Two requirements forced users of otherwise suitable containers to write a
wrapper, and neither was load-bearing.
array_t::capacity() was read in push_back(), emplace_back(), operator+=(), and
operator[](size_type), but set_parent() only looks at the value under
JSON_DIAGNOSTICS; without diagnostics it was computed and discarded. Read it
through array_capacity(), which reports unknown_size() when diagnostics are off
or when the array type has no capacity() at all, and treat an unknown capacity
as "the elements may have moved" so the parent pointers are refreshed
conservatively. std::deque now works as ArrayType, in both builds, and
capacity() is no longer named at all in a default build. Since the capacity is
now only meaningful for array insertions, it moves out of set_parent() into
set_parent_after_array_insert().
basic_json::erase(iterator) assigned the object's erase() return value, which
requires the container to return the following iterator. Abseil's hash maps
return void to avoid computing a successor the caller may not need. Detect that
and compute the successor before erasing; containers that return an iterator,
including the vector-backed ordered_map where a precomputed successor would be
wrong, keep the existing path.
Together these leave an Abseil hash map needing only an alias that restores the
template argument order, and no adapter at all for std::deque.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018hxZxz8svM54c6ATEvXp5E
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
The requirements that basic_json places on its eleven template parameters
were only implied by how the library uses the resulting object_t, array_t,
string_t, etc. Consumers had to discover them by trial and error.
Add "Template Parameter Requirements" collecting them, split into what is
always required and what is only required when a particular part of the API
is instantiated. Notable findings that were previously undocumented:
- ObjectType must provide a key_compare member type (actual_object_comparator
names object_t::key_compare in both arms of a std::conditional), and its
third template parameter is used as a comparator, so std::unordered_map
cannot be used without a wrapper.
- ArrayType must provide capacity() -- push_back(), emplace_back(),
operator+=(), and operator[](size_type) call it unconditionally -- and
needs random-access iterators, so std::deque and std::list do not work.
- StringType needs contiguous, null-terminated data(), a one-byte value_type,
and either assignability from std::to_string or an ADL int_to_string().
- NumberFloatType must be float, double, or long double for parsing and
serialization; the integer types must satisfy std::is_integral.
- AllocatorType must be stateless, support incomplete types, and use plain
pointers.
- BooleanType and the number types are union members and must be trivial.
Link the new page from the basic_json overview, the types feature page, and
the individual type alias pages, and correct the container examples given for
ObjectType (std::unordered_map) and ArrayType (std::list), which do not work.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018hxZxz8svM54c6ATEvXp5E
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels LohmannandClaude Opus 5