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nlohmann
74 lines
4.3 KiB
Markdown
74 lines
4.3 KiB
Markdown
# `nlohmann` Namespace
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The 3.11.0 release introduced an [inline namespace](https://en.cppreference.com/w/cpp/language/namespace#Inline_namespaces) to allow different parts of a codebase to safely use different versions of the JSON library as long as they never exchange instances of library types.
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## Structure
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The complete default namespace name is derived as follows:
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- The root namespace is always `nlohmann`.
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- The inline namespace starts with `json_abi` and is followed by several optional ABI tags according to the value of these ABI-affecting macros, in order:
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- [`JSON_DIAGNOSTICS`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md) defined non-zero appends `_diag`.
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- [`JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](https://json.nlohmann.me/api/macros/json_use_legacy_discarded_value_comparison/index.md) defined non-zero appends `_ldvcmp`.
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- The inline namespace ends with the suffix `_v` followed by the 3 components of the version number separated by underscores. To omit the version component, see [Disabling the version component](#disabling-the-version-component) below.
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For example, the namespace name for version 3.11.2 with `JSON_DIAGNOSTICS` defined to `1` is:
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```
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nlohmann::json_abi_diag_v3_11_2
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```
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## Purpose
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Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different definitions of [`JSON_DIAGNOSTICS`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md). This is illustrated in the diagram below.
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```
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graph
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json["<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=0"]
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json_diag["<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=1"]
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library["<strong>some library</strong>"]
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app["<strong>application</strong>"]
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library --> json
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app --> json_diag
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app --> library
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```
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In releases prior to 3.11.0, mixing any version of the JSON library with different `JSON_DIAGNOSTICS` settings would result in a crashing application. If `some_library` never passes instances of JSON library types to the application, this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.
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## Limitations
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Neither the compiler nor the linker will issue as much as a warning when translation units – intended to be linked together and that include different versions and/or configurations of the JSON library – exchange and use library types.
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There is an exception when forward declarations are used (i.e., when including `json_fwd.hpp`) in which case the linker may complain about undefined references.
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## Disabling the version component
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Different versions are not necessarily ABI-incompatible, but the project does not actively track changes in the ABI and recommends that all parts of a codebase exchanging library types be built with the same version. Users can, **at their own risk**, disable the version component of the inline namespace, allowing different versions – but not configurations – to be used in cases where the linker would otherwise output undefined reference errors.
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To do so, define [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](https://json.nlohmann.me/api/macros/nlohmann_json_namespace_no_version/index.md) to `1`.
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This applies to version 3.11.2 and above only; versions 3.11.0 and 3.11.1 can apply the technique described in the next section to emulate the effect of the `NLOHMANN_JSON_NAMESPACE_NO_VERSION` macro.
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Use at your own risk
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Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!
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## Disabling the inline namespace completely
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When interoperability with code using a pre-3.11.0 version of the library is required, users can, **at their own risk** restore the old namespace layout by redefining [`NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END`](https://json.nlohmann.me/api/macros/nlohmann_json_namespace_begin/index.md) as follows:
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```
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#define NLOHMANN_JSON_NAMESPACE_BEGIN namespace nlohmann {
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#define NLOHMANN_JSON_NAMESPACE_END }
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```
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Use at your own risk
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Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!
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## Version history
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- Introduced inline namespace (`json_v3_11_0[_abi-tag]*`) in version 3.11.0.
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- Changed structure of inline namespace in version 3.11.2.
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