This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 114a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-04-18

54. Static_cast from private base to derived class

Section: 7.6.1.9  [expr.static.cast]     Status: CD1     Submitter: Steve Adamczyk     Date: 13 Oct 1998

[Voted into WP at October 2004 meeting.]

Is it okay to use a static_cast to cast from a private base class to a derived class? That depends on what the words "valid standard conversion" in paragraph 8 mean — do they mean the conversion exists, or that it would not get an error if it were done? I think the former was intended — and therefore a static_cast from a private base to a derived class would be allowed.

Rationale (04/99): A static_cast from a private base to a derived class is not allowed outside a member from the derived class, because 7.3.12 [conv.ptr] paragraph 3 implies that the conversion is not valid. (Classic style casts work.)

Reopened September 2003:

Steve Adamczyk: It makes some sense to disallow the inverse conversion that is pointer-to-member of derived to pointer-to-member of private base. There's less justification for the pointer-to-private-base to pointer-to-derived case. EDG, g++ 3.2, and MSVC++ 7.1 allow the pointer case and disallow the pointer-to-member case. Sun disallows the pointer case as well.

  struct B {};
  struct D : private B {};
  int main() {
    B *p = 0;
    static_cast<D *>(p);  // Pointer case: should be allowed
    int D::*pm = 0;
    static_cast<int B::*>(pm);  // Pointer-to-member case: should get error
  }

There's a tricky case with old-style casts: because the static_cast interpretation is tried first, you want a case like the above to be considered a static_cast, but then issue an error, not be rejected as not a static cast; if you did the latter, you would then try the cast as a reinterpret_cast.

Ambiguity and casting to a virtual base should likewise be errors after the static_cast interpretation is selected.

Notes from the October 2003 meeting:

There was lots of sentiment for making things symmetrical: the pointer case should be the same as the pointer-to-member case. g++ 3.3 now issues errors on both cases.

We decided an error should be issued on both cases. The access part of the check should be done later; by some definition of the word the static_cast is valid, and then later an access error is issued. This is similar to the way standard conversions work.

Proposed Resolution (October 2003):

Replace paragraph 7.6.1.9 [expr.static.cast]/6:

The inverse of any standard conversion sequence ( 7.3 [conv]), other than the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), function-to-pointer (7.3.4 [conv.func]), and boolean (7.3.14 [conv.fctptr]) conversions, can be performed explicitly using static_cast. The lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) conversions are applied to the operand. Such a static_cast is subject to the restriction that the explicit conversion does not cast away constness (7.6.1.11 [expr.const.cast]), and the following additional rules for specific cases:

with two paragraphs:

The inverse of any standard conversion sequence ( 7.3 [conv]), other than the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), function-to-pointer (7.3.4 [conv.func]), and boolean (7.3.14 [conv.fctptr]) conversions, can be performed explicitly using static_cast. A program is ill-formed if it uses static_cast to perform the inverse of an ill-formed standard conversion sequence.[Example: 

struct B {};
struct D : private B {};
void f() {
  static_cast<D*>((B*)0); // Error: B is a private base of D.
  static_cast<int B::*>((int D::*)0); // Error: B is a private base of D.
}
--- end example]

The lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) conversions are applied to the operand. Such a static_cast is subject to the restriction that the explicit conversion does not cast away constness (7.6.1.11 [expr.const.cast]), and the following additional rules for specific cases:

In addition, modify the second sentence of 7.6.3 [expr.cast]/5. The first two sentences of 7.6.3 [expr.cast]/5 presently read:

The conversions performed by can be performed using the cast notation of explicit type conversion. The same semantic restrictions and behaviors apply.

Change the second sentence to read:

The same semantic restrictions and behaviors apply, with the exception that in performing a static_cast in the following situations the conversion is valid even if the base class is inaccessible:

Remove paragraph 7.6.3 [expr.cast]/7, which presently reads:

In addition to those conversions, the following static_cast and reinterpret_cast operations (optionally followed by a const_cast operation) may be performed using the cast notation of explicit type conversion, even if the base class type is not accessible: