fix: treat single-element brace-init as copy/move instead of wrapping in array (#5074)#5090
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ssam18 wants to merge 2 commits intonlohmann:developfrom
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fix: treat single-element brace-init as copy/move instead of wrapping in array (#5074)#5090ssam18 wants to merge 2 commits intonlohmann:developfrom
ssam18 wants to merge 2 commits intonlohmann:developfrom
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See the first entry in the FAQ. https://json.nlohmann.me/home/faq/#brace-initialization-yields-arrays |
🔴 Amalgamation check failed! 🔴The source code has not been amalgamated. @ssam18 |
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 nlohmann#5074
Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>
- Add missing comment from include/nlohmann/json.hpp explaining the single-element brace-init fix (issue nlohmann#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>
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🔴 Amalgamation check failed! 🔴The source code has not been amalgamated. @ssam18 |
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Owner
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There is currently no way to fix this behavior without breaking existing code. |
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Summary
When a value is passed using single-element brace initialization e.g.,
json j{someObj}or via a by-value parametervoid foo(json j)called asfoo({someObj}). C++ always prefers theinitializer_listconstructor over the copy/move constructor. This caused the value to be silently wrapped in a single-element array.This bug was previously compiler-dependent: GCC wrapped the value while Clang did not. As of Clang 20, Clang now matches GCC behavior, making this a universal regression.
Validation
All existing unit tests pass with the fix applied.
Fixes the issue #5074.