Re: [v3 PATCH] Implement 2801, Default-constructibility of unique_ptr.

[Jonathan Wakely <jwakely at redhat dot com>]

On 20/12/16 22:52 +0200, Ville Voutilainen wrote:
> diff --git a/libstdc++-v3/include/bits/unique_ptr.h b/libstdc++-v3/include/bits/unique_ptr.h
> index 56e6ec0..63dff37 100644
> --- a/libstdc++-v3/include/bits/unique_ptr.h
> +++ b/libstdc++-v3/include/bits/unique_ptr.h
> @@ -175,10 +175,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       // Constructors.
>
>       /// Default constructor, creates a unique_ptr that owns nothing.
> +      template <typename _Up = deleter_type,
> +		typename enable_if<
> +		  __and_<__not_<is_pointer<_Up>>,
> +			 is_default_constructible<_Up>>::value,
> +		  bool>::type = false>

Instead of repeating this condition half a dozen times, we could put
it in the __uniq_ptr_impl class template and reuse it, as in the
attached patch (and similarly for the unique_ptr<t[]> specialization).
What do you think?

>       constexpr unique_ptr() noexcept
>       : _M_t()
> -      { static_assert(!is_pointer<deleter_type>::value,
> -		     "constructed with null function pointer deleter"); }
> +      { }

The bodies of these constructors should be indented now that they're
templates.


> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/unique_ptr/cons/default.cc
> @@ -0,0 +1,40 @@
> +// { dg-do compile { target c++11 } }
> +
> +// Copyright (C) 2011-2016 Free Software Foundation, Inc.

Is this substantially copied from an existing file, or should it just
be 2016? (Not that it really matters, as I don't think we should have
copyright notices on tests like this at all, but that's something to
worry about another time.

diff --git a/libstdc++-v3/include/bits/unique_ptr.h b/libstdc++-v3/include/bits/unique_ptr.h
index 56e6ec0..f9ab9a6 100644
--- a/libstdc++-v3/include/bits/unique_ptr.h
+++ b/libstdc++-v3/include/bits/unique_ptr.h
@@ -130,6 +130,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	};
 
     public:
+      using _DeleterConstraint = enable_if<
+	__and_<__not_<is_pointer<_Dp>, is_default_constructible<_Dp>>>::value>;
+
       using pointer = typename _Ptr<_Tp, _Dp>::type;
 
       __uniq_ptr_impl() = default;
@@ -152,6 +155,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template <typename _Tp, typename _Dp = default_delete<_Tp>>
     class unique_ptr
     {
+      using _DeleterConstraint = __uniq_ptr_impl<_Tp, _Dp>::_DeleterConstraint;
+
       __uniq_ptr_impl<_Tp, _Dp> _M_t;
 
     public:
@@ -175,10 +180,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // Constructors.
 
       /// Default constructor, creates a unique_ptr that owns nothing.
-      constexpr unique_ptr() noexcept
-      : _M_t()
-      { static_assert(!is_pointer<deleter_type>::value,
-		     "constructed with null function pointer deleter"); }
+      template<typename = typename _DeleterConstraint::type>
+	constexpr unique_ptr() noexcept
+	: _M_t()
+	{ }
 
       /** Takes ownership of a pointer.
        *
@@ -186,11 +191,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        * The deleter will be value-initialized.
        */
-      explicit
-      unique_ptr(pointer __p) noexcept
-      : _M_t(__p)
-      { static_assert(!is_pointer<deleter_type>::value,
-		     "constructed with null function pointer deleter"); }
+      template<typename = typename _DeleterConstraint::type>
+	explicit
+	unique_ptr(pointer __p) noexcept
+	: _M_t(__p)
+	{ }
 
       /** Takes ownership of a pointer.
        *
@@ -218,7 +223,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		      "rvalue deleter bound to reference"); }
 
       /// Creates a unique_ptr that owns nothing.
-      constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }
+      template<typename = typename _DeleterConstraint::type>
+	constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }
 
       // Move constructors.