JSON for Modern C++  3.0.0

◆ iterator_wrapper() [1/2]

template<template< typename, typename, typename... > class ObjectType = std::map, template< typename, typename... > 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 > class AllocatorType = std::allocator, template< typename, typename=void > class JSONSerializer = adl_serializer>
static iteration_proxy<iterator> nlohmann::basic_json::iterator_wrapper ( reference  ref)
inlinestatic

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 "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 }
basic_json<> json
default JSON class
Definition: json.hpp:14353
static iteration_proxy< iterator > iterator_wrapper(reference ref)
wrapper to access iterator member functions in range-based for
Definition: json.hpp:11359
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 -Isrc 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.

Definition at line 11359 of file json.hpp.