basic_json() [5/10]

template<template< typename U, typename V, typename... Args > class ObjectType = std::map, template< typename U, typename... Args > class ArrayType = std::vector, class StringType = std::string, class BooleanType = bool, class NumberIntegerType = std::int64_t, class NumberUnsignedType = std::uint64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator, template< typename T, typename SFINAE=void > class JSONSerializer = adl_serializer, class BinaryType = std::vector<std::uint8_t>>

nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::basic_json ( initializer_list_t  init, bool  type_deduction = true, value_t  manual_type = value_t::array  )

inline

create a container (array or object) from an initializer list

Creates a JSON value of type array or object from the passed initializer list init. In case type_deduction is true (default), the type of the JSON value to be created is deducted from the initializer list init according to the following rules:

1. If the list is empty, an empty JSON object value {} is created.
2. If the list consists of pairs whose first element is a string, a JSON object value is created where the first elements of the pairs are treated as keys and the second elements are as values.
3. In all other cases, an array is created.

The rules aim to create the best fit between a C++ initializer list and JSON values. The rationale is as follows:

1. The empty initializer list is written as {} which is exactly an empty JSON object.
2. C++ has no way of describing mapped types other than to list a list of pairs. As JSON requires that keys must be of type string, rule 2 is the weakest constraint one can pose on initializer lists to interpret them as an object.
3. In all other cases, the initializer list could not be interpreted as JSON object type, so interpreting it as JSON array type is safe.

With the rules described above, the following JSON values cannot be expressed by an initializer list:

• the empty array ([]): use array(initializer_list_t) with an empty initializer list in this case
• arrays whose elements satisfy rule 2: use array(initializer_list_t) with the same initializer list in this case

Note

When used without parentheses around an empty initializer list, basic_json() is called instead of this function, yielding the JSON null value.

Parameters

[in]	init	initializer list with JSON values
[in]	type_deduction	internal parameter; when set to true, the type of the JSON value is deducted from the initializer list init; when set to false, the type provided via manual_type is forced. This mode is used by the functions array(initializer_list_t) and object(initializer_list_t).
[in]	manual_type	internal parameter; when type_deduction is set to false, the created JSON value will use the provided type (only value_t::array and value_t::object are valid); when type_deduction is set to true, this parameter has no effect

Exceptions

type_error.301

if type_deduction is false, manual_type is value_t::object, but init contains an element which is not a pair whose first element is a string. In this case, the constructor could not create an object. If type_deduction would have be true, an array would have been created. See object(initializer_list_t) for an example.

@complexity Linear in the size of the initializer list init.

@exceptionsafety Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

@liveexample{The example below shows how JSON values are created from initializer lists.,basic_json__list_init_t}

See also

see array(initializer_list_t) – create a JSON array value from an initializer list

see object(initializer_list_t) – create a JSON object value from an initializer list

Since

version 1.0.0

Definition at line 19433 of file json.hpp.

    {
        // check if each element is an array with two elements whose first
        // element is a string
        bool is_an_object = std::all_of(init.begin(), init.end(),
                                        [](const detail::json_ref<basic_json> &element_ref)
        {
            return element_ref->is_array() && element_ref->size() == 2 && (*element_ref)[0].is_string();
        });
 
        // adjust type if type deduction is not wanted
        if (!type_deduction)
        {
            // if array is wanted, do not create an object though possible
            if (manual_type == value_t::array)
            {
                is_an_object = false;
            }
 
            // if object is wanted but impossible, throw an exception
            if (JSON_HEDLEY_UNLIKELY(manual_type == value_t::object && !is_an_object))
            {
                JSON_THROW(type_error::create(301, "cannot create object from initializer list", basic_json()));
            }
        }
 
        if (is_an_object)
        {
            // the initializer list is a list of pairs -> create object
            m_type = value_t::object;
            m_value = value_t::object;
 
            for (auto &element_ref : init)
            {
                auto element = element_ref.moved_or_copied();
                m_value.object->emplace(
                    std::move(*((*element.m_value.array)[0].m_value.string)),
                    std::move((*element.m_value.array)[1]));
            }
        }
 
        else
        {
            // the initializer list describes an array -> create array
            m_type = value_t::array;
            m_value.array = create<array_t>(init.begin(), init.end());
        }
 
        set_parents();
        assert_invariant();
    }