mirror of
https://github.com/nlohmann/json.git
synced 2024-12-13 04:36:45 +08:00
156 lines
4.0 KiB
Markdown
156 lines
4.0 KiB
Markdown
# Iterators
|
|
|
|
## Overview
|
|
|
|
A `basic_json` value is a container and allows access via iterators. Depending on the value type, `basic_json` stores zero or more values.
|
|
|
|
As for other containers, `begin()` returns an iterator to the first value and `end()` returns an iterator to the value following the last value. The latter iterator is a placeholder and cannot be dereferenced. In case of null values, empty arrays, or empty objects, `begin()` will return `end()`.
|
|
|
|
|
|
|
|
### Iteration order for objects
|
|
|
|
When iterating over objects, values are ordered with respect to the `object_comparator_t` type which defaults to `std::less`. See the [types documentation](types/index.md#key-order) for more information.
|
|
|
|
??? example
|
|
|
|
```cpp
|
|
// create JSON object {"one": 1, "two": 2, "three": 3}
|
|
json j;
|
|
j["one"] = 1;
|
|
j["two"] = 2;
|
|
j["three"] = 3;
|
|
|
|
for (auto it = j.begin(); it != j.end(); ++it)
|
|
{
|
|
std::cout << *it << std::endl;
|
|
}
|
|
```
|
|
|
|
Output:
|
|
|
|
```json
|
|
1
|
|
3
|
|
2
|
|
```
|
|
|
|
The reason for the order is the lexicographic ordering of the object keys "one", "three", "two".
|
|
|
|
### Access object key during iteration
|
|
|
|
The JSON iterators have two member functions, `key()` and `value()` to access the object key and stored value, respectively. When calling `key()` on a non-object iterator, an [invalid_iterator.207](../home/exceptions.md#jsonexceptioninvalid_iterator207) exception is thrown.
|
|
|
|
??? example
|
|
|
|
```cpp
|
|
// create JSON object {"one": 1, "two": 2, "three": 3}
|
|
json j;
|
|
j["one"] = 1;
|
|
j["two"] = 2;
|
|
j["three"] = 3;
|
|
|
|
for (auto it = j.begin(); it != j.end(); ++it)
|
|
{
|
|
std::cout << it.key() << " : " << it.value() << std::endl;
|
|
}
|
|
```
|
|
|
|
Output:
|
|
|
|
```json
|
|
one : 1
|
|
three : 3
|
|
two : 2
|
|
```
|
|
|
|
### Range-based for loops
|
|
|
|
C++11 allows to use range-based for loops to iterate over a container.
|
|
|
|
```cpp
|
|
for (auto it : j_object)
|
|
{
|
|
// "it" is of type json::reference and has no key() member
|
|
std::cout << "value: " << it << '\n';
|
|
}
|
|
```
|
|
|
|
For this reason, the `items()` 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.
|
|
|
|
```cpp
|
|
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):
|
|
|
|
```cpp
|
|
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. 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.
|
|
|
|
### Reverse iteration order
|
|
|
|
`rbegin()` and `rend()` return iterators in the reverse sequence.
|
|
|
|
|
|
|
|
??? example
|
|
|
|
```cpp
|
|
json j = {1, 2, 3, 4};
|
|
|
|
for (auto it = j.begin(); it != j.end(); ++it)
|
|
{
|
|
std::cout << *it << std::endl;
|
|
}
|
|
```
|
|
|
|
Output:
|
|
|
|
```json
|
|
4
|
|
3
|
|
2
|
|
1
|
|
```
|
|
|
|
### Iterating strings and binary values
|
|
|
|
Note that "value" means a JSON value in this setting, not values stored in the underlying containers. That is, `*begin()` returns the complete string or binary array and is also safe the underlying string or binary array is empty.
|
|
|
|
??? example
|
|
|
|
```cpp
|
|
json j = "Hello, world";
|
|
for (auto it = j.begin(); it != j.end(); ++it)
|
|
{
|
|
std::cout << *it << std::endl;
|
|
}
|
|
```
|
|
|
|
Output:
|
|
|
|
```json
|
|
"Hello, world"
|
|
```
|
|
|
|
## Iterator invalidation
|
|
|
|
| Operations | invalidated iterators |
|
|
| ---------- | --------------------- |
|
|
| `clear` | all |
|