Niels LohmannandClaude Sonnet 5
6a406ee141
Closes #3106. set(JSON_Diagnostics ON) before find_package() has no effect on a package built and installed elsewhere (Homebrew, vcpkg, a system package, etc.) -- the compile definition is baked into the exported nlohmann_jsonTargets.cmake at install time and the generated config script never re-reads that variable. Verified empirically against the real Homebrew-installed 3.12.0 package: the exported target carries a fixed $<$<BOOL:OFF>:JSON_DIAGNOSTICS=1>, and the suggested set(JSON_Diagnostics ON) snippet produces no change in exception output. Documents the actual working fix (overriding the imported target's INTERFACE_COMPILE_DEFINITIONS property after find_package()) and the multi-target "JSON_DIAGNOSTICS redefined" pitfall reported earlier in the issue thread. Signed-off-by: Niels Lohmann <mail@nlohmann.me> Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
96 lines
2.9 KiB
Markdown
96 lines
2.9 KiB
Markdown
# JSON_DIAGNOSTICS
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```cpp
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#define JSON_DIAGNOSTICS /* value */
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```
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This macro enables [extended diagnostics for exception messages](../../home/exceptions.md#extended-diagnostic-messages).
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Possible values are `1` to enable or `0` to disable (default).
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When enabled, exception messages contain a [JSON Pointer](../json_pointer/json_pointer.md) to the JSON value that
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triggered the exception. Note that enabling this macro increases the size of every JSON value by one pointer and adds
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some runtime overhead.
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## Default definition
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The default value is `0` (extended diagnostics are switched off).
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```cpp
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#define JSON_DIAGNOSTICS 0
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```
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When the macro is not defined, the library will define it to its default value.
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## Notes
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!!! note "ABI compatibility"
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As of version 3.11.0, this macro is no longer required to be defined consistently throughout a codebase to avoid
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One Definition Rule (ODR) violations, as the value of this macro is encoded in the namespace, resulting in distinct
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symbol names.
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This allows different parts of a codebase to use different versions or configurations of this library without
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causing improper behavior.
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Where possible, it is still recommended that all code define this the same way for maximum interoperability.
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!!! hint "CMake option"
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Diagnostic messages can also be controlled with the CMake option
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[`JSON_Diagnostics`](../../integration/cmake.md#json_diagnostics) (`OFF` by default)
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which defines `JSON_DIAGNOSTICS` accordingly. Note this only applies when building the
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library from source — see the pre-installed-package caveat on that page.
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## Examples
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??? example "Example 1: default behavior"
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```cpp
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--8<-- "examples/diagnostics_standard.cpp"
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```
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Output:
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```
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--8<-- "examples/diagnostics_standard.output"
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```
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This exception can be hard to debug if storing the value `#!c "12"` and accessing it is further apart.
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??? example "Example 2: extended diagnostic messages"
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```cpp
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--8<-- "examples/diagnostics_extended.cpp"
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```
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Output:
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```
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--8<-- "examples/diagnostics_extended.output"
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```
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Now the exception message contains a JSON Pointer `/address/housenumber` that indicates which value has the wrong type.
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??? example "Example 3: using only diagnostic positions in exceptions"
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```cpp
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--8<-- "examples/diagnostic_positions_exception.cpp"
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```
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Output:
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```
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--8<-- "examples/diagnostic_positions_exception.output"
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```
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The output shows the exception with start/end positions only.
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## See also
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- [:simple-cmake: JSON_Diagnostics](../../integration/cmake.md#json_diagnostics) - CMake option to control the macro
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- [JSON_DIAGNOSTIC_POSITIONS](json_diagnostic_positions.md) - macro to access positions of elements
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## Version history
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- Added in version 3.10.0.
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- As of version 3.11.0, the definition is allowed to vary between translation units.
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