On Mon, 27 Apr 2026, 08:21 André Offringa via Std-Proposals, <
std-proposals_at_[hidden]> wrote:

> Hi Steve,
>
> Thanks for the feedback. You're suggesting helper friend classes as an
> alternative, but there are some issues with a helper friend class to enable
> translation-unit-local functions. First of all, to make the functions
> translation-unit-local, it must be placed in an anonymous namespace. To do
> this, it must be done so before the befriending, so something like this:
>
> == header file ==
>
> namespace {
> struct FooFriend;
> }
>
> class Foo {
> int a;
> friend FooFriend;
> };
>
> == cpp file ==
>
> namespace {
> struct FooFriend {
> static void SetA(Foo& foo) {
> foo.a = 3;
> }
> };
> }
>
> I think this is an awkward construct. The Google C++ style explicitly
> forbids this, as it doesn't allow unnamed namespaces in header files.
>
It breaks the ODR. The friend declaration refers to a different FooFriend
in each translation unit.

So the FooFriend solution doesn't mix well with the desire to have internal
linkage for all the helpers.



The SetA() function is also now publicly available, and even though that is
> limited to a single translation unit, it is still easy to accidentally
> break a class invariant in this way. The classname of FooFriend is leaked
> out of the scope of the unit file (style guides often mandate to put it in
> yet another subnamespace like 'details' in that case). Finally, the syntax
> of the call and use of private variables is more verbose. Compare it to:
>
> == header file ==
>
> class Foo {
> int a;
> };
>
> == cpp file ==
>
> private Foo::SetA() {
> a = 3;
> }
>
>
> So, while there are alternatives for private functions, I don't think the
> friend classes are really a solution for the issue that the proposal tries
> to solve.
>
> Kind regards,
> André
> On 4/26/26 11:05 PM, Steve Weinrich via Std-Proposals wrote:
>
> This is purely subjective, but I don't find that a single line, "friend
> class Helper;" is clutter. It can be put at the very bottom of the class.
> It gives Helper full access for whatever purpose. No committee required!
>
> Cheers,
> Steve
>
> On Sun, Apr 26, 2026, 00:17 André Offringa via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> On 4/25/26 10:56 PM, Steve Weinrich via Std-Proposals wrote:
>>
>> I may have missed something, but one can declare:
>>
>> class Foo
>> {
>> friend class Helper;
>>
>> friend void FooHelper (Foo *);
>> };
>>
>> Both the class Helper and the function FooHelper() can access all private
>> members of Foo. Thus, only the names and/or the function signature is
>> declared in the interface.
>>
>>
>> What I'm trying to solve:
>>
>> - Wen using friend declarations, these clutters the class with
>> implementation details that do not affect the layout of the class.
>>
>> - For FooHelper(), it would require the return and parameters types to be
>> available at the place of the friend declaration, creating extra
>> dependencies. Your example takes only Foo*, but I think it's common for a
>> helper function to use other parameters (and/or have a return value), which
>> causes the dependencies.
>> Regards,
>> André
>>
>>
>>
>> On Sat, Apr 25, 2026, 14:46 André Offringa via Std-Proposals <
>> std-proposals_at_[hidden]> wrote:
>>
>>> Hi all,
>>>
>>> I was wondering what people think of the following idea. The problem I'm
>>> trying to address is that if we want to introduce a helper method for a
>>> class, we have to declare this helper function in the class, e.g. assume
>>> this situation:
>>>
>>> == Header file: ==
>>>
>>> class Foo {
>>> public:
>>> void A();
>>>
>>> private:
>>> void Helper();
>>>
>>> int value_;
>>> };
>>>
>>> == Unit file: ==
>>>
>>> void Foo::A() {
>>> ...
>>> Helper();
>>> ...
>>> }
>>>
>>> void Foo::Helper() {
>>> ...
>>> value_ = ...;
>>> ...
>>> }
>>>
>>> I think it would be useful if there would be a way to skip the
>>> declaration of the Helper method inside the class (in the header file),
>>> and make it translation local just like a static function or function
>>> inside an anonymous namespace would be. From the compiler's point of
>>> view, it could then act as a translation-unit-local function, except
>>> with the possibility to access (private) class fields.
>>>
>>> The benefit is that the method is no longer part of the "interface" of
>>> the class, and this is useful because it is, after all, an
>>> implementation detail of the class. This makes it also no longer
>>> necessary to have the parameter types and return value type declared in
>>> the header file, which decreases dependencies between files.
>>>
>>> An example of how this could look like, could be to use the keyword
>>> 'private' and let it act as an identifier for declaring such a function,
>>> e.g.:
>>>
>>> == Header file: ==
>>>
>>> class Foo {
>>> public:
>>> void A();
>>>
>>> private:
>>> int value_;
>>> }
>>>
>>> == Unit file: ==
>>>
>>> private void Foo::Helper() {
>>> ...
>>> value_ = ...;
>>> ...
>>> }
>>>
>>> void Foo::A() {
>>> ...
>>> Helper();
>>> ...
>>> }
>>>
>>> Of course the syntax is open for discussion. The idea is that Helper()
>>> is now a private translation-unit-local function that receives the
>>> 'this' pointer and access to private fields. The function itself acts in
>>> name lookup as a free function, to avoid participating in member lookup,
>>> but is only visible inside class member functions or other private
>>> translation-unit-local functions, and is not accessible outside of that.
>>> This makes it somewhat between a member function and a free function.
>>> With such an approach, it can not be used to access private fields from
>>> a scope that does not allow access to those fields. Hence, the class
>>> data remains encapsulated. It should not modify the layout of the class
>>> and not change its ABI. There are more details to think through.
>>>
>>> Thinking of alternatives, another direction to solve this would be to
>>> change the standard such that friend functions can be declared as friend
>>> outside of the class definition, instead of by introducing a function
>>> with special visibility rules. They would then behave as normal
>>> functions, which simplify some details. This makes private data too
>>> widely usable, so I don't see a good solution in that direction.
>>>
>>> Syntax aside, the problem I'm trying to solve is to have a function that:
>>> - has access to private members
>>> - is defined only in the unit file
>>> - does not require any declaration in the header
>>> - does not become part of the class interface
>>>
>>> I think the best existing alternative for this situation is to declare a
>>> static free function in the unit file that takes as parameter the class
>>> members it needs. In complex situations, this is not as nice. In pimpl
>>> implementations it is a reasonable solution, but a pimpl pattern is not
>>> always desired.
>>>
>>> I'm curious to hear what people think about the idea of private
>>> translation-unit-local functions.
>>>
>>> Kind regards,
>>> André Offringa
>>>
>>> --
>>> Std-Proposals mailing list
>>> Std-Proposals_at_[hidden]
>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>>
>>
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