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nlohmann Namespace

The 3.11.0 release introduced an inline namespace 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.


The complete default namespace name is derived as follows:

  • The root namespace is always nlohmann.
  • The inline namespace starts with json_abi and is followed by serveral optional ABI tags according to the value of these ABI-affecting macros, in order:
  • 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 below.

For example, the namespace name for version 3.11.2 with JSON_DIAGNOSTICS defined to 1 is:



Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different definitions of JSON_DIAGNOSTICS. This is illustrated in the diagram below.

uml diagram

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.


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.

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.

Disabling the version component

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 linline namespace, allowing different versions – but not configurations – to be used in cases where the linker would otherwise output undefined reference errors.


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.

Use at your own risk

Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!

Disabling the inline namespace completely

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 as follows:

#define NLOHMANN_JSON_NAMESPACE_BEGIN  namespace nlohmann {

Use at your own risk

Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!

Version history

  • Introduced inline namespace (json_v3_11_0[_abi-tag]*) in version 3.11.0.
  • Changed structure of inline namespace in version 3.11.2.

Last update: August 10, 2022