jk... unless?

A library for performing jq-ish queries on C++ objects.

Supported language

The parser compiles directly into coroutine filters; it does not use a VM. In addition to navigation, construction, arithmetic, comparison, and the existing collection/math builtins, it supports:

• if … then … elif … then … else … end; an omitted else is identity.
• select, and, or, and // with jq truthiness and stream semantics. // handles false/null/empty results, not errors.
• try EXP catch HANDLER, try EXP, optional navigation, error, and error(payload). Catch handlers receive the original error value.
• any/all with zero, one, or two arguments; first/last with zero or one; isempty(filter); nth(index) and nth(index; filter); limit(count; filter), skip(count; filter), and all three range arities.
• Semicolon-separated filter arguments, pipelines in object values, expression-valued slice bounds, nested string interpolation ("x=\(.x)"), and # comments, including backslash continuation.

Filters preserve output before an error. Early consumers such as first and limit do not evaluate the unused remainder of a stream, including streams produced by object construction. Slices and splitting strings with an empty separator operate on UTF-8 codepoints.

Compatibility and execution limits

This is a jq subset, not a replacement for the jq command-line application. Variables, user-defined functions, reduce/foreach, assignment/update, modules, regex, and I/O are not implemented. Objects use sorted keys rather than jq's insertion order. Integer comparisons preserve signed 64-bit precision; general arithmetic uses binary64, not jq's full decimal-literal representation. Runtime error wording is not byte-for-byte jq compatible.

Parsing allocates on the normal heap and compiled filters own stable smallfun nodes. Evaluation can use an evaluation_context: coroutine frames and owning temporary values then use its caller-supplied storage, while pass-through values are borrowed until a collecting or escaping boundary. The default bounded context throws std::bad_alloc when exhausted; an explicit upstream resource can preserve unbounded compatibility. Results are valid until their producer advances. Use yielded::take() to retain a result inside the selected allocation context, or yielded::persist() to detach it onto normal heap storage before the context resets.

This removes per-message heap allocation for supported bounded workloads; it does not impose a work budget, and runtime errors still allocate persistent exception diagnostics and payloads. Therefore evaluation is not yet hard-real-time safe.

Verification

The standalone build requires C++20, Catch2 3, fmt, and SmallFunction:

cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build
ctest --test-dir build --output-on-failure

tests/cases.txt is the supported-behavior regression corpus. tests/gen-cases.sh requires the official jq 1.8.2 executable; set JQ to its path if necessary. Regenerate tests/conformance_cases.inc with that script. The generated oracle compares sorted-key JSON output, stream order, and compile/runtime failure status, including output emitted before failure.