json/api/basic_json_view/contains.md

# <small>nlohmann::basic_json_view::</small>contains

```cpp
// (1)
bool contains(string_view_t key) const;
bool contains(const char* key) const;
bool contains(const string_t& key) const;

// (2)
bool contains(const json_pointer& ptr) const;
```

1. Checks whether the value is an object with a member with key `key`.
2. Checks whether a JSON pointer `ptr` can be resolved, starting at this value.

## Parameters

`key` (in)
:   key value to check its existence

`ptr` (in)
:   JSON pointer to check its existence

## Return value

1. `#!cpp true` if the value is an object and has a member with key `key`, `#!cpp false` otherwise
2. `#!cpp true` if `ptr` can be resolved to a value starting at this view, `#!cpp false` otherwise

## Exception safety

No-throw guarantee: this function never throws exceptions.

## Complexity

1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
   another, in document order, stopping at the first match. Each comparison first checks the
   key's length -- already known from the index, without reading the key bytes -- before comparing its content.
   Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes
   constant time on average.
2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
   that level or the index into the array -- as for [`operator[]`](operator[].md#complexity) and
   [`at`](at.md#complexity) with a JSON pointer.

## Notes

!!! warning "Duplicate keys: the first member wins"

    If the source text repeats a key, this resolves to the *first* member with it, not to the last one that
    [`materialize()`](materialize.md) and `parse()` keep. See the [Notes on duplicate keys](operator[].md#notes) of
    `operator[]` and [Duplicate keys](../../features/json_view.md#duplicate-keys) for the reasons and for how to get
    the last value.

Overload 1 always returns `#!cpp false` when the value is not an object -- including a [discarded](is_discarded.md)
view.

!!! info "Postconditions"

    If `#!cpp v.contains(key)` returns `#!cpp true`, then `#!cpp v[key]` is not [discarded](is_discarded.md) and
    `#!cpp v.at(key)` does not throw. If
    `#!cpp v.contains(ptr)` returns `#!cpp true`, then `#!cpp v[ptr]` is not discarded and `#!cpp v.at(ptr)` does not
    throw.

!!! info "Overload 2 never throws"

    Unlike [`BasicJsonType::contains(const json_pointer&)`](../basic_json/contains.md), which can throw for certain
    malformed pointers (for instance an empty array-index reference token), overload 2 never throws: a missing key,
    an out-of-range or malformed array index, a `#!cpp "-"` index, or a reference token used on a primitive all
    simply make it return `#!cpp false`.

## Examples

??? example "Example: (1) check with key"

    The example below counts how many of a batch of records carry an optional `retry_of` field, using `contains()`
    to check without ever materializing a single record of the batch.

    ```cpp
    #include <iostream>
    #include <nlohmann/json_view.hpp>
    
    using json_document = nlohmann::json_document;
    
    int main()
    {
        // count how many of many incoming records carry an optional "retry_of"
        // field -- contains() only walks the flat index, so scanning a large
        // batch like this never builds a single nlohmann::json value
        json_document batch = json_document::parse(R"(
          [
            {"id": 1},
            {"id": 2, "retry_of": 1},
            {"id": 3},
            {"id": 4, "retry_of": 3}
          ]
        )");
    
        const auto records = batch.root();
        std::size_t retries = 0;
        for (std::size_t i = 0; i < records.size(); ++i)
        {
            if (records[i].contains("retry_of"))
            {
                ++retries;
            }
        }
        std::cout << retries << " of " << records.size() << " records are retries\n";
    }
    
    ```

    Output:

    ```json
    2 of 4 records are retries
    
    ```

??? example "Example: (2) check with JSON pointer"

    The example below checks an optional, nested field with a JSON pointer, and shows two pointers that
    `#!cpp contains()` resolves to `#!cpp false` without throwing.

    ```cpp
    #include <iostream>
    #include <nlohmann/json_view.hpp>
    
    using json_document = nlohmann::json_document;
    using json_pointer = nlohmann::json::json_pointer;
    
    int main()
    {
        // "retry_of" is only present on some records, nested a level down
        // inside "meta"
        json_document batch = json_document::parse(R"(
          [
            {"id": 1, "meta": {}},
            {"id": 2, "meta": {"retry_of": 1}}
          ]
        )");
    
        const auto records = batch.root();
        const json_pointer retry_of("/meta/retry_of");
        for (std::size_t i = 0; i < records.size(); ++i)
        {
            const auto record = records[i];
            if (record.contains(retry_of))
            {
                std::cout << "record " << i << " is a retry of " << record[retry_of].get<int>() << '\n';
            }
            else
            {
                std::cout << "record " << i << " is original\n";
            }
        }
    
        // contains() with a JSON pointer never throws -- not even for a
        // pointer that indexes into a primitive ("/0/id/x") or uses a
        // malformed array index ("/01"), either of which would need a
        // try/catch with json::contains(ptr)
        std::cout << std::boolalpha << records.contains(json_pointer("/0/id/x")) << '\n';
        std::cout << std::boolalpha << records.contains(json_pointer("/01")) << '\n';
    }
    
    ```

    Output:

    ```json
    record 0 is original
    record 1 is a retry of 1
    false
    false
    
    ```

## See also

- [find](find.md) - find a value in an object
- [count](count.md) - returns the number of occurrences of a key
- [at](at.md), [operator[]](operator[].md) - resolve a JSON pointer and throw, or return a discarded view
- [`BasicJsonType::contains`](../basic_json/contains.md) - the corresponding function of `basic_json`

## Version history

- Added in version 3.13.0.