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Niels Lohmann b730946432 Fix key types convertible to std::string_view breaking lookups (#5689 )

Since #4958, a key type implicitly convertible to std::string_view was
accepted by is_usable_as_basic_json_key_type without checking that the
object's comparator can actually compare object_t::key_type with that
key type. The key was then forwarded unchanged to the underlying map,
so const operator[], at, find, count, contains, erase and value failed
to compile (a hard error inside <map>) for a key convertible only to
std::string_view, and value() rejected such keys outright. For keys
convertible to both std::string and std::string_view, the KeyType&&
templates now won overload resolution over the object_t::key_type
overloads and then failed the same way, a regression from 3.12.0. Only
the non-const operator[] worked, because it uses emplace(), which
constructs a std::string from the key explicitly. ordered_json was not
affected, since ordered_map checks comparability itself.

Add a trait, is_string_view_convertible_key_type, that recognizes a key
type that is convertible to std::string_view but not directly
comparable with the object's key type, provided std::string_view itself
is comparable with it. at(), operator[], find(), count(), contains(),
erase() and value() now route such keys through a new lookup_key()
helper that converts them to std::string_view before they reach the
object, matching how the object's transparent comparator already
supports std::string_view lookups.

Fixes #5663.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-10-04 11:46:32 +02:00

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nlohmann::basic_json::contains

// (1)
bool contains(const typename object_t::key_type& key) const;

// (2)
template<typename KeyType>
bool contains(KeyType&& key) const;

// (3)
bool contains(const json_pointer& ptr) const;

Check whether an element exists in a JSON object with a key equivalent to key. If the element is not found or the JSON value is not an object, #!cpp false is returned.
See 1. This overload is only available if KeyType is comparable with #!cpp typename object_t::key_type and #!cpp typename object_comparator_t::is_transparent denotes a type.
Check whether the given JSON pointer ptr can be resolved in the current JSON value.

Template parameters

KeyType: A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17).

Parameters

key (in): key value to check its existence.
ptr (in): JSON pointer to check its existence.

Return value

#!cpp true if an element with specified key exists. If no such element with such a key is found or the JSON value is not an object, #!cpp false is returned.
See 1.
#!cpp true if the JSON pointer can be resolved to a stored value, #!cpp false otherwise.

Exception safety

Strong exception safety: if an exception occurs, the original value stays intact.

Exceptions

The function does not throw exceptions.
The function does not throw exceptions.
The function does not throw exceptions.

Complexity

Logarithmic in the size of the JSON object.

Notes

This method always returns #!cpp false when executed on a JSON type that is not an object.
This method can be executed on any JSON value type.
Calling this function with an integer argument (for example, #!cpp contains(0)) does not compile: such an argument would otherwise implicitly convert to a null #!cpp const char* and, from there, cause undefined behavior when constructing a #!cpp std::string for the object key. To check for an array element instead, use at, operator[], or compare against size.

!!! info "Postconditions"

If `#!cpp j.contains(x)` returns `#!c true` for a key or JSON pointer `x`, then it is safe to call `j[x]`.

!!! warning "Deprecation"

Overload (3) also accepts a [`json_pointer`](../json_pointer/index.md) whose template argument is a `basic_json`
specialization (e.g., `nlohmann::json_pointer<nlohmann::json>`) instead of a string type. This is deprecated since
version 3.11.0 and will be removed in a future major version; use `basic_json::json_pointer` (for `json`,
`nlohmann::json_pointer<std::string>`) instead.

You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated
function.

See the [migration guide](../../integration/migration_guide.md#json-pointers) for how to update existing code.

Examples

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

The example shows how `contains()` is used.

```cpp
--8<-- "examples/contains__object_t_key_type.cpp"
```

Output:

```json
--8<-- "examples/contains__object_t_key_type.output"
```

??? example "Example: (2) check with key using string_view"

The example shows how `contains()` is used.

```cpp
--8<-- "examples/contains__keytype.c++17.cpp"
```

Output:

```json
--8<-- "examples/contains__keytype.c++17.output"
```

??? example "Example: (3) check with JSON pointer"

The example shows how `contains()` is used.

```cpp
--8<-- "examples/contains__json_pointer.cpp"
```

Output:

```json
--8<-- "examples/contains__json_pointer.output"
```

Version history

Added in version 3.11.0.
Added in version 3.6.0. Extended template KeyType to support comparable types in version 3.11.0. Fixed in version 3.13.0 to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
Added in version 3.7.0.
Deleted overloads for integral key types added in version 3.13.0 to reject such calls at compile time instead of causing undefined behavior at runtime.

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