Unverified Commit 521fe49f authored by Théo DELRIEU's avatar Théo DELRIEU
Browse files

Add basic_json::get_to function.

Takes an lvalue reference, and returns the same reference.

This allows non-default constructible types to be converted without
specializing adl_serializer.
This overload does not require CopyConstructible either.

Implements #1227
parent 680a4ab6
......@@ -227,12 +227,15 @@ json j_string = "this is a string";
std::string cpp_string = j_string;
// retrieve the string value (explicit JSON to std::string conversion)
auto cpp_string2 = j_string.get<std::string>();
// retrieve the string value (alternative explicit JSON to std::string conversion)
std::string cpp_string3;
j_string.get_to(cpp_string3);
// retrieve the serialized value (explicit JSON serialization)
std::string serialized_string = j_string.dump();
// output of original string
std::cout << cpp_string << " == " << cpp_string2 << " == " << j_string.get<std::string>() << '\n';
std::cout << cpp_string << " == " << cpp_string2 << " == " << cpp_string3 << " == " << j_string.get<std::string>() << '\n';
// output of serialized value
std::cout << j_string << " == " << serialized_string << std::endl;
```
......@@ -643,15 +646,15 @@ namespace ns {
}
void from_json(const json& j, person& p) {
p.name = j.at("name").get<std::string>();
p.address = j.at("address").get<std::string>();
p.age = j.at("age").get<int>();
j.at("name").get_to(p.name);
j.at("address").get_to(p.address);
j.at("age").get_to(p.age);
}
} // namespace ns
```
That's all! When calling the `json` constructor with your type, your custom `to_json` method will be automatically called.
Likewise, when calling `get<your_type>()`, the `from_json` method will be called.
Likewise, when calling `get<your_type>()` or `get_to(your_type&)`, the `from_json` method will be called.
Some important things:
......@@ -659,9 +662,8 @@ Some important things:
* Those methods **MUST** be available (e.g., properly headers must be included) everywhere you use the implicit conversions. Look at [issue 1108](https://github.com/nlohmann/json/issues/1108) for errors that may occur otherwise.
* When using `get<your_type>()`, `your_type` **MUST** be [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). (There is a way to bypass this requirement described later.)
* In function `from_json`, use function [`at()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a93403e803947b86f4da2d1fb3345cf2c.html#a93403e803947b86f4da2d1fb3345cf2c) to access the object values rather than `operator[]`. In case a key does not exist, `at` throws an exception that you can handle, whereas `operator[]` exhibits undefined behavior.
* In case your type contains several `operator=` definitions, code like `your_variable = your_json;` [may not compile](https://github.com/nlohmann/json/issues/667). You need to write `your_variable = your_json.get<decltype your_variable>();` instead.
* In case your type contains several `operator=` definitions, code like `your_variable = your_json;` [may not compile](https://github.com/nlohmann/json/issues/667). You need to write `your_variable = your_json.get<decltype(your_variable)>();` or `your_json.get_to(your_variable);` instead.
* You do not need to add serializers or deserializers for STL types like `std::vector`: the library already implements these.
* Be careful with the definition order of the `from_json`/`to_json` functions: If a type `B` has a member of type `A`, you **MUST** define `to_json(A)` before `to_json(B)`. Look at [issue 561](https://github.com/nlohmann/json/issues/561) for more details.
#### How do I convert third-party types?
......
#include <iostream>
#include <unordered_map>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create a JSON value with different types
json json_types =
{
{"boolean", true},
{
"number", {
{"integer", 42},
{"floating-point", 17.23}
}
},
{"string", "Hello, world!"},
{"array", {1, 2, 3, 4, 5}},
{"null", nullptr}
};
bool v1;
int v2;
short v3;
float v4;
int v5;
std::string v6;
std::vector<short> v7;
std::unordered_map<std::string, json> v8;
// use explicit conversions
json_types["boolean"].get_to(v1);
json_types["number"]["integer"].get_to(v2);
json_types["number"]["integer"].get_to(v3);
json_types["number"]["floating-point"].get_to(v4);
json_types["number"]["floating-point"].get_to(v5);
json_types["string"].get_to(v6);
json_types["array"].get_to(v7);
json_types.get_to(v8);
// print the conversion results
std::cout << v1 << '\n';
std::cout << v2 << ' ' << v3 << '\n';
std::cout << v4 << ' ' << v5 << '\n';
std::cout << v6 << '\n';
for (auto i : v7)
{
std::cout << i << ' ';
}
std::cout << "\n\n";
for (auto i : v8)
{
std::cout << i.first << ": " << i.second << '\n';
}
}
......@@ -2623,6 +2623,52 @@ class basic_json
return JSONSerializer<ValueTypeCV>::from_json(*this);
}
/*!
@brief get a value (explicit)
Explicit type conversion between the JSON value and a compatible value.
The value is filled into the input parameter by calling the @ref json_serializer<ValueType>
`from_json()` method.
The function is equivalent to executing
@code {.cpp}
ValueType v;
JSONSerializer<ValueType>::from_json(*this, v);
@endcode
This overloads is chosen if:
- @a ValueType is not @ref basic_json,
- @ref json_serializer<ValueType> has a `from_json()` method of the form
`void from_json(const basic_json&, ValueType&)`, and
@tparam ValueType the input parameter type.
@return the input parameter, allowing chaining calls.
@throw what @ref json_serializer<ValueType> `from_json()` method throws
@liveexample{The example below shows several conversions from JSON values
to other types. There a few things to note: (1) Floating-point numbers can
be converted to integers\, (2) A JSON array can be converted to a standard
`std::vector<short>`\, (3) A JSON object can be converted to C++
associative containers such as `std::unordered_map<std::string\,
json>`.,get_to}
@since version 3.3.0
*/
template<typename ValueType,
detail::enable_if_t <
not detail::is_basic_json<ValueType>::value and
detail::has_from_json<basic_json_t, ValueType>::value,
int> = 0>
ValueType & get_to(ValueType& v) const noexcept(noexcept(
JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), v)))
{
JSONSerializer<ValueType>::from_json(*this, v);
return v;
}
/*!
@brief get a pointer value (explicit)
......
......@@ -13696,6 +13696,52 @@ class basic_json
return JSONSerializer<ValueTypeCV>::from_json(*this);
}
 
/*!
@brief get a value (explicit)
Explicit type conversion between the JSON value and a compatible value.
The value is filled into the input parameter by calling the @ref json_serializer<ValueType>
`from_json()` method.
The function is equivalent to executing
@code {.cpp}
ValueType v;
JSONSerializer<ValueType>::from_json(*this, v);
@endcode
This overloads is chosen if:
- @a ValueType is not @ref basic_json,
- @ref json_serializer<ValueType> has a `from_json()` method of the form
`void from_json(const basic_json&, ValueType&)`, and
@tparam ValueType the input parameter type.
@return the input parameter, allowing chaining calls.
@throw what @ref json_serializer<ValueType> `from_json()` method throws
@liveexample{The example below shows several conversions from JSON values
to other types. There a few things to note: (1) Floating-point numbers can
be converted to integers\, (2) A JSON array can be converted to a standard
`std::vector<short>`\, (3) A JSON object can be converted to C++
associative containers such as `std::unordered_map<std::string\,
json>`.,get_to}
@since version 3.3.0
*/
template<typename ValueType,
detail::enable_if_t <
not detail::is_basic_json<ValueType>::value and
detail::has_from_json<basic_json_t, ValueType>::value,
int> = 0>
ValueType & get_to(ValueType& v) const noexcept(noexcept(
JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), v)))
{
JSONSerializer<ValueType>::from_json(*this, v);
return v;
}
/*!
@brief get a pointer value (explicit)
 
......
......@@ -298,6 +298,19 @@ TEST_CASE("basic usage", "[udt]")
CHECK(book == parsed_book);
}
SECTION("via explicit calls to get_to")
{
udt::person person;
udt::name name;
json person_json = big_json["contacts"][0]["person"];
CHECK(person_json.get_to(person) == sfinae_addict);
// correct reference gets returned
person_json["name"].get_to(name).m_val = "new name";
CHECK(name.m_val == "new name");
}
SECTION("implicit conversions")
{
const udt::contact_book parsed_book = big_json;
......
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