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nlohmann
192 lines
8.1 KiB
Markdown
192 lines
8.1 KiB
Markdown
# Converting values
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A `basic_json` value stores JSON data, but most of the time you want to move that data into ordinary C++ types (an `int`, a `std::string`, a `std::vector`, or one of your own structs) and back. This page describes how these conversions work.
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## Getting values out
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The [`get`](https://json.nlohmann.me/api/basic_json/get/index.md) function template returns a copy of the stored value converted to the requested type:
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```
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json j = R"({"name": "Mary", "age": 42, "hobbies": ["hiking", "reading"]})"_json;
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auto name = j["name"].get<std::string>(); // "Mary"
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auto age = j["age"].get<int>(); // 42
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auto hobbies = j["hobbies"].get<std::vector<std::string>>(); // {"hiking", "reading"}
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```
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Getting a string without quotes
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A frequent point of confusion: use [`get`](https://json.nlohmann.me/api/basic_json/get/index.md), **not** [`dump`](https://json.nlohmann.me/features/serialization/index.md), to read a string value. `j["name"].get<std::string>()` yields `Mary`, whereas `j["name"].dump()` yields the JSON text `"Mary"` (**with** quotes), because `dump` always produces a JSON text.
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Alternatively, [`get_to`](https://json.nlohmann.me/api/basic_json/get_to/index.md) writes into an existing variable and deduces the target type, which avoids repeating it:
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Example
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```
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#include <iostream>
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#include <unordered_map>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create a JSON value with different types
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json json_types =
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{
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{"boolean", true},
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{
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"number", {
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{"integer", 42},
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{"floating-point", 17.23}
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}
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},
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{"string", "Hello, world!"},
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{"array", {1, 2, 3, 4, 5}},
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{"null", nullptr}
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};
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bool v1;
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int v2;
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short v3;
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float v4;
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int v5;
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std::string v6;
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std::vector<short> v7;
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std::unordered_map<std::string, json> v8;
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// use explicit conversions
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json_types["boolean"].get_to(v1);
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json_types["number"]["integer"].get_to(v2);
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json_types["number"]["integer"].get_to(v3);
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json_types["number"]["floating-point"].get_to(v4);
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json_types["number"]["floating-point"].get_to(v5);
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json_types["string"].get_to(v6);
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json_types["array"].get_to(v7);
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json_types.get_to(v8);
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// print the conversion results
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std::cout << v1 << '\n';
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std::cout << v2 << ' ' << v3 << '\n';
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std::cout << v4 << ' ' << v5 << '\n';
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std::cout << v6 << '\n';
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for (auto i : v7)
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{
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std::cout << i << ' ';
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}
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std::cout << "\n\n";
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for (auto i : v8)
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{
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std::cout << i.first << ": " << i.second << '\n';
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}
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}
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```
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Output:
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```
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1
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42 42
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17.23 17
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Hello, world!
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1 2 3 4 5
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string: "Hello, world!"
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number: {"floating-point":17.23,"integer":42}
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null: null
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boolean: true
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array: [1,2,3,4,5]
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```
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The library already knows how to convert to and from the scalar types and the STL containers (such as `std::vector`, `std::map`, `std::array`, `std::optional`, and many more). Converting a JSON object back to a `std::map` or a JSON array back to a `std::vector` therefore works without any extra code:
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```
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json j = {{"one", 1}, {"two", 2}};
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auto m = j.get<std::map<std::string, int>>(); // {{"one", 1}, {"two", 2}}
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```
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## Implicit conversions
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By default, a JSON value implicitly converts to a compatible C++ type, so the explicit `get` call can often be omitted:
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```
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json j = "Hello";
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std::string s = j; // implicit conversion, same as j.get<std::string>()
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```
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Implicit conversions are convenient but can be surprising (for example, in overload resolution or with `auto`). They can be disabled by defining [`JSON_USE_IMPLICIT_CONVERSIONS`](https://json.nlohmann.me/api/macros/json_use_implicit_conversions/index.md) to `0`, which forces the explicit `get` form and can catch unintended conversions at compile time.
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Conversions do not range-check numbers
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Just like C++ itself, the `get` family performs numeric conversions without range checks — retrieving a floating-point value as an integer truncates it, and narrowing conversions may overflow. See [number conversion](https://json.nlohmann.me/features/types/number_handling/#number-conversion) for details and how to guard against it.
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std::optional direct construction from JSON null throws
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Constructing or assigning `std::optional<T>` directly from a JSON value does not correctly produce `std::nullopt` for a JSON `null`:
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```
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json j_null;
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std::optional<std::string> opt = j_null; // ❌ throws type_error 302
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```
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This is due to C++ language rules: `std::optional<T>` has its own converting constructor that is chosen over `basic_json::operator T()` when both are viable. Use `get<std::optional<T>>()` or `get_to()` instead:
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```
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auto opt = j_null.get<std::optional<std::string>>(); // ✅ std::nullopt
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j_null.get_to(opt); // ✅ std::nullopt
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```
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`static_cast` and `get<std::optional<T>>()` are not guaranteed equivalent
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`operator ValueType()` (used by `static_cast` and implicit conversions) intentionally excludes `std::optional<T>` from delegating to `get<T>()`, to avoid a constructor ambiguity with `std::optional<T>`'s own converting constructor from `basic_json`. As a result, `static_cast<std::optional<T>>(json_value)` goes through `std::optional<T>`'s own converting constructor rather than through `get<std::optional<T>>()`, which can behave differently -- for example, with a custom `adl_serializer<std::optional<T>>` specialization. Prefer `get<std::optional<T>>()`/`get_to()` over `static_cast` for optional types.
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Converting to a fixed-size `std::array` does not check length
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Converting a JSON array to `std::array<T, N>` does not check that the JSON array's size matches `N`: if the JSON array is longer, the extra elements are silently dropped; if it is shorter, the remaining `std::array` elements are left default-constructed. No exception is thrown in either case.
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```
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json j = {1, 2, 3, 4, 5};
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auto a = j.get<std::array<int, 3>>(); // {1, 2, 3} -- elements 4 and 5 silently dropped
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```
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## Omitting a field when serializing `std::optional`
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By default, `to_json` for `std::optional<T>` writes either the value or `null` -- there is no built-in way to make a field disappear from the serialized object entirely when the `std::optional` is `std::nullopt`. Because a specialization of `adl_serializer<std::optional<T>>` only controls how the *value* is converted (it cannot prevent the containing object's `to_json` from inserting the key in the first place), omission has to be implemented in the *containing* type's `to_json`:
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```
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struct person {
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std::string name;
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std::optional<int> age;
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};
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void to_json(json& j, const person& p) {
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j = json{{"name", p.name}};
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if (p.age) {
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j["age"] = *p.age; // key is only inserted when the optional has a value
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}
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}
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```
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## Putting values in
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The reverse direction works the same way: assigning or constructing a `json` from a C++ value converts it to JSON.
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```
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std::vector<int> numbers = {1, 2, 3};
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json j = numbers; // [1,2,3]
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```
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## Your own types
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The conversions above are built in for standard types. To make the same syntax work for **your own** types, provide `to_json`/`from_json` functions (or use one of the convenience macros). This is described in detail on the [arbitrary types conversions](https://json.nlohmann.me/features/arbitrary_types/index.md) page. Enums can be mapped to strings as described in [specializing enum conversion](https://json.nlohmann.me/features/enum_conversion/index.md).
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## See also
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- [`get`](https://json.nlohmann.me/api/basic_json/get/index.md) - get a copy converted to a given type
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- [`get_to`](https://json.nlohmann.me/api/basic_json/get_to/index.md) - convert into an existing variable
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- [`get_ref`](https://json.nlohmann.me/api/basic_json/get_ref/index.md) / [`get_ptr`](https://json.nlohmann.me/api/basic_json/get_ptr/index.md) - access the stored value without copying
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- [Arbitrary types conversions](https://json.nlohmann.me/features/arbitrary_types/index.md) - support your own types
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- [`JSON_USE_IMPLICIT_CONVERSIONS`](https://json.nlohmann.me/api/macros/json_use_implicit_conversions/index.md) - toggle implicit conversions
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