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Re: [PATCH] 77864 Fix noexcept conditions for map/set default constructors
On 08/10/16 22:55 +0200, François Dumont wrote:
On 06/10/2016 23:34, Jonathan Wakely wrote:
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?
_Rb_tree_node_base is used in 2 context. As member of _Rb_tree_impl in
which case we need the new default constructor. And also as base class
of _Rb_tree_node which is never constructed. Nodes are being allocated
and then associated value is being constructed through the allocator,
the node default constructor itself is never invoked.
In C++03 mode that is true, but it's only valid because the type is
trivially-constructible. If the type requires "non-vacuous
initialization" then it's not valid to allocate memory for it and
start using it without invoking a constructor. If you add a
non-trivial constructor then we can't do that any more.
In C++11 and later, see line 550 in <bits/stl_tree.h>
::new(__node) _Rb_tree_node<_Val>;
This default-constructs a tree node. Currently there is no
user-provided default constructor, so default-construction does no
initialization. Adding your constructor would mean it is used for
every node.
If you think it is cleaner to create an intermediate type that
will take care of this initialization through its default constructor
I can do that.
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?
It would be a major compiler bug if making a constructor default was
making it trivial.
I must be misunderstanding you, because this is not a bug:
#include <type_traits>
struct A {
A() { }
};
static_assert( !std::is_trivially_default_constructible<A>::value, "" );
struct B {
B() = default;
};
static_assert( std::is_trivially_default_constructible<B>::value, "" );