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Niels Lohmann 77acd4563c Add element access, iteration, values, and JSON pointers to json_view

Give basic_json_view the read-only access functions of basic_json:
operator[] and at() with keys and indices, front()/back(), find(),
contains(), count(), begin()/end() and cbegin()/cend(), items()
with structured bindings from C++17 on, and type_name().

Exceptions have the ids and messages of the const functions of
basic_json. Where basic_json has undefined behavior the view
answers safely: operator[] with a missing key or an out-of-range
index returns a discarded view, and front()/back() of an empty
container throw invalid_iterator.214. Objects are iterated in
document order, and all members are visited; duplicate-key lookups
find the first member (as yyjson and simdjson do), while parse(),
materialize(), and the map conversions keep the last value, as
parse() does. Keys of up to 16 bytes are compared with two
overlapping loads.

Add value conversions: get<T>()/get_to() for arithmetic types,
bool, nullptr_t, strings (std::basic_string copied,
string_view_t without a copy), BasicJsonType, views, std::vector,
and maps with string keys; get_string() for the string without a
copy; number_token() for the number exactly as written in the
source; value() with keys and JSON pointers; and operator[]/at()/
contains() with JSON pointers. Everything else, including types
with from_json(), goes through materialize() of that subtree.
get<T>() of arithmetic types is inlined down to the conversion, so
reading an integer needs no call.

detail::json_pointer_access exposes a pointer's reference tokens
to code outside basic_json.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-10-07 16:42:26 +02:00

2.5 KiB

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nlohmann::basic_json::get_to

template<typename ValueType>
ValueType& get_to(ValueType& v) const noexcept(
    noexcept(JSONSerializer<ValueType>::from_json(
        std::declval<const basic_json_t&>(), v)));

Explicit type conversion between the JSON value and a compatible value. The value is filled into the input parameter by calling the json_serializer<ValueType> from_json() method.

The function is equivalent to executing

ValueType v;
JSONSerializer<ValueType>::from_json(*this, v);

This overload is chosen if:

ValueType is not basic_json,
json_serializer<ValueType> has a from_json() method of the form void from_json(const basic_json&, ValueType&)

v must not be const. Passing a const object is a compile-time error. For types such as arithmetic types, enums, and C arrays, the error is a static_assert that names the problem. For other types, the overload is not viable, and the compiler reports that no matching get_to was found.

Template parameters

ValueType: the value type to return

Return value

the input parameter, allowing chaining calls

Exception safety

Depends on what json_serializer<ValueType> from_json() method throws; the JSON value itself is never modified, since get_to() is a #!cpp const member function.

Exceptions

Depends on what json_serializer<ValueType> from_json() method throws

Complexity

Depends on the json_serializer<ValueType>::from_json() implementation.

Examples

??? example

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
`#!cpp std::vector<short>`, (3) A JSON object can be converted to C++ associative containers such as
`#!cpp std::map<std::string, json>`.
    
```cpp
--8<-- "examples/get_to.cpp"
```

Output:

```json
--8<-- "examples/get_to.output"
```

Version history

Since version 3.3.0.
Added a static_assert with a clear message for const arguments in version 3.13.0.

2.5 KiB Raw Blame History