I'm a bit confused. Section 8.5 paragraph 6
> says that "Every object of static storage duration shall be zero-initialized at
> program startup before any other initialization takes place." But it also says
> in paragraph 9 of the same section that "If no initializer is specified for an
> object, ... if the object is of const-qualified type, the underlying class type
> shall have a user-declared default constructor."
> 
> My intuition tells me that static objects get zero-initialized first,
> then default-constructed a bit later, all during program startup. Is this the
> correct interpretation of the standard?

Yes.

> If so, it sounds like it goes against
> the C++ principle of not paying for things you don't use. Why zero-init an
> object if it's just going to be default-initialized immediately after?

Default-initialization doesn't necessarily set anything to zero. Besides, the zero-initialization for static objects tends to be free.

> Of course, if this is the case, then clang is correct, and g++ & Bullet are
> not standard-conforming.

Yes, Clang is correct to diagnose the requirement in p9 that a const-qualified object of class type have a user-declared default constructor.

	- Doug