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>
3.2 KiB
nlohmann::basic_json_view::get_string
string_view_t get_string() const;
Returns the string value as a string_view_t, without copying it.
Return value
The string, as a string_view_t that points either into the document's
source() text (a string with no escape sequences), or into the document's own
buffer of decoded strings (a string that contains escape sequences, such as #!json "\n" or #!json "\u00e9", which
had to be decoded once when the document was parsed).
Exception safety
Strong guarantee: there are no changes to the view or the document it refers to.
Exceptions
Throws type_error.302 if the value is not a string; example:
"type must be string, but is array".
This exception does not carry a JSON_DIAGNOSTICS path: the view has no
BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with
JSON_DIAGNOSTICS enabled.
Complexity
Constant.
Notes
basic_json_view has no BasicJsonType value stored anywhere, so unlike BasicJsonType, it has no get_ref() to
hand out a reference to a stored string_t. get_string() (equivalently, get<string_view_t>()) is the
zero-copy alternative: BasicJsonType::get_ref<const string_t&>() is its closest
counterpart, except that it returns a view instead of a reference to a value that must already exist.
get_string() borrows the string: nothing is copied, whereas get<std::string>() copies it. For a borrowed
document, the returned string_view_t is valid as long as the caller's buffer; for an owned
one, as long as the document.
The returned string_view_t is valid exactly as long as the view that produced it -- see the
validity rules of basic_json_view -- and, for a string with no escapes, for as long as the document's
source text, which for a borrowed document must remain valid and unmodified for as long as the document exists,
since floating-point values and unescaped strings are read from the source lazily.
Examples
??? example
The example below pulls one field out of a JSON text that stands in for a large API response, and shows that no
`#!cpp std::string` was allocated for it: the returned view still points inside the original buffer. A field that
contains an escape sequence cannot point into the original text -- it was decoded once into the document's own
buffer instead -- but still avoids a per-field allocation.
```cpp
--8<-- "examples/basic_json_view__get_string.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__get_string.output"
```
See also
- get - convert the value to a given type (
#!cpp get<string_view_t>()is equivalent to this function) - number_token - a number's token text, without a copy
BasicJsonType::get_ref- the closest counterpart ofbasic_json
Version history
- Added in version 3.13.0.