Add element access, iteration, values, and JSON pointers to json_view

Give basic_json_view the read-only access functions of basic_json:
operator[] and at() with keys and indices, front()/back(), find(),
contains(), count(), begin()/end() and cbegin()/cend(), items()
with structured bindings from C++17 on, and type_name().

Exceptions have the ids and messages of the const functions of
basic_json. Where basic_json has undefined behavior the view
answers safely: operator[] with a missing key or an out-of-range
index returns a discarded view, and front()/back() of an empty
container throw invalid_iterator.214. Objects are iterated in
document order, and all members are visited; duplicate-key lookups
find the first member (as yyjson and simdjson do), while parse(),
materialize(), and the map conversions keep the last value, as
parse() does. Keys of up to 16 bytes are compared with two
overlapping loads.

Add value conversions: get<T>()/get_to() for arithmetic types,
bool, nullptr_t, strings (std::basic_string copied,
string_view_t without a copy), BasicJsonType, views, std::vector,
and maps with string keys; get_string() for the string without a
copy; number_token() for the number exactly as written in the
source; value() with keys and JSON pointers; and operator[]/at()/
contains() with JSON pointers. Everything else, including types
with from_json(), goes through materialize() of that subtree.
get<T>() of arithmetic types is inlined down to the conversion, so
reading an integer needs no call.

detail::json_pointer_access exposes a pointer's reference tokens
to code outside basic_json.

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

docs/mkdocs/docs/api/basic_json/at.md

@@ -233,6 +233,7 @@ Strong exception safety: if an exception occurs, the original value stays intact
 - documentation on [checked access](../../features/element_access/checked_access.md)
 - [`operator[]`](operator%5B%5D.md) for unchecked access by reference
 - [`value`](value.md) for access with default value
+- [basic_json_view::at](../basic_json_view/at.md) - the same access on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/back.md

@@ -58,6 +58,7 @@ Constant.
 ## See also
 
 - [front](front.md) to access the first element
+- [basic_json_view::back](../basic_json_view/back.md) - the same access on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/begin.md

@@ -44,6 +44,8 @@ Constant.
 - [rbegin](rbegin.md) returns a reverse iterator to the last element
 - [items](items.md) returns an iteration proxy to access keys and values during range-based for loops
 - [Iterators](../../features/iterators.md) - the article on iterators
+- [basic_json_view::begin](../basic_json_view/begin.md) - the same iteration on a zero-copy view (in document order,
+  not sorted by key)
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/cbegin.md

@@ -42,6 +42,7 @@ Constant.
 - [cend](cend.md) returns a const iterator to one past the last element
 - [crbegin](crbegin.md) returns a const reverse iterator to the last element
 - [Iterators](../../features/iterators.md) - the article on iterators
+- [basic_json_view::cbegin](../basic_json_view/cbegin.md) - the same iteration on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/cend.md

@@ -42,6 +42,7 @@ Constant.
 - [cbegin](cbegin.md) returns a const iterator to the first element
 - [crend](crend.md) returns a const reverse iterator to one before the first element
 - [Iterators](../../features/iterators.md) - the article on iterators
+- [basic_json_view::cend](../basic_json_view/cend.md) - the same iteration on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/contains.md

@@ -127,6 +127,7 @@ Logarithmic in the size of the JSON object.
 
 - [find](find.md) find a value in an object
 - [count](count.md) returns the number of occurrences of a key
+- [basic_json_view::contains](../basic_json_view/contains.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/count.md

@@ -80,6 +80,7 @@ Logarithmic in the size of the JSON object.
 
 - [find](find.md) find a value in an object
 - [contains](contains.md) checks whether a key exists
+- [basic_json_view::count](../basic_json_view/count.md) - the same check on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/end.md

@@ -43,6 +43,7 @@ Constant.
 - [cend](cend.md) returns a const iterator to one past the last element
 - [rend](rend.md) returns a reverse iterator to one before the first element
 - [Iterators](../../features/iterators.md) - the article on iterators
+- [basic_json_view::end](../basic_json_view/end.md) - the same iteration on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/find.md

@@ -84,6 +84,7 @@ Logarithmic in the size of the JSON object.
 
 - [count](count.md) returns the number of occurrences of a key
 - [contains](contains.md) checks whether a key exists
+- [basic_json_view::find](../basic_json_view/find.md) - the same lookup on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/front.md

@@ -51,6 +51,7 @@ Constant.
 ## See also
 
 - [back](back.md) to access the last element
+- [basic_json_view::front](../basic_json_view/front.md) - the same access on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/get.md

@@ -183,6 +183,8 @@ overload (3).
 - [get_ref](get_ref.md) get a reference to the stored value
 - [operator ValueType](operator_ValueType.md) get a value via implicit conversion
 - [Converting values](../../features/conversions.md) - the type conversions article
+- [basic_json_view::get](../basic_json_view/get.md) - the same conversion on a zero-copy view (many types are
+  converted without ever building a `basic_json` value)
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/get_ref.md

@@ -61,6 +61,8 @@ Constant.
 ## See also
 
 - [get_ptr()](get_ptr.md) get a pointer value
+- [basic_json_view::get_string](../basic_json_view/get_string.md) - the closest counterpart on a zero-copy view: a
+  string without a copy, but as a view rather than a reference to a value that must already exist
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/get_to.md

@@ -72,6 +72,7 @@ Depends on the `json_serializer<ValueType>::from_json()` implementation.
 - [get_ref](get_ref.md) get a reference to the stored value
 - [get_ptr](get_ptr.md) get a pointer to the stored value
 - [Converting values](../../features/conversions.md) - the type conversions article
+- [basic_json_view::get_to](../basic_json_view/get_to.md) - the same conversion on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/items.md

@@ -99,6 +99,8 @@ When iterating over an array, `key()` will return the index of the element as st
 
 - [begin](begin.md) returns an iterator to the first element
 - [end](end.md) returns an iterator to one past the last element
+- [basic_json_view::items](../basic_json_view/items.md) - the same range on a zero-copy view (`#!cpp const auto`,
+  not `#!cpp const auto&`: items are produced on the fly)
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/operator[].md

@@ -275,6 +275,8 @@ Strong exception safety: if an exception occurs, the original value stays intact
 - documentation on [runtime assertions](../../features/assertions.md)
 - see [`at`](at.md) for access by reference with range checking
 - see [`value`](value.md) for access with default value
+- [basic_json_view::operator[]](../basic_json_view/operator%5B%5D.md) - the same access on a zero-copy view (always
+  returns a discarded view instead of assuming undefined behavior)
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/type_name.md

@@ -56,6 +56,7 @@ Constant.
 
 - [type](type.md) returns the type of the JSON value
 - [value_t](value_t.md) the enumeration of JSON types
+- [basic_json_view::type_name](../basic_json_view/type_name.md) - the same function on a zero-copy view
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json/value.md

@@ -216,6 +216,7 @@ changes to any JSON value.
 
 - see [`at`](at.md) for access by reference with range checking
 - see [`operator[]`](operator%5B%5D.md) for unchecked access by reference
+- [basic_json_view::value](../basic_json_view/value.md) - the same access on a zero-copy view
 
 ## Version history