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fix: treat single-element brace-init as copy/move instead of wrapping in array (#5074) (#5090)

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* fix: treat single-element brace-init as copy/move

When passing a json value using brace initialization with a single element
(e.g., `json j{someObj}` or `foo({someJson})`), C++ always prefers the
initializer_list constructor over the copy/move constructor. This caused
the value to be unexpectedly wrapped in a single-element array.

This bug was previously compiler-dependent (GCC wrapped, Clang did not),
but Clang 20 started matching GCC behavior, making it a universal issue.

Fix: In the initializer_list constructor, when type deduction is enabled
and the list has exactly one element, copy/move it directly instead of
creating a single-element array.

Before:
  json obj = {{"key", 1}};
  json j{obj};   // -> [{"key":1}]  (wrong: array)
  foo({obj});    // -> [{"key":1}]  (wrong: array)

After:
  json j{obj};   // -> {"key":1}   (correct: copy)
  foo({obj});    // -> {"key":1}   (correct: copy)

To explicitly create a single-element array, use json::array({value}).

Fixes the issue #5074

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* fix: regenerate amalgamated single_include/nlohmann/json.hpp

- Add missing comment from include/nlohmann/json.hpp explaining the
  single-element brace-init fix (issue #5074)
- Fix extra 4-space indentation in embedded json_fwd.hpp section

Regenerated by running: make amalgamate

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* Revert brace-init semantics change and fix amalgamation

The single-element brace-init change was a breaking change that cannot be accepted upstream. Reverted all related source, test, and doc changes, then regenerated single_include with correct indentation to pass the amalgamation CI check.

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* Fix: add JSON_BRACE_INIT_COPY_SEMANTICS opt-in macro for issue #5074

Single-element brace initialization wrapping in an array cannot be fixed without breaking existing code. Added JSON_BRACE_INIT_COPY_SEMANTICS as an opt-in macro (default 0) so users can enable copy/move semantics for single-element brace init without affecting anyone relying on the current behavior.

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* docs: add dedicated macro page and CI test target for JSON_BRACE_INIT_COPY_SEMANTICS

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* fix: remove compiler-dependent assertions from #5074 regression test

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* fix: use defined() guard for JSON_BRACE_INIT_COPY_SEMANTICS to satisfy -Wundef

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* docs: fix section name in json_brace_init_copy_semantics.md to pass style check

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

* docs: move Default definition section before Notes to fix style check order

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>

---------

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>
This commit is contained in:
SamareshSingh
2026-05-08 01:20:24 -05:00
committed by GitHub
parent fd17b0889e
commit 62f3b41b30
9 changed files with 196 additions and 0 deletions
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tests/src/unit-regression2.cpp
+37
View File
@@ -1203,4 +1203,41 @@ TEST_CASE_TEMPLATE("issue #4798 - nlohmann::json::to_msgpack() encode float NaN
CHECK(json::from_cbor(cbor_z_3).get<T>() == -std::numeric_limits<T>::infinity());
}
TEST_CASE("regression test #5074 - portable workaround for single-element brace init")
{
json const j_obj = {{"key", "value"}};
json const j = json::array({j_obj});
CHECK(j.is_array());
CHECK(j.size() == 1);
CHECK(j[0] == j_obj);
}
#if defined(JSON_BRACE_INIT_COPY_SEMANTICS) && (JSON_BRACE_INIT_COPY_SEMANTICS == 1)
TEST_CASE("regression test #5074 - single-element brace init with JSON_BRACE_INIT_COPY_SEMANTICS")
{
// with JSON_BRACE_INIT_COPY_SEMANTICS: single-element brace init copies/moves
json const j_obj = {{"key", "value"}, {"num", 42}};
json const j_arr = {1, 2, 3};
// object: brace init copies instead of wrapping
json const j1{j_obj};
CHECK(j1.is_object());
CHECK(j1 == j_obj);
// array: brace init copies instead of wrapping
json const j2{j_arr};
CHECK(j2.is_array());
CHECK(j2.size() == 3);
CHECK(j2 == j_arr);
// primitives still work as initializer lists
json const j3{true};
CHECK(j3.is_boolean());
json const j4{42};
CHECK(j4.is_number_integer());
}
#endif
DOCTEST_CLANG_SUPPRESS_WARNING_POP