operator[]() [5/8]

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>>

reference nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::operator[] ( size_type  idx )

inline

access specified array element

Returns a reference to the element at specified location idx.

Note

If idx is beyond the range of the array (i.e., idx >= size()), then the array is silently filled up with null values to make idx a valid reference to the last stored element.

Parameters

[in]

idx

index of the element to access

Returns

reference to the element at index idx

Exceptions

type_error.305

if the JSON value is not an array or null; in that cases, using the [] operator with an index makes no sense.

@complexity Constant if idx is in the range of the array. Otherwise linear in idx - size().

@liveexample{The example below shows how array elements can be read and written using [] operator. Note the addition of null values.,operatorarray__size_type}

Since

version 1.0.0

Definition at line 21385 of file json.hpp.

    {
        // implicitly convert null value to an empty array
        if (is_null())
        {
            m_type = value_t::array;
            m_value.array = create<array_t>();
            assert_invariant();
        }
 
        // operator[] only works for arrays
        if (JSON_HEDLEY_LIKELY(is_array()))
        {
            // fill up array with null values if given idx is outside range
            if (idx >= m_value.array->size())
            {
#if JSON_DIAGNOSTICS
                // remember array size before resizing
                const auto previous_size = m_value.array->size();
#endif
                m_value.array->resize(idx + 1);
#if JSON_DIAGNOSTICS
                // set parent for values added above
                set_parents(begin() + static_cast<typename iterator::difference_type>(previous_size), static_cast<typename iterator::difference_type>(idx + 1 - previous_size));
#endif
            }
 
            return m_value.array->operator[](idx);
        }
 
        JSON_THROW(type_error::create(305, "cannot use operator[] with a numeric argument with " + std::string(type_name()), *this));
    }