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80 lines
3.4 KiB
Markdown
80 lines
3.4 KiB
Markdown
# JSON_USE_IMPLICIT_CONVERSIONS
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```
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#define JSON_USE_IMPLICIT_CONVERSIONS /* value */
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```
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When defined to `0`, implicit conversions are switched off. By default, implicit conversions are switched on. The value directly affects [`operator ValueType`](https://json.nlohmann.me/api/basic_json/operator_ValueType/index.md) and the [converting constructor](https://json.nlohmann.me/api/basic_json/basic_json/index.md) from a `basic_json` specialization with a different string type (overload 4).
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## Default definition
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By default, implicit conversions are enabled.
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```
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#define JSON_USE_IMPLICIT_CONVERSIONS 1
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```
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## Notes
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Future behavior change
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Implicit conversions will be switched off by default in the next major release of the library.
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You can prepare existing code by already defining `JSON_USE_IMPLICIT_CONVERSIONS` to `0` and replace any implicit conversions with calls to [`get`](https://json.nlohmann.me/api/basic_json/get/index.md).
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See the [migration guide](https://json.nlohmann.me/integration/migration_guide/#replace-implicit-conversions) for how to update existing code.
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Automatic migration
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The community-maintained clang-tidy check `modernize-nlohmann-json-explicit-conversions` rewrites implicit conversions into explicit calls to [`get`](https://json.nlohmann.me/api/basic_json/get/index.md); for example, `int i = j;` becomes `int i = j.get<int>();`. The check is not part of clang-tidy itself, and it does not catch every case (for example, constructing a `std::optional` from a JSON value), so review the result. See [discussion #4610](https://github.com/nlohmann/json/discussions/4610) for how to build and use it.
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CMake option
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Implicit conversions can also be controlled with the CMake option [`JSON_ImplicitConversions`](https://json.nlohmann.me/integration/cmake/#json_implicitconversions) (`ON` by default) which defines `JSON_USE_IMPLICIT_CONVERSIONS` accordingly.
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## Examples
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Example: implicit conversion
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This is an example for an implicit conversion:
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```
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json j = "Hello, world!";
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std::string s = j;
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```
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When `JSON_USE_IMPLICIT_CONVERSIONS` is defined to `0`, the code above does no longer compile. Instead, it must be written like this:
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```
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json j = "Hello, world!";
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auto s = j.get<std::string>();
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```
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Example: conversion between `basic_json` specializations
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A `basic_json` specialization with a different string type is also no longer converted implicitly when `JSON_USE_IMPLICIT_CONVERSIONS` is defined to `0`:
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```
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using wjson = nlohmann::basic_json<std::map, std::vector, std::wstring>;
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void load(const nlohmann::json& j);
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wjson wj = /* ... */;
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load(wj); // error: no implicit conversion
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load(nlohmann::json(wj)); // OK: explicit conversion
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load(wj.get<nlohmann::json>()); // OK: explicit conversion
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```
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Specializations that share the same string type, such as `json` and `ordered_json`, remain implicitly convertible.
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## See also
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- [**operator ValueType**](https://json.nlohmann.me/api/basic_json/operator_ValueType/index.md) - get a value (implicit)
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- [**get**](https://json.nlohmann.me/api/basic_json/get/index.md) - get a value (explicit)
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- [JSON_ImplicitConversions](https://json.nlohmann.me/integration/cmake/#json_implicitconversions) - CMake option to control the macro
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## Version history
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- Added in version 3.9.0.
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- Also affects the conversion between `basic_json` specializations with different string types since version 3.13.0 unreleased.
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