JSON for Modern C++  3.6.1

◆ 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::empty ( ) const
inlinenoexcept

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
object result of function object_t::empty()
array result of function array_t::empty()
Example
The following code uses empty() to check if a JSON object contains any elements.
1 #include <iostream>
2 #include <nlohmann/json.hpp>
3 
4 using json = nlohmann::json;
5 
6 int main()
7 {
8  // create JSON values
9  json j_null;
10  json j_boolean = true;
11  json j_number_integer = 17;
12  json j_number_float = 23.42;
13  json j_object = {{"one", 1}, {"two", 2}};
14  json j_object_empty(json::value_t::object);
15  json j_array = {1, 2, 4, 8, 16};
16  json j_array_empty(json::value_t::array);
17  json j_string = "Hello, world";
18 
19  // call empty()
20  std::cout << std::boolalpha;
21  std::cout << j_null.empty() << '\n';
22  std::cout << j_boolean.empty() << '\n';
23  std::cout << j_number_integer.empty() << '\n';
24  std::cout << j_number_float.empty() << '\n';
25  std::cout << j_object.empty() << '\n';
26  std::cout << j_object_empty.empty() << '\n';
27  std::cout << j_array.empty() << '\n';
28  std::cout << j_array_empty.empty() << '\n';
29  std::cout << j_string.empty() << '\n';
30 }
basic_json<> json
default JSON class
Definition: json.hpp:871

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 -Isingle_include doc/examples/empty.cpp -o empty 
Complexity
Constant, as long as array_t and object_t satisfy the Container concept; that is, their empty() functions have constant complexity.
Iterator validity
No changes.
Exception safety
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.
Requirements
This function helps basic_json satisfying the Container requirements:
  • The complexity is constant.
  • Has the semantics of begin() == end().
See also
size() – returns the number of elements
Since
version 1.0.0

Definition at line 17254 of file json.hpp.