json/home/debugging/index.md

Debugging

This page collects the library's built-in debugger integrations and other debugging-related features. They are not linked from a single place elsewhere in the docs, so are collected here.

Visual Studio (natvis)

The repository ships nlohmann_json.natvis at its root, a Natvis file that gives json/ordered_json values a friendly, key/value debugger view instead of showing raw internal fields, when debugging with the MSVC debug engine (cppvsdbg) in Visual Studio or VS Code.

The checked-in visualizers cover namespace variants with zero, one, or two ABI tags. Configurations with three or more ABI tags are not covered and may show raw internal fields instead. To generate a Natvis file for such a configuration:

1. Check out the release tag of the library version you use.

2. List the ABI tags and the macros that enable them:

   make natvis NATVIS_ARGS="--list-tags"
   

3. Generate a Natvis file with the tags of your configuration, for example for JSON_DIAGNOSTICS, JSON_DIAGNOSTIC_POSITIONS, and JSON_VIEW_NO_SIMD:

   make natvis NATVIS_ARGS="--abi-tags _diag,_dp,_vns --output my_nlohmann_json.natvis"
   

4. Use the generated file instead of nlohmann_json.natvis.

--abi-tags can be repeated for several configurations, and --all generates every combination (about 60 MB for 12 ABI tags, doubling with each further tag). See tools/generate_natvis for all options.

Debug engines that wrap LLDB instead of the MSVC debug engine (for example, codelldb in VS Code) only have partial/experimental Natvis support, and commonly fall back to showing raw internal fields even with the .natvis file present. Switching to cppvsdbg where available, or checking your debug extension's own Natvis support/version, are the next things to try if this happens. There is currently no bundled LLDB-native pretty-printer script in this repository.

GDB

The repository ships a GDB Python pretty printer under tools/gdb_pretty_printer, with its own usage instructions in that directory's README.md.

Extended exception diagnostics

Defining JSON_DIAGNOSTICS before including the library augments type_error/out_of_range-style exceptions with a JSON Pointer to the offending value, which can help pinpoint where in a large document a runtime error occurred. This only applies to exceptions thrown after a value exists (e.g. during element access); parse errors, which happen before any value exists to point at, are not covered by this mechanism -- see Parsing and exceptions for how parse errors report their own location instead.