Re: [PATCH] 77864 Fix noexcept conditions for map/set default constructors

[Jonathan Wakely <jwakely at redhat dot com>]

On 06/10/16 22:17 +0200, François Dumont wrote:
> Another approach is to rely on existing compiler ability to compute 
> conditional noexcept when defaulting implementations. This is what I 
> have done in this patch.
>
> The new default constructor on _Rb_tree_node_base is not a problem as 
> it is not used to build _Rb_tree_node.

Why not?

> I'll try to do the same for copy constructor/assignment and move 
> constructor/assignment.

We need to make sure we don't change whether any of those operations
are trivial (which shouldn't be a problem for copy/move, because they
are definitely very non-trivial and will stay that way!)

Does this change the default constructors from non-trivial to trivial?


> --- a/libstdc++-v3/include/bits/stl_tree.h
> +++ b/libstdc++-v3/include/bits/stl_tree.h
> @@ -108,6 +108,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     _Base_ptr		_M_left;
>     _Base_ptr		_M_right;
>
> +    _Rb_tree_node_base() _GLIBCXX_NOEXCEPT
> +      : _M_color(_S_red), _M_parent(0), _M_left(this), _M_right(this)
> +    { }
> +
>     static _Base_ptr
>     _S_minimum(_Base_ptr __x) _GLIBCXX_NOEXCEPT
>     {

Another option would be:

   struct _Head_node : _Rb_tree_node_base {
     _Head_node() {
       _M_color = _S_red;
       _M_parent = _Base_ptr();
       _M_left = _M_right = this;
     }
   };

> @@ -603,23 +607,33 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         {
> 	  _Key_compare		_M_key_compare;
> 	  _Rb_tree_node_base 	_M_header;

         _Head_node _M_header;

That way *only* this node gets the zero-initialization, not all node
bases.

With either solution we can get rid of _M_header() in every
ctor-initializer-list.


> +#if __cplusplus < 201103L
> 	  size_type 		_M_node_count; // Keeps track of size of tree.
> +#else
> +	  size_type 		_M_node_count = 0; // Keeps track of size of tree.
> +#endif
>
> +#if __cplusplus < 201103L
> 	  _Rb_tree_impl()
> 	  : _Node_allocator(), _M_key_compare(), _M_header(),
> 	    _M_node_count(0)
> -	  { _M_initialize(); }
> +	  { }
> +#else
> +	  _Rb_tree_impl() = default;
> +#endif
>
> 	  _Rb_tree_impl(const _Key_compare& __comp, const _Node_allocator& __a)
> -	  : _Node_allocator(__a), _M_key_compare(__comp), _M_header(),
> -	    _M_node_count(0)
> -	  { _M_initialize(); }
> +	  : _Node_allocator(__a), _M_key_compare(__comp), _M_header()
> +#if __cplusplus < 201103L
> +	  , _M_node_count(0)
> +#endif

Doing this conditionally seems pointless, why not just set it here
unconditionally?