JSON for Modern C++ 2.1.1
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◆ size()

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>
size_type nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer >::size ( ) const
inlinenoexcept

Returns the number of elements in a JSON value.

Returns
The return value depends on the different types and is defined as follows:
Value type return value
null 0
boolean 1
string 1
number 1
object result of function object_t::size()
array result of function array_t::size()
Note
This function does not return the length of a string stored as JSON value - it returns the number of elements in the JSON value which is 1 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 size() functions have constant complexity.
Requirements\n This function helps basic_json satisfying the
Container requirements:
  • The complexity is constant.
  • Has the semantics of std::distance(begin(), end()).
Example\n The following code calls size() on the different value
types.
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 size()
19 std::cout << j_null.size() << '\n';
20 std::cout << j_boolean.size() << '\n';
21 std::cout << j_number_integer.size() << '\n';
22 std::cout << j_number_float.size() << '\n';
23 std::cout << j_object.size() << '\n';
24 std::cout << j_object_empty.size() << '\n';
25 std::cout << j_array.size() << '\n';
26 std::cout << j_array_empty.size() << '\n';
27 std::cout << j_string.size() << '\n';
28}
size_type size() const noexcept
returns the number of elements
Definition json.hpp:5063
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):
0
1
1
1
2
0
5
0
1

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

Definition at line 5063 of file json.hpp.