null_resource is a singleton

doc/qbk/02_07_allocators.qbk

@@ -158,6 +158,14 @@ available when using the library:
 [table Functions and Types
 [ [Name] [Description] ]
 [
+    [__get_null_resource__]
+    [
+        Returns a pointer to a memory resource instance which
+        always throws an exception upon allocation. This is
+        used to to achieve the invariant that no parsing or
+        container operation will dynamically allocate memory.
+    ]
+][
     [__is_deallocate_trivial__]
     [
         A customization point allowing a memory resource type
@@ -182,13 +190,6 @@ available when using the library:
         freed until the resource is destroyed, making it fast for
         parsing but not suited for performing modifications.
     ]
-][
-    [__null_resource__]
-    [
-        A memory resource always throws an exception upon allocation.
-        This is used to to achieve the invariant that no parsing
-        or container operation will dynamically allocate memory.
-    ]
 ][
     [__polymorphic_allocator__]
     [
@@ -285,11 +286,12 @@ To use the resource, construct it with a local buffer:
 
 [heading Null Resource]
 
-The __null_resource__ can only be default constructed. It offers a simple
-invariant: all calls to allocate will throw the exception `std::bad_alloc`.
-An instance of the null resource can be used to make parsing guarantee
-that allocations from the heap are never made. This is explored in more
-detail in a later section.
+The function __get_null_resource__ returns a global instance of the
+null resource. This resource offers a simple invariant: all calls to
+allocate will throw the exception `std::bad_alloc`. An instance of
+the null resource can be used to make parsing guarantee that
+allocations from the heap are never made. This is explored
+in more detail in a later section.
 
 [/-----------------------------------------------------------------------------]
 

doc/qbk/main.qbk

@@ -52,6 +52,7 @@
 [def __error_code__             [link json.ref.boost__json__error_code `error_code`]]
 [def __error_condition__        [link json.ref.boost__json__error_condition `error_condition`]]
 [def __get__                    [link json.ref.boost__json__get `get`]]
+[def __get_null_resource__      [link json.ref.boost__json__get_null_resource `get_null_resource`]]
 [def __has_value_from__         [link json.ref.boost__json__has_value_from `has_value_from`]]
 [def __has_value_to__           [link json.ref.boost__json__has_value_to `has_value_to`]]
 [def __is_deallocate_trivial__  [link json.ref.boost__json__is_deallocate_trivial `is_deallocate_trivial`]]
@@ -60,7 +61,6 @@
 [def __make_counted_resource__  [link json.ref.boost__json__make_counted_resource `make_counted_resource`]]
 [def __memory_resource__        [link json.ref.boost__json__memory_resource `memory_resource`]]
 [def __monotonic_resource__     [link json.ref.boost__json__monotonic_resource `monotonic_resource`]]
-[def __null_resource__          [link json.ref.boost__json__null_resource `null_resource`]]
 [def __number_cast__            [link json.ref.boost__json__number_cast `number_cast`]]
 [def __object__                 [link json.ref.boost__json__object `object`]]
 [def __parse__                  [link json.ref.boost__json__parse `parse`]]

doc/qbk/quickref.xml

@@ -24,7 +24,6 @@
           <member><link linkend="json.ref.boost__json__basic_parser">basic_parser</link></member>
           <member><link linkend="json.ref.boost__json__key_value_pair">key_value_pair</link></member>
           <member><link linkend="json.ref.boost__json__monotonic_resource">monotonic_resource</link></member>
-          <member><link linkend="json.ref.boost__json__null_resource">null_resource</link></member>
           <member><link linkend="json.ref.boost__json__object">object</link></member>
           <member><link linkend="json.ref.boost__json__parser">parser</link></member>
           <member><link linkend="json.ref.boost__json__parse_options">parse_options</link></member>
@@ -41,6 +40,7 @@
         <bridgehead renderas="sect3">Functions</bridgehead>
         <simplelist type="vert" columns="1">
           <member><link linkend="json.ref.boost__json__get">get</link></member>
+          <member><link linkend="json.ref.boost__json__get_null_resource">get_null_resource</link></member>
           <member><link linkend="json.ref.boost__json__make_counted_resource">make_counted_resource</link></member>
           <member><link linkend="json.ref.boost__json__number_cast">number_cast</link></member>
           <member><link linkend="json.ref.boost__json__parse">parse</link></member>

fuzzing/fuzz_parser.cpp

@@ -67,8 +67,7 @@ struct FuzzHelper {
     void useDynLess() {
         // this is on the heap because the size is chosen dynamically
         std::unique_ptr<unsigned char[]> temp(new unsigned char[memlimit1]);
-        null_resource nr;
-        parser p(&nr,
+        parser p(get_null_resource(),
                  opt,
                  temp.get(),
                  memlimit1);

include/boost/json/impl/null_resource.ipp

@@ -15,32 +15,85 @@
 
 BOOST_JSON_NS_BEGIN
 
-void*
-null_resource::
-do_allocate(
-    std::size_t,
-    std::size_t)
-{
-    detail::throw_bad_alloc(
-        BOOST_CURRENT_LOCATION);
-}
+namespace detail {
 
-void
-null_resource::
-do_deallocate(
-    void*,
-    std::size_t,
-    std::size_t)
-{
-    // do nothing
-}
+/** A resource which always fails.
 
-bool
-null_resource::
-do_is_equal(
-    memory_resource const& mr) const noexcept
+    This memory resource always throws the exception
+    `std::bad_alloc` in calls to `allocate`.
+*/
+class null_resource final
+    : public memory_resource
+{   
+public:
+    /// Copy constructor (deleted)
+    null_resource(
+        null_resource const&) = delete;
+
+    /// Copy assignment (deleted)
+    null_resource& operator=(
+        null_resource const&) = delete;
+
+    /** Destructor
+
+        This destroys the resource.
+
+        @par Complexity
+        Constant.
+
+        @part Exception Safety
+        No-throw guarantee.
+    */
+    ~null_resource() noexcept = default;
+
+    /** Constructor
+
+        This constructs the resource.
+
+        @par Complexity
+        Constant.
+
+        @par Exception Safety
+        No-throw guarantee.
+    */
+    /** @{ */
+    null_resource() noexcept = default;
+
+protected:
+    void*
+    do_allocate(
+        std::size_t,
+        std::size_t) override
+    {
+        detail::throw_bad_alloc(
+            BOOST_CURRENT_LOCATION);
+    }
+
+    void
+    do_deallocate(
+        void*,
+        std::size_t,
+        std::size_t) override
+    {
+        // do nothing
+    }
+
+    bool
+    do_is_equal(
+        memory_resource const& mr
+            ) const noexcept override
+    {
+        return this == &mr;
+    }
+};
+
+} // detail
+
+memory_resource*
+get_null_resource() noexcept
 {
-    return this == &mr;
+    static detail::null_resource mr;
+    return &mr;
 }
 
 BOOST_JSON_NS_END

include/boost/json/null_resource.hpp

@@ -15,77 +15,20 @@
 
 BOOST_JSON_NS_BEGIN
 
-//----------------------------------------------------------
-
-/** A resource which always fails.
+/** Return a pointer to the null resource.
 
     This memory resource always throws the exception
     `std::bad_alloc` in calls to `allocate`.
+
+    @par Complexity
+    Constant.
+
+    @par Exception Safety
+    No-throw guarantee.
 */
-class BOOST_JSON_CLASS_DECL
-    null_resource final
-    : public memory_resource
-{   
-public:
-    /// Copy constructor (deleted)
-    null_resource(
-        null_resource const&) = delete;
-
-    /// Copy assignment (deleted)
-    null_resource& operator=(
-        null_resource const&) = delete;
-
-    /** Destructor
-
-        This destroys the resource.
-
-        @par Complexity
-        Constant.
-
-        @part Exception Safety
-        No-throw guarantee.
-    */
-    ~null_resource() noexcept = default;
-
-    /** Constructor
-
-        This constructs the resource.
-
-        @par Complexity
-        Constant.
-
-        @par Exception Safety
-        No-throw guarantee.
-    */
-    /** @{ */
-    null_resource() noexcept = default;
-
-protected:
-#ifndef BOOST_JSON_DOCS
-    void*
-    do_allocate(
-        std::size_t n,
-        std::size_t align) override;
-
-    void
-    do_deallocate(
-        void* p,
-        std::size_t n,
-        std::size_t align) override;
-
-    bool
-    do_is_equal(
-        memory_resource const& mr
-            ) const noexcept override;
-#endif
-};
-
-template<>
-struct is_deallocate_trivial<
-    null_resource>
-{
-    static constexpr bool value = true;
-};
+BOOST_JSON_DECL
+memory_resource*
+get_null_resource() noexcept;
 
 BOOST_JSON_NS_END
 

include/boost/json/value.hpp

@@ -1053,7 +1053,14 @@ public:
         return *this = value(init, storage());
     }
 
-    /** Assignment.
+    /** Assignment
+
+        Assigns `t` to `*this`.
+
+        @par Effects
+        @code
+        *this = value( std::forward<T>(t), this->storage() );
+        @endcode
 
         @par Constraints
         @code

test/doc_parsing.cpp

@@ -195,9 +195,11 @@ parser p(
 template< class Handler >
 void do_rpc( string_view s, Handler&& handler )
 {
-    null_resource mr1;                          // The null resource guarantees we will never dynamically allocate
     unsigned char temp[ 4096 ];                 // The parser will use this storage for its temporary needs
-    parser p( &mr1, parse_options(), temp );    // Construct a strict parser using the temp buffer and no dynamic memory
+    parser p(                                   // Construct a strict parser using the temp buffer and no dynamic memory
+        get_null_resource(),                    // The null resource guarantees we will never dynamically allocate
+        parse_options(),                        // Default constructed parse options allow only standard JSON
+        temp );
 
     unsigned char buf[ 16384 ];                 // Now we need a buffer to hold the actual JSON values
     static_resource mr2( buf );                 // The static resource is monotonic, using only a caller-provided buffer

test/null_resource.cpp

@@ -10,6 +10,8 @@
 // Test that header file is self-contained.
 #include <boost/json/null_resource.hpp>
 
+#include <boost/json/storage_ptr.hpp>
+
 #include "test_suite.hpp"
 
 BOOST_JSON_NS_BEGIN
@@ -20,17 +22,15 @@ public:
     void
     test()
     {
-        null_resource mr;
+        auto& mr = *get_null_resource();
         BOOST_TEST_THROWS(
             mr.allocate(16),
             std::bad_alloc);
         char buf[128];
         // no-op
         mr.deallocate(&buf[0], 128);
-
-        BOOST_TEST(mr == mr);
-        null_resource mr2;
-        BOOST_TEST(mr != mr2);
+        BOOST_TEST(
+            mr == *get_null_resource());
     }
 
     void