Copy a pair-shaped array value under JSON_BRACE_INIT_COPY_SEMANTICS

With JSON_BRACE_INIT_COPY_SEMANTICS enabled, single-element brace
initialization from a JSON value decided whether to copy the value or
build an object by inspecting the value's runtime shape: a two-element
array whose first element is a string, such as ["key", 42], was turned
into an object instead of being copied. This made the behavior depend
on the element's content, and it did not distinguish an existing value
of this shape from a nested braced pair written in the source, such as
the inner {"key", "value"} of {{"key", "value"}}.

json_ref now records whether it was constructed from a braced list
(true only for the std::initializer_list<json_ref> constructor used
for nested braced lists) or from a value. The initializer-list
constructor uses this to copy or move a single non-braced-list element
before deciding whether the list describes an object, so a JSON value
is always copied regardless of its shape, while a braced pair written
in the source still creates an object.

Fixes #5662.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/from_bjdata.md

@@ -111,12 +111,3 @@ Linear in the size of the input.
 - Added in version 3.11.0.
 - Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
 - Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
-
-!!! warning "Deprecation"
-
-    - Overload (2) replaces calls to `from_bjdata` with a pointer and a length as first two parameters, which has been
-      deprecated in version 3.13.0. This overload will be removed in version 4.0.0. Please replace all calls like
-      `#!cpp from_bjdata(ptr, len, ...);` with `#!cpp from_bjdata(ptr, ptr+len, ...);`.
-
-    You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated
-    function.

docs/mkdocs/docs/api/basic_json/from_bon8.md

@@ -106,12 +106,3 @@ Linear in the size of the input.
 ## Version history
 
 - Added in version 3.13.0.
-
-!!! warning "Deprecation"
-
-    - Overload (2) replaces calls to `from_bon8` with a pointer and a length as first two parameters, which has been
-      deprecated in version 3.13.0. This overload will be removed in version 4.0.0. Please replace all calls like
-      `#!cpp from_bon8(ptr, len, ...);` with `#!cpp from_bon8(ptr, ptr+len, ...);`.
-
-    You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated
-    function.

docs/mkdocs/docs/api/macros/json_brace_init_copy_semantics.md

@@ -50,9 +50,11 @@ The default value is `0` (disabled — existing behavior is preserved).
     ```
 
     Code that relies on these producing arrays must use `json::array()` instead (see below). Lists with more than one
-    element, and a single `[string, value]` pair such as `{{"key", "value"}}`, which still creates an object, are not
-    affected. The library's own conversions are not affected either: for example, `std::tuple<int>{5}` still becomes
-    `[5]`.
+    element, and a single `[string, value]` pair *written as a braced list*, such as `{{"key", "value"}}`, which still
+    creates an object, are not affected. This exception is based on how the pair is written, not on the shape of its
+    value: an existing JSON value that happens to be a two-element array with a string as its first element, such as
+    `json arr = {"key", 42};`, is still copied by `json j{arr};` rather than turned into an object. The library's own
+    conversions are not affected either: for example, `std::tuple<int>{5}` still becomes `[5]`.
 
 !!! note "ABI compatibility"
 

include/nlohmann/detail/json_ref.hpp

@@ -34,6 +34,7 @@ class json_ref
 
     json_ref(std::initializer_list<json_ref> init)
         : owned_value(init)
+        , braced_list(true)
     {}
 
     template <
@@ -69,9 +70,17 @@ class json_ref
         return &** this;
     }
 
+    /// whether the value was written as a braced list, such as {"key", 1},
+    /// rather than given as a value
+    bool is_braced_list() const noexcept
+    {
+        return braced_list;
+    }
+
   private:
     mutable value_type owned_value = nullptr;
     value_type const* value_ref = nullptr;
+    bool braced_list = false;
 };
 
 }  // namespace detail

include/nlohmann/json.hpp

@@ -1672,6 +1672,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                bool type_deduction = true,
                value_t manual_type = value_t::array)
     {
+#if JSON_BRACE_INIT_COPY_SEMANTICS
+        // a single element that is a value rather than a braced list is
+        // copied or moved as is, whatever its content looks like
+        if (type_deduction && init.size() == 1 && !init.begin()->is_braced_list())
+        {
+            *this = init.begin()->moved_or_copied();
+            set_parents();
+            assert_invariant();
+            return;
+        }
+#endif
+
         // check if each element is an array with two elements whose first
         // element is a string
         bool is_an_object = std::all_of(init.begin(), init.end(),
@@ -5547,16 +5559,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return result;
     }
 
-    template<typename T>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
-    JSON_HEDLEY_DEPRECATED_FOR(3.13.0, from_bjdata(ptr, ptr + len))
-    static basic_json from_bjdata(const T* ptr, std::size_t len,
-                                  const bool strict = true,
-                                  const bool allow_exceptions = true)
-    {
-        return from_bjdata(ptr, ptr + len, strict, allow_exceptions);
-    }
-
     /// @brief create a JSON value from an input in BON8 format
     /// @sa https://json.nlohmann.me/api/basic_json/from_bon8/
     template<typename InputType>
@@ -5594,16 +5596,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return result;
     }
 
-    template<typename T>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
-    JSON_HEDLEY_DEPRECATED_FOR(3.13.0, from_bon8(ptr, ptr + len))
-    static basic_json from_bon8(const T* ptr, std::size_t len,
-                                const bool strict = true,
-                                const bool allow_exceptions = true)
-    {
-        return from_bon8(ptr, ptr + len, strict, allow_exceptions);
-    }
-
     /// @brief create a JSON value from an input in BSON format
     /// @sa https://json.nlohmann.me/api/basic_json/from_bson/
     template<typename InputType>

single_include/nlohmann/json.hpp

@@ -20047,6 +20047,7 @@ class json_ref
 
     json_ref(std::initializer_list<json_ref> init)
         : owned_value(init)
+        , braced_list(true)
     {}
 
     template <
@@ -20082,9 +20083,17 @@ class json_ref
         return &** this;
     }
 
+    /// whether the value was written as a braced list, such as {"key", 1},
+    /// rather than given as a value
+    bool is_braced_list() const noexcept
+    {
+        return braced_list;
+    }
+
   private:
     mutable value_type owned_value = nullptr;
     value_type const* value_ref = nullptr;
+    bool braced_list = false;
 };
 
 }  // namespace detail
@@ -27753,6 +27762,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                bool type_deduction = true,
                value_t manual_type = value_t::array)
     {
+#if JSON_BRACE_INIT_COPY_SEMANTICS
+        // a single element that is a value rather than a braced list is
+        // copied or moved as is, whatever its content looks like
+        if (type_deduction && init.size() == 1 && !init.begin()->is_braced_list())
+        {
+            *this = init.begin()->moved_or_copied();
+            set_parents();
+            assert_invariant();
+            return;
+        }
+#endif
+
         // check if each element is an array with two elements whose first
         // element is a string
         bool is_an_object = std::all_of(init.begin(), init.end(),
@@ -31628,16 +31649,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return result;
     }
 
-    template<typename T>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
-    JSON_HEDLEY_DEPRECATED_FOR(3.13.0, from_bjdata(ptr, ptr + len))
-    static basic_json from_bjdata(const T* ptr, std::size_t len,
-                                  const bool strict = true,
-                                  const bool allow_exceptions = true)
-    {
-        return from_bjdata(ptr, ptr + len, strict, allow_exceptions);
-    }
-
     /// @brief create a JSON value from an input in BON8 format
     /// @sa https://json.nlohmann.me/api/basic_json/from_bon8/
     template<typename InputType>
@@ -31675,16 +31686,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return result;
     }
 
-    template<typename T>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
-    JSON_HEDLEY_DEPRECATED_FOR(3.13.0, from_bon8(ptr, ptr + len))
-    static basic_json from_bon8(const T* ptr, std::size_t len,
-                                const bool strict = true,
-                                const bool allow_exceptions = true)
-    {
-        return from_bon8(ptr, ptr + len, strict, allow_exceptions);
-    }
-
     /// @brief create a JSON value from an input in BSON format
     /// @sa https://json.nlohmann.me/api/basic_json/from_bson/
     template<typename InputType>

tests/src/unit-bjdata.cpp

@@ -4572,31 +4572,3 @@ TEST_CASE("BJData roundtrips" * doctest::skip())
         }
     }
 }
-
-TEST_CASE("issue #5648 - from_bjdata(ptr, len) must read len bytes, not treat ptr as a C string")
-{
-    // to_bjdata() encodes the integer 0 as the two bytes 'i' 0x00 (a BJData
-    // type marker followed by the value byte 0x00), so the packed data
-    // below contains a 0x00 byte before its end.
-    const json j = {{"a", 0}};
-    const std::vector<std::uint8_t> packed = json::to_bjdata(j);
-    bool contains_nul = false;
-    for (const auto byte : packed)
-    {
-        contains_nul |= (byte == 0x00);
-    }
-    REQUIRE(contains_nul);
-
-    // before the fix, from_bjdata had no (ptr, len) overload, so this call
-    // bound to from_bjdata(InputType&&, bool strict) instead: ptr was read
-    // as a NUL-terminated C string (stopping at the embedded 0x00 byte), and
-    // len was silently converted to the strict flag. The deprecated
-    // overload added for this issue forwards to from_bjdata(ptr, ptr + len,
-    // ...) instead, like from_ubjson's deprecated (ptr, len) overload does.
-    json result;
-    CHECK_NOTHROW(result = json::from_bjdata(packed.data(), packed.size()));
-    CHECK(result == j);
-
-    // len must not collapse into the strict flag either
-    CHECK(json::from_bjdata(packed.data(), packed.size(), false) == j);
-}

tests/src/unit-bon8.cpp

@@ -1010,34 +1010,6 @@ TEST_CASE("BON8 roundtrips" * doctest::skip())
     }
 }
 
-TEST_CASE("issue #5648 - from_bon8(ptr, len) must read len bytes, not treat ptr as a C string")
-{
-    // to_bon8() encodes the integer 40 as the two bytes 0xC2 0x00 (a BON8
-    // lead byte followed by a continuation byte of 0x00), so the packed data
-    // below contains a 0x00 byte before its end.
-    const json j = {{"a", 40}, {"b", "x"}};
-    const std::vector<std::uint8_t> packed = json::to_bon8(j);
-    bool contains_nul = false;
-    for (const auto byte : packed)
-    {
-        contains_nul |= (byte == 0x00);
-    }
-    REQUIRE(contains_nul);
-
-    // before the fix, from_bon8 had no (ptr, len) overload, so this call
-    // bound to from_bon8(InputType&&, bool strict) instead: ptr was read as
-    // a NUL-terminated C string (stopping at the embedded 0x00 byte), and
-    // len was silently converted to the strict flag. The deprecated
-    // overload added for this issue forwards to from_bon8(ptr, ptr + len,
-    // ...) instead, like from_cbor's deprecated (ptr, len) overload does.
-    json result;
-    CHECK_NOTHROW(result = json::from_bon8(packed.data(), packed.size()));
-    CHECK(result == j);
-
-    // len must not collapse into the strict flag either
-    CHECK(json::from_bon8(packed.data(), packed.size(), false) == j);
-}
-
 #ifdef JSON_HAS_CPP_17
 TEST_CASE("BON8 with std::byte")
 {

tests/src/unit-brace-init-copy-semantics.cpp

@@ -72,6 +72,28 @@ TEST_CASE("JSON_BRACE_INIT_COPY_SEMANTICS")
         CHECK(j7 == json::array({1, 2}));
     }
 
+    SECTION("single-element brace initialization copies a pair-shaped array value (#5662)")
+    {
+        // a JSON value that happens to be a 2-element array whose first
+        // element is a string must still be copied, not turned into an
+        // object; only a braced list written in the source, such as the
+        // inner {"key", "value"} of {{"key", "value"}}, describes an object
+        json const pair_shaped = json::array({"key", 42});
+
+        json const j1{pair_shaped};
+        CHECK(j1.is_array());
+        CHECK(j1 == pair_shaped);
+
+        json const j2 = {pair_shaped};
+        CHECK(j2.is_array());
+        CHECK(j2 == pair_shaped);
+
+        // the same holds for an rvalue of the same shape
+        json const j3{json::array({"key", 42})};
+        CHECK(j3.is_array());
+        CHECK(j3 == pair_shaped);
+    }
+
     SECTION("what the macro does not change")
     {
         // lists with more than one element are unaffected