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Reject integral keys for contains(), find(), and count() at compile time (#5705)

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j.contains(0), j.find(0), and j.count(0) used to compile: the literal 0 is
a null pointer constant, so it converts to a null const char*, and the
overloads taking const typename object_t::key_type& accepted it by
constructing a std::string from that null pointer, which is undefined
behavior (a crash with both libc++ and libstdc++). value(0, default_value)
had the same problem in C++11, where the object comparator is not
transparent.

Add deleted overloads for integral arguments to contains(), find()
(const and non-const), count(), and value() so that these calls are
compile errors in every supported language mode instead of crashing.
Calls with string, string_view, json_pointer, and size-typed element
access (at(), operator[](), erase()) are unaffected.

Fixes #5657.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-30 20:07:45 +02:00
committed by GitHub
parent 5bd766aa50
commit 6ae17630a4
7 changed files with 140 additions and 3 deletions
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12
docs/mkdocs/docs/api/basic_json/value.md
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@@ -52,6 +52,14 @@ This is equivalent to Python's `dict.get(key, default)`.
- Unlike [`operator[]`](operator[].md), this function does not implicitly add an element to the position defined by
`key`/`ptr` key. This function is furthermore also applicable to const objects.
!!! note "Integer keys"
Calling this function with an integer `key` argument (for example, `#!cpp value(0, 1)`) does not compile in
C++11, where `object_comparator_t` is not transparent: such an argument would otherwise implicitly convert to a
null `#!cpp const char*` and, from there, cause undefined behavior when constructing a `#!cpp std::string` for the
object key. To access an array element with a default value, use [`at`](at.md) together with a `#!cpp try`/`#!cpp
catch` block, or compare against [`size`](size.md) instead.
## Template parameters
`KeyType`
@@ -184,7 +192,9 @@ changes to any JSON value.
## Version history
1. Added in version 1.0.0. Changed parameter `default_value` type from `const ValueType&` to `ValueType&&` in version 3.11.0.
1. Added in version 1.0.0. Changed parameter `default_value` type from `const ValueType&` to `ValueType&&` in version
3.11.0. Deleted overload for integral key types added in version 3.13.0 to reject such calls at compile time
instead of causing undefined behavior at runtime.
2. Added in version 3.11.0. Made `ValueType` the first template parameter in version 3.11.2.
3. Added in version 2.0.2. Extended to work with arrays in version 3.13.0, including fixing an issue where resolving
`ptr` through an array unexpectedly threw `out_of_range` instead of returning the resolved element (or