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Niels Lohmann
7b755737b4
From gregmarr's review of #5622: - count() counts every member with the key, so count(key) > 1 detects duplicate keys; it was contains() as 0/1 before. - get_to() passes types that get<T>() does not convert directly to from_json with the existing value, as basic_json::get_to() does, so that a from_json which sets only the fields present keeps the others. - array_index() checks the empty token and "-" first; the results are unchanged. - Docs: no "if an exception is thrown" where a view never changes anything; the borrowed source must also stay unmodified; items() on a temporary view is safe; back()/front() notes shortened; the basic_json see-also entries say how the view's results can differ. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
1.4 KiB
1.4 KiB
nlohmann::basic_json::front
reference front();
const_reference front() const;
Returns a reference to the first element in the container. For a JSON container #!cpp c, the expression
#!cpp c.front() is equivalent to #!cpp *c.begin().
Return value
In the case of a structured type (array or object), a reference to the first element is returned. In the case of number, string, boolean, or binary values, a reference to the value is returned.
Exception safety
Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
Exceptions
If the JSON value is #!json null, exception
invalid_iterator.214 is thrown.
Complexity
Constant.
Notes
!!! info "Precondition"
The array or object must not be empty. Calling `front` on an empty array or object yields undefined behavior.
Examples
??? example
The following code shows an example for `front()`.
```cpp
--8<-- "examples/front.cpp"
```
Output:
```json
--8<-- "examples/front.output"
```
See also
- back to access the last element
- basic_json_view::front - the same access on a zero-copy view (document order, not sorted; duplicate keys are visited)
Version history
- Added in version 1.0.0.
- Adjusted code to return reference to binary values in version 3.8.0.