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96 lines
2.4 KiB
Markdown
96 lines
2.4 KiB
Markdown
# NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END
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```cpp
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#define NLOHMANN_JSON_NAMESPACE_BEGIN /* value */ // (1)
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#define NLOHMANN_JSON_NAMESPACE_END /* value */ // (2)
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```
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These macros can be used to open and close the `nlohmann` namespace. See
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[`nlohmann` Namespace](../../features/namespace.md#structure) for details.
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1. Opens the namespace.
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2. Closes the namespace.
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## Default definition
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The default definitions open and close the `nlohmann` namespace. The precise definition of
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`NLOHMANN_JSON_NAMESPACE_BEGIN` varies as described [here](../../features/namespace.md#structure).
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1. Default definition of `NLOHMANN_JSON_NAMESPACE_BEGIN`:
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```cpp
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namespace nlohmann
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{
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inline namespace json_abi_v3_12_0
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{
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```
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2. Default definition of `NLOHMANN_JSON_NAMESPACE_END`:
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```cpp
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} // namespace json_abi_v3_12_0
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} // namespace nlohmann
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```
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When these macros are not defined, the library will define them to their default definitions.
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## Examples
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??? example
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The example shows how to use `NLOHMANN_JSON_NAMESPACE_BEGIN`/`NLOHMANN_JSON_NAMESPACE_END` from the
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[How do I convert third-party types?](../../features/arbitrary_types.md#how-do-i-convert-third-party-types) page.
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```cpp
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#include <iostream>
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#include <optional>
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#include <nlohmann/json.hpp>
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// partial specialization (see https://json.nlohmann.me/features/arbitrary_types/)
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NLOHMANN_JSON_NAMESPACE_BEGIN
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template <typename T>
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struct adl_serializer<std::optional<T>>
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{
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static void to_json(json& j, const std::optional<T>& opt)
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{
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if (opt == std::nullopt)
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{
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j = nullptr;
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}
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else
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{
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j = *opt;
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}
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}
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};
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NLOHMANN_JSON_NAMESPACE_END
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int main()
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{
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std::optional<int> o1 = 1;
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std::optional<int> o2 = std::nullopt;
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NLOHMANN_JSON_NAMESPACE::json j;
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j.push_back(o1);
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j.push_back(o2);
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std::cout << j << std::endl;
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}
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```
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Output:
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```json
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[1,null]
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```
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## See also
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- [`nlohmann` Namespace](../../features/namespace.md)
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- [NLOHMANN_JSON_NAMESPACE](nlohmann_json_namespace.md)
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- [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](nlohmann_json_namespace_no_version.md)
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## Version history
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- Added in version 3.11.0. Changed inline namespace name in version 3.11.2.
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