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1# `nlohmann` Namespace
2
3The 3.11.0 release introduced an
4[inline namespace](https://en.cppreference.com/w/cpp/language/namespace#Inline_namespaces) to allow different parts of
5a codebase to safely use different versions of the JSON library as long as they never exchange instances of library
6types.
7
8## Structure
9
10The complete default namespace name is derived as follows:
11
12- The root namespace is always `nlohmann`.
13- The inline namespace starts with `json_abi` and is followed by serveral optional ABI tags according to the value of
14  these ABI-affecting macros, in order:
15    - [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md) defined non-zero appends `_diag`.
16    - [`JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](../api/macros/json_use_legacy_discarded_value_comparison.md)
17      defined non-zero appends `_ldvcmp`.
18- The inline namespace ends with the suffix `_v` followed by the 3 components of the version number separated by
19  underscores. To omit the version component, see [Disabling the version component](#disabling-the-version-component)
20  below.
21
22For example, the namespace name for version 3.11.2 with `JSON_DIAGNOSTICS` defined to `1` is:
23
24```cpp
25nlohmann::json_abi_diag_v3_11_2
26```
27
28## Purpose
29
30Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different
31definitions of [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md). This is illustrated in the diagram below.
32
33```plantuml
34[**nlohmann_json (v3.10.5)**\nJSON_DIAGNOSTICS=0] as [json]
35[**nlohmann_json (v3.10.5)**\nJSON_DIAGNOSTICS=1] as [json_diag]
36[**some_library**] as [library]
37[**application**] as [app]
38
39[library] ..|> [json]
40[app] ..|> [json_diag]
41[app] ..|>[library]
42```
43
44In releases prior to 3.11.0, mixing any version of the JSON library with different `JSON_DIAGNOSTICS` settings would
45result in a crashing application. If `some_library` never passes instances of JSON library types to the application,
46this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.
47
48## Limitations
49
50Neither the compiler nor the linker will issue as much as a warning when translation units – intended to be linked
51together and that include different versions and/or configurations of the JSON library – exchange and use library
52types.
53
54There is an exception when forward declarations are used (i.e., when including `json_fwd.hpp`) in which case the linker
55may complain about undefined references.
56
57## Disabling the version component
58
59Different versions are not necessarily ABI-incompatible, but the project does not actively track changes in the ABI and
60recommends that all parts of a codebase exchanging library types be built with the same version. Users can, **at their
61own risk**, disable the version component of the linline namespace, allowing different versions – but not
62configurations – to be used in cases where the linker would otherwise output undefined reference errors.
63
64To do so, define [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](../api/macros/nlohmann_json_namespace_no_version.md) to `1`.
65
66This 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
67section to emulate the effect of the `NLOHMANN_JSON_NAMESPACE_NO_VERSION` macro.
68
69!!! danger "Use at your own risk"
70
71    Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect
72    behavior. You have been warned!
73## Disabling the inline namespace completely
74
75When interoperability with code using a pre-3.11.0 version of the library is required, users can, **at their own risk**
76restore the old namespace layout by redefining
77[`NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END`](../api/macros/nlohmann_json_namespace_begin.md) as
78follows:
79
80```cpp
81#define NLOHMANN_JSON_NAMESPACE_BEGIN  namespace nlohmann {
82#define NLOHMANN_JSON_NAMESPACE_END    }
83```
84
85!!! danger "Use at your own risk"
86
87    Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You
88    have been warned!
89
90## Version history
91
92- Introduced inline namespace (`json_v3_11_0[_abi-tag]*`) in version 3.11.0.
93- Changed structure of inline namespace in version 3.11.2.
94