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.

On Sat, Apr 25, 2026, 14:46 André Offringa via Std-Proposals <std-proposals@lists.isocpp.org> 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

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