starred/json
1
0
Fork 0
mirror of https://github.com/nlohmann/json.git synced 2026-10-05 14:21:05 +01:00
Code Issues Packages Projects Releases 10 Wiki Activity

Test JSON_BRACE_INIT_COPY_SEMANTICS for real, and fix one-element tuples under it

Browse Source 
The opt-in JSON_BRACE_INIT_COPY_SEMANTICS was never exercised by CI:

- Its only test, in unit-regression3.cpp, was guarded by
  `#if defined(JSON_BRACE_INIT_COPY_SEMANTICS)` after the #include. The
  header #undefs the macro unconditionally in macro_unscope.hpp, so the
  guard was always false and the test compiled to nothing, whatever -D
  flag was passed.
- The ci_test_brace_init_copy_semantics target that passes the flag was
  not named by any workflow.

Move the test into its own translation unit that defines the macro before
including the header, as unit-diagnostics.cpp does for JSON_DIAGNOSTICS.
It now runs in every CI job and for every standard. Remove the unused
target: it ran the whole suite with the macro, and that suite deliberately
relies on default brace-init semantics in about 90 places
(e.g. `json({1})` meaning `[1]`), so it could never pass.

Running the whole suite with the macro did find one library bug:
to_json for std::tuple builds `j = { std::get<Idx>(t)... }`, so with copy
semantics a one-element tuple became its element. `json(std::tuple<int>{5})`
was `5` instead of `[5]`, and `get<std::tuple<int>>()` threw type_error.302
on the result. Under the macro, a one-element tuple now builds exactly what
the default deduction builds. Without the macro nothing changes.

The new tests also pin that the library's other conversions produce the
same values with and without the macro. The macro page now says that the
macro affects every single-element list (`json j = {1}` is `1`), and that
all translation units must agree on it, since it has no ABI tag.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-24 07:16:56 +02:00
parent ed513715a8
commit 2534db3570
6 changed files with 213 additions and 42 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/conversions/to_json.hpp
+17
View File
@@ -471,6 +471,23 @@ inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<
j = { std::get<Idx>(t)... };
}
#if JSON_BRACE_INIT_COPY_SEMANTICS
// JSON_BRACE_INIT_COPY_SEMANTICS makes a one-element braced list copy its
// element instead of wrapping it, which would serialize std::tuple<int>{5} as 5
// rather than [5]. Build what the default deduction builds instead: an object
// if the element is a [string, value] pair, a one-element array otherwise.
template<typename BasicJsonType, typename Tuple>
inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<0> /*unused*/)
{
BasicJsonType element(std::get<0>(t));
// same test as the initializer-list constructor, including the cast that
// keeps a string type constructible from 0 from selecting operator[](key)
const bool is_member = element.is_array() && element.size() == 2
&& element[static_cast<typename BasicJsonType::size_type>(0)].is_string();
j = is_member ? BasicJsonType::object({std::move(element)}) : BasicJsonType::array({std::move(element)});
}
#endif
template<typename BasicJsonType, typename Tuple>
inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& /*unused*/, index_sequence<> /*unused*/)
{