JSON for Modern C++  3.7.0

◆ iterator_wrapper() [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>
static iteration_proxy<iterator> nlohmann::basic_json::iterator_wrapper ( reference  ref)
inlinestaticnoexcept

This function allows to access iterator::key() and iterator::value() during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.

For loop without iterator_wrapper:

for (auto it = j_object.begin(); it != j_object.end(); ++it)
{
std::cout << "key: " << it.key() << ", value:" << it.value() << '\n';
}

Range-based for loop without iterator proxy:

for (auto it : j_object)
{
// "it" is of type json::reference and has no key() member
std::cout << "value: " << it << '\n';
}

Range-based for loop with iterator proxy:

for (auto it : json::iterator_wrapper(j_object))
{
std::cout << "key: " << it.key() << ", value:" << it.value() << '\n';
}
Note
When iterating over an array, key() will return the index of the element as string (see example).
Parameters
[in]refreference to a JSON value
Returns
iteration proxy object wrapping ref with an interface to use in range-based for loops
Example
The following code shows how the wrapper is used
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_object = {{"one", 1}, {"two", 2}};
10  json j_array = {1, 2, 4, 8, 16};
11 
12  // example for an object
13  for (auto& x : json::iterator_wrapper(j_object))
14  {
15  std::cout << "key: " << x.key() << ", value: " << x.value() << '\n';
16  }
17 
18  // example for an array
19  for (auto& x : json::iterator_wrapper(j_array))
20  {
21  std::cout << "key: " << x.key() << ", value: " << x.value() << '\n';
22  }
23 }
static iteration_proxy< iterator > iterator_wrapper(reference ref) noexcept
wrapper to access iterator member functions in range-based for
Definition: json.hpp:18808
basic_json<> json
default JSON class
Definition: json.hpp:2355

Output (play with this example online):
key: one, value: 1
key: two, value: 2
key: 0, value: 1
key: 1, value: 2
key: 2, value: 4
key: 3, value: 8
key: 4, value: 16
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/iterator_wrapper.cpp -o iterator_wrapper 
Exception safety
Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
Complexity
Constant.
Note
The name of this function is not yet final and may change in the future.
Deprecated:
This stream operator is deprecated and will be removed in future 4.0.0 of the library. Please use items() instead; that is, replace json::iterator_wrapper(j) with j.items().

Definition at line 18808 of file json.hpp.