Remove the conversion to bool from json_view

explicit operator bool meant "refers to a value", which silently differs
from what a basic_json converts to. Use !v.is_discarded() instead.

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

docs/mkdocs/docs/api/basic_json_view/basic_json_view.md

@@ -4,8 +4,8 @@
 basic_json_view() noexcept = default;
 ```
 
-Creates an invalid (discarded) view: [`type()`](type.md) is `#!cpp value_t::discarded`,
-[`is_discarded()`](is_discarded.md) is `#!cpp true`, and `#!cpp explicit operator bool()` is `#!cpp false`.
+Creates an invalid (discarded) view: [`type()`](type.md) is `#!cpp value_t::discarded` and
+[`is_discarded()`](is_discarded.md) is `#!cpp true`.
 
 This is the only constructor a caller can use directly. Every other view is obtained from a
 [`basic_json_document`](../basic_json_document/index.md), via [`root()`](../basic_json_document/root.md) or (once
@@ -43,7 +43,6 @@ placeholder for "no value yet" and later be assigned a real view.
 ## See also
 
 - [is_discarded](is_discarded.md) - return whether the view is invalid
-- [operator bool](operator_bool.md) - return whether the view refers to a value
 - [root](../basic_json_document/root.md) - the view of a document's root value
 
 ## Version history

docs/mkdocs/docs/api/basic_json_view/index.md

@@ -62,7 +62,6 @@ element access, iteration, `get<T>()`, JSON Pointer support, `dump()`, or compar
 - [**is_primitive**](is_primitive.md) - return whether the type is primitive
 - [**is_structured**](is_structured.md) - return whether the type is structured
 - [**is_discarded**](is_discarded.md) - return whether the view is invalid
-- [**operator bool**](operator_bool.md) - return whether the view refers to a value
 
 ### Capacity
 

docs/mkdocs/docs/api/basic_json_view/is_discarded.md

@@ -23,8 +23,9 @@ Constant.
 
 ## Notes
 
-`#!cpp v.is_discarded()` and `#!cpp !static_cast<bool>(v)` are equivalent; use whichever reads better at the call
-site.
+A `basic_json_view` is not convertible to `#!cpp bool`: such a conversion would mean "refers to a value", whereas
+`basic_json` converts to the `#!cpp bool` it holds, so the same code would silently behave differently. Test
+`#!cpp !v.is_discarded()` explicitly.
 
 ## Examples
 
@@ -45,7 +46,6 @@ site.
 
 ## See also
 
-- [operator bool](operator_bool.md) - return whether the view refers to a value
 - [(constructor)](basic_json_view.md) - the default constructor creates a discarded view
 - [is_discarded (basic_json_document)](../basic_json_document/is_discarded.md) - return whether the last parse failed
 - [`BasicJsonType::is_discarded`](../basic_json/is_discarded.md) - the corresponding function of `basic_json`

docs/mkdocs/docs/api/basic_json_view/operator_bool.md

@@ -1,46 +0,0 @@
-# <small>nlohmann::basic_json_view::</small>operator bool
-
-```cpp
-explicit operator bool() const noexcept;
-```
-
-Returns whether this view refers to a value, i.e. the negation of [`is_discarded()`](is_discarded.md). Being
-`#!cpp explicit`, this conversion is only considered in a boolean context (`#!cpp if (v)`, `#!cpp !v`, `#!cpp v &&
-...`), not for implicit conversions to other types.
-
-## Return value
-
-`#!cpp true` if the view refers to a value, `#!cpp false` if it is [discarded](is_discarded.md).
-
-## Exception safety
-
-No-throw guarantee: this function never throws exceptions.
-
-## Complexity
-
-Constant.
-
-## Examples
-
-??? example
-
-    The example below classifies several parsed documents by the type of their root value, without materializing any
-    of them into a `BasicJsonType` value.
-
-    ```cpp
-    --8<-- "examples/basic_json_view__type_predicates.cpp"
-    ```
-
-    Output:
-
-    ```json
-    --8<-- "examples/basic_json_view__type_predicates.output"
-    ```
-
-## See also
-
-- [is_discarded](is_discarded.md) - return whether the view is invalid
-
-## Version history
-
-- Added in version 3.13.0.

docs/mkdocs/docs/examples/basic_json_view__basic_json_view.cpp

@@ -8,10 +8,10 @@ int main()
     // the default constructor is the only public one: it creates an invalid
     // (discarded) view, useful as a "no value yet" placeholder
     nlohmann::json_view v;
-    std::cout << static_cast<bool>(v) << ' ' << v.is_discarded() << '\n';
+    std::cout << v.is_discarded() << '\n';
 
     // views are trivially copyable handles (two pointers); the document owns
     // the actual data
     nlohmann::json_view copy = v;
-    std::cout << static_cast<bool>(copy) << '\n';
+    std::cout << copy.is_discarded() << '\n';
 }

docs/mkdocs/docs/examples/basic_json_view__basic_json_view.output

@@ -1,2 +1,2 @@
-false true
-false
+true
+true

docs/mkdocs/docs/examples/basic_json_view__type_predicates.cpp

@@ -35,8 +35,8 @@ int main()
     // parse without exceptions, are both discarded
     nlohmann::json_view invalid;
     json_document failed = json_document::parse("not json", /* allow_exceptions */ false);
-    std::cout << static_cast<bool>(invalid) << ' ' << invalid.is_discarded() << '\n';
-    std::cout << static_cast<bool>(failed.root()) << ' ' << failed.root().is_discarded() << '\n';
+    std::cout << invalid.is_discarded() << '\n';
+    std::cout << failed.root().is_discarded() << '\n';
 
     // type() returns the same value_t enumeration as basic_json::type()
     std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\n';

docs/mkdocs/docs/examples/basic_json_view__type_predicates.output

@@ -7,6 +7,6 @@ true
 true true
 true false
 false
-false true
-false true
+true
+true
 true

docs/mkdocs/mkdocs.yml

@@ -266,7 +266,6 @@ nav:
           - 'is_string': api/basic_json_view/is_string.md
           - 'is_structured': api/basic_json_view/is_structured.md
           - 'materialize': api/basic_json_view/materialize.md
-          - 'operator bool': api/basic_json_view/operator_bool.md
           - 'size': api/basic_json_view/size.md
           - 'source_offset': api/basic_json_view/source_offset.md
           - 'type': api/basic_json_view/type.md

include/nlohmann/json_view.hpp

@@ -156,12 +156,6 @@ class basic_json_view
         return type() == value_t::discarded;
     }
 
-    /// false for discarded views
-    explicit operator bool() const noexcept
-    {
-        return m_node != nullptr;
-    }
-
     //////////////
     // capacity //
     //////////////

tests/src/unit-json_view.cpp

@@ -41,6 +41,8 @@ template<typename... Args>
 using accept_call_t = decltype(json_document::accept(std::declval<Args>()...));
 template<typename... Args>
 using read_call_t = decltype(std::declval<json_document&>().read(std::declval<Args>()...));
+template<typename View>
+using bool_conversion_t = decltype(static_cast<bool>(std::declval<View>()));
 
 #if !defined(JSON_NOEXCEPTION)
 // the exception parse() throws for a text, or "" if it accepts it
@@ -155,7 +157,6 @@ TEST_CASE("json_view")
             CHECK(v.is_primitive() == j.is_primitive());
             CHECK(v.is_structured() == j.is_structured());
             CHECK(!v.is_discarded());
-            CHECK(static_cast<bool>(v));
             CHECK(v.size() == j.size());
             CHECK(v.empty() == j.empty());
             CHECK(v.materialize() == j);
@@ -163,7 +164,6 @@ TEST_CASE("json_view")
 
         const json_view invalid{};
         CHECK(invalid.is_discarded());
-        CHECK(!static_cast<bool>(invalid));
         CHECK(invalid.type() == json::value_t::discarded);
         CHECK(invalid.size() == 0);
         CHECK(invalid.empty());
@@ -380,6 +380,10 @@ TEST_CASE("json_view")
         static_assert(is_detected<read_call_t, const char*, bool>::value, "read(ptr, bool) is valid");
         static_assert(!is_detected<read_call_t, const char*, std::size_t>::value, "read(ptr, len) must not compile");
 
+        // json_view has no conversion to bool: unlike basic_json's, it would
+        // mean "exists", not "is not null"; use is_discarded()
+        static_assert(!is_detected<bool_conversion_t, json_view>::value, "json_view must not convert to bool");
+
         // the valid calls still work
         const char* const text = "[1]";
         CHECK(json_document::parse(text, true).root().is_array());