C++ named requirements: NullablePointer (since C++11)
Specifies that the type is a pointerlike object which can be compared to std::nullptr_t objects.
Contents 
[edit] Requirements
The type must meet all of the following requirements:
In addition, a valueinitialized object of the type must produce a null value of that type. This null value shall only be equivalent to itself. Default initialization of the type may have an indeterminate or erroneous(since C++26) value.
A value of the type must be contextually convertible to bool. The effect of this conversion returns false if the value is equivalent to its null value and true otherwise.
None of the operations that this type performs may throw exceptions.
The type must satisfy the following additional expressions, given two values p and q that are of the type, and that np is a value of std::nullptr_t type (possibly constqualified):
Declaration  Effects  
Type p(np);
Type p = np; 
Afterwards, p is equivalent to nullptr  
Expression  Effects  
Type(np)  A temporary object that is equivalent to nullptr  
p = np  Must return a Type& , and afterwards, p is equivalent to nullptr
 
p != q 
The effect is !(p == q)  
p == np
np == p 
The effect is (p == Type())  
p != np
np != p 
The effect is !(p == np) 
[edit] Notes
Note that dereferencing (operator* or operator>) is not required for a NullablePointer type. A minimalistic type that satisfies these requirements is
class handle { int id = 0; public: handle() = default; handle(std::nullptr_t) {} explicit operator bool() const { return id != 0; } friend bool operator==(handle l, handle r) { return l.id == r.id; } friend bool operator!=(handle l, handle r) { return !(l == r); } // or only a defaulted operator== (since C++20) };
[edit] Standard library
The following types satisfy NullablePointer:
The following types must satisfy NullablePointer in order to communicate with standard library components:
 The member types
X::pointer
,X::const_pointer
,X::void_pointer
andX::const_void_pointer
of every Allocator typeX
.  The member type
pointer
of std::unique_ptr.

(since C++23) 
[edit] Defect reports
The following behaviorchanging defect reports were applied retroactively to previously published C++ standards.
DR  Applied to  Behavior as published  Correct behavior 

LWG 2114 (P2167R3) 
C++11  contextual convertibility to bool was too weak to reflect the expectation of implementations  requirements strengthened 