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Niels Lohmann 6ae17630a4 Reject integral keys for contains(), find(), and count() at compile time (#5705 )

j.contains(0), j.find(0), and j.count(0) used to compile: the literal 0 is
a null pointer constant, so it converts to a null const char*, and the
overloads taking const typename object_t::key_type& accepted it by
constructing a std::string from that null pointer, which is undefined
behavior (a crash with both libc++ and libstdc++). value(0, default_value)
had the same problem in C++11, where the object comparator is not
transparent.

Add deleted overloads for integral arguments to contains(), find()
(const and non-const), count(), and value() so that these calls are
compile errors in every supported language mode instead of crashing.
Calls with string, string_view, json_pointer, and size-typed element
access (at(), operator[](), erase()) are unaffected.

Fixes #5657.

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

2026-09-30 20:07:45 +02:00

2.8 KiB

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nlohmann::basic_json::find

// (1)
iterator find(const typename object_t::key_type& key);
const_iterator find(const typename object_t::key_type& key) const;

// (2)
template<typename KeyType>
iterator find(KeyType&& key);
template<typename KeyType>
const_iterator find(KeyType&& key) const;

Finds an element in a JSON object with a key equivalent to key. If the element is not found or the JSON value is not an object, end() 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.

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 of the element to search for.

Return value

Iterator to an element with a key equivalent to key. If no such element is found or the JSON value is not an object, a past-the-end iterator (see end()) is returned.

Exception safety

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

Complexity

Logarithmic in the size of the JSON object.

Notes

This method always returns end() when executed on a JSON type that is not an object.
Calling this function with an integer argument (for example, #!cpp find(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 access an array element instead, use at, operator[], or compare against size.

Examples

??? example "Example: (1) find object element by key"

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

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

Output:

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

??? example "Example: (2) find object element by key using string_view"

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

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

Output:

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

Version history

Added in version 3.11.0.
Added in version 1.0.0. Changed to support comparable types in version 3.11.0.
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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