6 Basic concepts [basic]

6.7 Storage duration [basic.stc]

6.7.4 Dynamic storage duration [basic.stc.dynamic]

6.7.4.3 Safely-derived pointers [basic.stc.dynamic.safety]

(1.1)
an object of an object pointer type ([basic.compound]), or
(1.2)
an object of an integral type that is at least as large as std::intptr_t, or
(1.3)
a sequence of elements in an array of narrow character type ([basic.fundamental]), where the size and alignment of the sequence match those of some object pointer type.

A pointer value is a safely-derived pointer to a dynamic object only if it has an object pointer type and it is one of the following:

(2.1)
the value returned by a call to the C++ standard library implementation of ::operator new(std::size_t) or ::operator new(std::size_t, std::align_val_t);40
(2.2)
the result of taking the address of an object (or one of its subobjects) designated by an lvalue resulting from indirection through a safely-derived pointer value;
(2.3)
the result of well-defined pointer arithmetic ([expr.add]) using a safely-derived pointer value;
(2.4)
the result of a well-defined pointer conversion ([conv.ptr], [expr.cast]) of a safely-derived pointer value;
(2.5)
the result of a reinterpret_cast of a safely-derived pointer value;
(2.6)
the result of a reinterpret_cast of an integer representation of a safely-derived pointer value;
(2.7)
the value of an object whose value was copied from a traceable pointer object, where at the time of the copy the source object contained a copy of a safely-derived pointer value.

An integer value is an integer representation of a safely-derived pointer only if its type is at least as large as std::intptr_t and it is one of the following:

(3.1)
the result of a reinterpret_cast of a safely-derived pointer value;
(3.2)
the result of a valid conversion of an integer representation of a safely-derived pointer value;
(3.3)
the value of an object whose value was copied from a traceable pointer object, where at the time of the copy the source object contained an integer representation of a safely-derived pointer value;
(3.4)
the result of an additive or bitwise operation, one of whose operands is an integer representation of a safely-derived pointer value P, if that result converted by reinterpret_cast<void*> would compare equal to a safely-derived pointer computable from reinterpret_cast<void*>(P).

An implementation may have relaxed pointer safety, in which case the validity of a pointer value does not depend on whether it is a safely-derived pointer value.

Alternatively, an implementation may have strict pointer safety, in which case a pointer value referring to an object with dynamic storage duration that is not a safely-derived pointer value is an invalid pointer value unless the referenced complete object has previously been declared reachable ([util.dynamic.safety]).

[ Note

The effect of using an invalid pointer value (including passing it to a deallocation function) is undefined, see [basic.stc.dynamic.deallocation].

This is true even if the unsafely-derived pointer value might compare equal to some safely-derived pointer value.

— end note

]

It is implementation-defined whether an implementation has relaxed or strict pointer safety.

40)

This section does not impose restrictions on indirection through pointers to memory not allocated by ::operator new.

This maintains the ability of many C++ implementations to use binary libraries and components written in other languages.

In particular, this applies to C binaries, because indirection through pointers to memory allocated by std::malloc is not restricted.