This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-11-11

195. Converting between function and object pointers

[Voted into WP at April 2005 meeting.]

It is currently not permitted to cast directly between a pointer to function type and a pointer to object type. This conversion is not listed in 7.6.1.9 [expr.static.cast] and 7.6.1.10 [expr.reinterpret.cast] and thus requires a diagnostic to be issued. However, if a sufficiently long integral type exists (as is the case in many implementations), it is permitted to cast between pointer to function types and pointer to object types using that integral type as an intermediary.

In C the cast results in undefined behavior and thus does not require a diagnostic, and Unix C compilers generally do not issue one. This fact is used in the definition of the standard Unix function dlsym, which is declared to return void* but in fact may return either a pointer to a function or a pointer to an object. The fact that C++ compilers are required to issue a diagnostic is viewed as a "competitive disadvantage" for the language.

Suggested resolution: Add wording to 7.6.1.10 [expr.reinterpret.cast] allowing conversions between pointer to function and pointer to object types, if the implementation has an integral data type that can be used as an intermediary.

Several points were raised in opposition to this suggestion:

1. Early C++ supported this conversion and it was deliberately removed during the standardization process.
2. The existence of an appropriate integral type is irrelevant to whether the conversion is "safe." The condition should be on whether information is lost in the conversion or not.
3. There are numerous ways to address the problem at an implementation level rather than changing the language. For example, the compiler could recognize the specific case of dlsym and omit the diagnostic, or the C++ binding to dlsym could be changed (using templates, for instance) to circumvent the violation.
4. The conversion is, in fact, not supported by C; the dlsym function is simply relying on non-mandated characteristics of C implementations, and we would be going beyond the requirements of C compatibility in requiring (some) implementations to support the conversion.
5. This issue is in fact not a defect (omitted or self-contradictory requirements) in the current Standard; the proposed change would actually be an extension and should not be considered until the full review of the IS.
6. dlsym appears not to be used very widely, and the declaration in the header file is not problematic, only calls to it. Since C code generally requires some porting to be valid C++ anyway, this should be considered one of those items that requires porting.

Martin O'Riordan suggested an alternative approach:

• Do not allow the conversions to be performed via old-style casts.
• Allow reinterpret_cast to perform the conversions with the C-style semantics (undefined behavior if not supported).
• Allow dynamic_cast to perform the conversions with well-defined behavior: the result would be a null pointer if the implementation could not support the requested conversion.

The advantage of this approach is that it would permit writing portable, well-defined programs involving such conversions. However, it breaks the current degree of compatibility between old and new casts, and it adds functionality to dynamic_cast which is not obviously related to its current meaning.

Notes from 04/00 meeting:

Andrew Koenig suggested yet another approach: specify that "no diagnostic is required" if the implementation supports the conversion.

Later note:

It was observed that conversion between function and data pointers is listed as a "common extension" in C99.

Notes on the 10/01 meeting:

It was decided that we want the conversion defined in such a way that it always exists but is always undefined (as opposed to existing only when the size relationship is appropriate, and being implementation-defined in that case). This would allow an implementation to issue an error at compile time if the conversion does not make sense.

Bill Gibbons notes that the definitions of the other similar casts are inconsistent in this regard. Perhaps they should be updated as well.

Proposed resolution (April 2003):

After 7.6.1.10 [expr.reinterpret.cast] paragraph 6, insert:

A pointer to a function can be explicitly converted to a pointer to a function of a different type. The effect of calling a function through a pointer to a function type (9.3.4.6 [dcl.fct]) that is not the same as the type used in the definition of the function is undefined. Except that converting an rvalue of type ``pointer to T1'' to the type ``pointer to T2'' (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified. [Note: see also 7.3.12 [conv.ptr] for more details of pointer conversions. ] It is implementation defined whether a conversion from pointer to object to pointer to function and/or a conversion from pointer to function to pointer to object exist.

An lvalue expression of type T1 can be cast to the type ``reference to T2'' if T1 and T2 are object types and an expression of type ``pointer to T1'' can be explicitly converted to the type ``pointer to T2'' using a reinterpret_cast. That is, a reference cast reinterpret_cast< T& >(x) has the same effect as the conversion *reinterpret_cast< T* >(&x) with the built-in & and * operators. The result is an lvalue that refers to the same object as the source lvalue, but with a different type. No temporary is created, no copy is made, and constructors (11.4.5 [class.ctor]) or conversion functions (11.4.8 [class.conv]) are not called.

Drafting Note:

If either conversion exists, the implementation already has to define the behavior (paragraph 3).

Notes from April 2003 meeting:

The new consensus is that if the implementation allows this cast, pointer-to-function converted to pointer-to-object converted back to the original pointer-to-function should work; anything else is undefined behavior. If the implementation does not allow the cast, it should be ill-formed.

Tom Plum is investigating a new concept, that of a "conditionally-defined" feature, which may be applicable here.

Proposed Resolution (October, 2004):

(See paper J16/04-0067 = WG21 N1627 for background material and rationale for this resolution. The resolution proposed here differs only editorially from the one in the paper.)

1. Insert the following into Clause 3 [intro.defs] and renumber all following definitions accordingly:

   1.3.2  conditionally-supported behavior

   behavior evoked by a program construct that is not a mandatory requirement of this International Standard. If a given implementation supports the construct, the behavior shall be as described herein; otherwise, the implementation shall document that the construct is not supported and shall treat a program containing an occurrence of the construct as ill-formed (Clause 3 [intro.defs]).

2. Add the indicated words to 4.1 [intro.compliance] paragraph 2, bullet 2:

   • If a program contains a violation of any diagnosable rule, or an occurrence of a construct described herein as “conditionally-supported” or as resulting in “conditionally-supported behavior” when the implementation does not in fact support that construct, a conforming implementation shall issue at least one diagnostic message, except that

3. Insert the following as a new paragraph following 7.6.1.10 [expr.reinterpret.cast] paragraph 7:

   Converting a pointer to a function to a pointer to an object type or vice versa evokes conditionally-supported behavior. In any such conversion supported by an implementation, converting from an rvalue of one type to the other and back (possibly with different cv-qualification) shall yield the original pointer value; mappings between pointers to functions and pointers to objects are otherwise implementation-defined.

4. Change 9.10 [dcl.asm] paragraph 1 as indicated:

   The meaning of an An asm declaration evokes conditionally-supported behavior. If supported, its meaning is implementation-defined.

5. Change 9.11 [dcl.link] paragraph 2 as indicated:

   The string-literal indicates the required language linkage. The meaning of the string-literal is implementation-defined. A linkage-specification with a string that is unknown to the implementation is ill-formed. This International Standard specifies the semantics of C and C++ language linkage. Other values of the string-literal evoke conditionally-supported behavior, with implementation-defined semantics. [Note: Therefore, a linkage-specification with a string-literal that is unknown to the implementation requires a diagnostic. When the string-literal in a linkage-specification names a programming language, the spelling of the programming language's name is implementation-defined. [Note: It is recommended that the spelling be taken from the document defining that language, for example Ada (not ADA) and Fortran or FORTRAN (depending on the vintage). The semantics of a language linkage other than C++ or C are implementation-defined. ]

6. Change Clause 13 [temp] paragraph 4 as indicated:

   A template, a template explicit specialization (13.9.4 [temp.expl.spec]), or a class template partial specialization shall not have C linkage. If the linkage of one of these is something other than C or C++, the behavior is implementation-defined result is conditionally-supported behavior, with implementation-defined semantics.