Is Pointer-to- " inner struct" member forbidden ?

Question

Hi! i have a nested struct and i'd like to have a pointer-to-member to one of the nested member:

is it legal?

struct InnerStruct
{
    bool c;
};
struct MyStruct {
    bool t;
    bool b;
    InnerStruct inner;
};

this:

MyStruct mystruct;
//...
bool MyStruct::* toto = &MyStruct::b;

is ok but:

bool MyStruct::* toto = &MyStruct::inner.c;

is not. any idea?

thanks

Here are some details Yes it is &MyStruct::b and not mystruct::b; The code is from a custom RTTI/Property system. For each specified class we keep an array of "Property", including a Ptr-to-member It is used like this:

//somewhere else in code...
( myBaseClassWithCustomRTTIPointer)->* toto = true;

Answer 1

The InnerStruct you care about happens to be contained in an instance of a MyStruct, but that doesn't affect how you get a pointer to the member of the InnerStruct.

bool InnerStruct::* toto2 = &InnerStruct::c;

Edit: Rereading your question, I'm guessing you want to define a pointer to member of the outer struct, and have it point directly at a member of the inner struct. That's simply not allowed. To get to the member of the inner struct that's contained in the outer struct, you'd have to create a pointer to the inner struct itselft, then to its member. To use it, you'd dereference both pointers to members:

// Pointer to inner member of MyStruct:
InnerStruct MyStruct::* toto = &MyStruct::inner;

// Pointer to c member of InnerStruct:
bool InnerStruct::* toto2 = &InnerStruct::c;

// Dereference both to get to the actual bool:
bool x = mystruct.*toto.*toto2;

Answer 2

Yes, it is forbidden. You are not the first to come up with this perfectly logical idea. In my opinion this is one of the obvious "bugs"/"omissions" in the specification of pointers-to-members in C++, but apparently the committee has no interest in developing the specification of pointers-to-members any further (as is the case with most of the "low-level" language features).

Note that everything necessary to implement the feature in already there, in the language. A pointer to a-data-member-of-a-member is in no way different from a pointer to an immediate data member. The only thing that's missing is the syntax to initialize such a pointer. However, the committee is apparently not interested in introducing such a syntax.

From the pure formal logic point of view, this should have been allowed in C++

struct Inner {
  int i;
  int j[10];
};

struct Outer {
  int i;
  int j[10];
  Inner inner;
};

Outer o;
int Outer::*p;

p = &Outer::i; // OK
o.*p = 0; // sets `o.i` to 0

p = &Outer::inner.i; // ERROR, but should have been supported
o.*p = 0; // sets `o.inner.i` to 0

p = &Outer::j[3]; // ERROR, but should have been supported
o.*p = 0; // sets `o.j[3]` to 0

p = &Outer::inner.j[5]; // ERROR, but should have been supported
o.*p = 0; // sets `o.inner.j[5]` to 0