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◆ items() [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, class BinaryType = std::vector<std::uint8_t>>
iteration_proxy<const_iterator> nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::items |
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inlinenoexcept |
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 items() function:
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 items() function:
for (auto it : j_object)
{
std::cout << "value: " << it << '\n';
}
Range-based for loop with items() function:
for (auto& el : j_object.items())
{
std::cout << "key: " << el.key() << ", value:" << el.value() << '\n';
}
The items() function also allows to use structured bindings (C++17):
for (auto& [key, val] : j_object.items())
{
std::cout << "key: " << key << ", value:" << val << '\n';
}
- Note
- When iterating over an array,
key() will return the index of the element as string (see example). For primitive types (e.g., numbers), key() returns an empty string.
- Warning
- Using
items() on temporary objects is dangerous. Make sure the object's lifetime exeeds the iteration. See https://github.com/nlohmann/json/issues/2040 for more information.
- Returns
- iteration proxy object wrapping ref with an interface to use in range-based for loops
- Example
- The following code shows how the function is used.
2 #include <nlohmann/json.hpp>
9 json j_object = {{ "one", 1}, { "two", 2}};
10 json j_array = {1, 2, 4, 8, 16};
13 for ( auto& x : j_object.items())
15 std::cout << "key: " << x.key() << ", value: " << x.value() << '\n';
19 for ( auto& x : j_array.items())
21 std::cout << "key: " << x.key() << ", value: " << x.value() << '\n';
basic_json<> json default JSON class
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 withg++ -std=c++11 -Isingle_include doc/examples/items.cpp -o items
- Exception safety
- Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
- Complexity
- Constant.
- Since
- version 3.1.0, structured bindings support since 3.5.0.
Definition at line 21374 of file json.hpp.
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