JSON for Modern C++  3.7.0

◆ back() [1/2]

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>
reference nlohmann::basic_json::back ( )
inline

Returns a reference to the last element in the container. For a JSON container c, the expression c.back() is equivalent to

auto tmp = c.end();
--tmp;
return *tmp;
Returns
In case of a structured type (array or object), a reference to the last element is returned. In case of number, string, or boolean values, a reference to the value is returned.
Complexity
Constant.
Precondition
The JSON value must not be null (would throw std::out_of_range) or an empty array or object (undefined behavior, guarded by assertions).
Postcondition
The JSON value remains unchanged.
Exceptions
invalid_iterator.214when called on a null value. See example below.
Example
The following code shows an example for back().
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_boolean = true;
10  json j_number_integer = 17;
11  json j_number_float = 23.42;
12  json j_object = {{"one", 1}, {"two", 2}};
13  json j_object_empty(json::value_t::object);
14  json j_array = {1, 2, 4, 8, 16};
15  json j_array_empty(json::value_t::array);
16  json j_string = "Hello, world";
17 
18  // call back()
19  std::cout << j_boolean.back() << '\n';
20  std::cout << j_number_integer.back() << '\n';
21  std::cout << j_number_float.back() << '\n';
22  std::cout << j_object.back() << '\n';
23  //std::cout << j_object_empty.back() << '\n'; // undefined behavior
24  std::cout << j_array.back() << '\n';
25  //std::cout << j_array_empty.back() << '\n'; // undefined behavior
26  std::cout << j_string.back() << '\n';
27 
28  // back() called on a null value
29  try
30  {
31  json j_null;
32  j_null.back();
33  }
34  catch (json::invalid_iterator& e)
35  {
36  std::cout << e.what() << '\n';
37  }
38 }
basic_json<> json
default JSON class
Definition: json.hpp:2355
detail::invalid_iterator invalid_iterator
exception indicating errors with iterators
Definition: json.hpp:14641

Output (play with this example online):
true
17
23.42
2
16
"Hello, world"
[json.exception.invalid_iterator.214] cannot get value
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/back.cpp -o back 
See also
front() – access the first element
Since
version 1.0.0

Definition at line 17999 of file json.hpp.