JSON for Modern C++ 2.1.1
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◆ 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>
bool nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer >::empty ( ) const
inlinenoexcept

Checks if a JSON value has no elements.

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
object result of function object_t::empty()
array result of function array_t::empty()
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.
Complexity\n Constant, as long as @ref array_t and @ref object_t satisfy
the Container concept; that is, their empty() functions have constant complexity.
Requirements\n This function helps basic_json satisfying the
Container requirements:
  • The complexity is constant.
  • Has the semantics of begin() == end().
Example\n The following code uses empty() to check if a JSON
object contains any elements.
1#include <json.hpp>
2
3using json = nlohmann::json;
4
5int main()
6{
7 // create JSON values
9 json j_boolean = true;
11 json j_number_float = 23.42;
12 json j_object = {{"one", 1}, {"two", 2}};
14 json j_array = {1, 2, 4, 8, 16};
16 json j_string = "Hello, world";
17
18 // call empty()
19 std::cout << std::boolalpha;
20 std::cout << j_null.empty() << '\n';
21 std::cout << j_boolean.empty() << '\n';
22 std::cout << j_number_integer.empty() << '\n';
23 std::cout << j_number_float.empty() << '\n';
24 std::cout << j_object.empty() << '\n';
25 std::cout << j_object_empty.empty() << '\n';
26 std::cout << j_array.empty() << '\n';
27 std::cout << j_array_empty.empty() << '\n';
28 std::cout << j_string.empty() << '\n';
29}
bool empty() const noexcept
checks whether the container is empty
Definition json.hpp:4995
static basic_json array(std::initializer_list< basic_json > init=std::initializer_list< basic_json >())
explicitly create an array from an initializer list
Definition json.hpp:2165
static basic_json object(std::initializer_list< basic_json > init=std::initializer_list< basic_json >())
explicitly create an object from an initializer list
Definition json.hpp:2205
a class to store JSON values
Definition json.hpp:1040
basic_json<> json
default JSON class
Definition json.hpp:12369

Output (play with this example online):
true
false
false
false
false
true
false
true
false

The example code above can be translated with
g++ -std=c++11 -Isrc doc/examples/empty.cpp -o empty 
See also
size() – returns the number of elements
Since
version 1.0.0

Definition at line 4995 of file json.hpp.