c++ - Conversion from Derived** to Base*const* -

please read question before linking parashift, can google search, different case.

this isn't allowed

child **cc; base ** bb = cc; 

because do

*bb = new otherchild; 

but if have

child **cc; const base *const *const bb = cc; 

i don't think const necessary example, sure..

i think minimum should work

base *const *bb = cc; 

then can't this

*bb = new otherchild; 

so should safe. why isn't allowed?

you're confusing 2 cases:

• the addition of const
• upcasts

while formally (in computer science theory) both of these deal subclassing, reality c++ rules these different, because representation of const t , t guaranteed same, while representations of base* , derived* differ offset (but may radically different when virtual inheritance involved).

in 3.9.3, standard declares that

the cv-qualified or cv-unqualified versions of type distinct types; however, shall have same representation , alignment requirements

given:

struct base {}; struct derived : base {}; derived* pd = nullptr; base* pb = pd; 

const can indeed added in way suggest.

base const* const* const cpcpcb = &pb; base* const* pcpb = &pb; // legal, pointer can't changed base const* * ppcb = &pb; // illegal, 1 try rebind pointer                           // const object,                           // use pb mutate const object 

but there no is-a relationship between derived* , base*. conversion exists, derived* variable not contain address of base object (the base subobject within derived object may have different address). , therefore both line you're complaining about, , line question assumed valid, illegal:

base const* const* const cpcpcd = &pd; // error, there's no address of base                                        // found in pd base* const* pcpd = &pd; // error: again, there's no address of base                          // stored in pd 

formally, standard describes in 4.10:

a prvalue of type "pointer cv d”, d class type, can converted prvalue of type "pointer cv b", b base class of d. if b inaccessible or ambiguous base class of d, program necessitates conversion ill-formed. result of conversion pointer base class subobject of derived class object. null pointer value converted null pointer value of destination type.

the result of conversion prvalue, doesn't have address, , can't create pointer it.