empty()

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>>

bool nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::empty ( ) const

inlinenoexcept

checks whether the container is empty.

Checks if a JSON value has no elements (i.e. whether its size is 0).

Returns

The return value depends on the different types and is defined as follows:

Value type	return value
null	true
boolean	false
string	false
number	false
binary	false
object	result of function object_t::empty()
array	result of function array_t::empty()

@liveexample{The following code uses empty() to check if a JSON object contains any elements.,empty}

@complexity Constant, as long as array_t and object_t satisfy the Container concept; that is, their empty() functions have constant complexity.

@iterators No changes.

@exceptionsafety No-throw guarantee: this function never throws exceptions.

Note

This function does not return whether a string stored as JSON value is empty - it returns whether the JSON container itself is empty which is false in the case of a string.

@requirement This function helps basic_json satisfying the Container requirements:

• The complexity is constant.
• Has the semantics of begin() == end().

See also

see size() – returns the number of elements

Since

version 1.0.0

Definition at line 22832 of file json.hpp.

    {
        switch (m_type)
        {
            case value_t::null:
            {
                // null values are empty
                return true;
            }
 
            case value_t::array:
            {
                // delegate call to array_t::empty()
                return m_value.array->empty();
            }
 
            case value_t::object:
            {
                // delegate call to object_t::empty()
                return m_value.object->empty();
            }
 
            case value_t::string:
            case value_t::boolean:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::number_float:
            case value_t::binary:
            case value_t::discarded:
            default:
            {
                // all other types are nonempty
                return false;
            }
        }
    }