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.
- 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';
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 19077 of file json.hpp.