basic_json destructor allocates memory, violating noexcept semantics

Description

The destructor of basic_json invokes json_value::destroy, which may allocate a large std::vector to flatten out its object/array hierarchy in order to avoid recursion. In the event of an allocation failure, the vector will throw an uncatchable std::bad_alloc, terminating the program.

(I noticed this while debugging a related issue, where destroying an invalidated JSON object caused a four-exabyte allocation.)

Reproduction steps

• Create a complex JSON document by nesting a few million arrays/objects in an arbitrary structure.
• Artificially exhaust the application's available memory.
• Destroy your JSON document.

Expected vs. actual results

Allocating a bunch of memory in a destructor is weird and widely understood to be unsafe.

Normal recursive destruction would be suitable for my use-case. If that is unacceptable, there are methods (such as forming an ad-hoc linked list) that could be used to avoid recursion without allocating. If those methods are unacceptable, a non-throwing allocator could be used with a fallback to recursive destruction in the event that no memory is available.

The latter approach is implemented by PR #4654.

Minimal code example

Error messages

Compiler and operating system

Win11/MSVC

Library version

3.12.0

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

Start by reading json_value::destroy and reviewing PR #4654, which the issue identifies as an implementation of one proposed approach. Check how destruction behaves when allocation fails, then compile and run the repository's unit tests. Done means basic_json destruction no longer performs a potentially throwing allocation or terminates on allocation failure.