We make a non-member function a friend of a class when we want it to access that class's private members. This gives it the same access rights as a static member function would have. Both alternatives would give you a function that is not associated with any instance of that class.
When must we use a friend function? When must we use a static function? If both are viable options to solve a problem, how do we weigh up their suitability? Is there one that should be preferred by default?
For example, when implementing a factory that creates instances of class foo which only has a private constructor, should that factory function be a static member of foo (you would call foo::create()) or should it be a friend function (you would call create_foo())?
Section 11.5 "The C++ Programming Language" by Bjarne Stroustrup states that ordinary member functions get 3 things:
access to internals of class
are in the scope of the class
must be invoked on an instance
friends get only 1.
static functions get 1 and 2.
The question seems to address the situation where the programmer needs to introduce a function that does not work on any instance of a class (hence the possibility of choosing a static member function). Therefore, I will limit this answer to the following design situation, where the choice is between a static function f() and a friend free function f():
struct A
{
static void f(); // Better this...
private:
friend void f(); // ...or this?
static int x;
};
int A::x = 0;
void A::f() // Defines static function
{
cout << x;
}
void f() // Defines friend free function
{
cout << A::x;
}
int main()
{
A::f(); // Invokes static function
f(); // Invokes friend free function
}
Without knowing anything in advance about the semantics of f() and A (I'll come back to this later), this limited scenario has an easy answer: the static function is preferable. I see two reasons for this.
GENERIC ALGORITHMS:
The main reason is that a template such as the following can be written:
template<typename T> void g() { T::f(); }
If we had two or more classes that have a static function f() on their interface, this would allow us writing one single function that invokes f() generically on any such class.
There is no way to write an equivalent generic function if we make f() a free, non-member function. Although it is true that we could put f() into a namespace, so that the N::f() syntax could be used to mimic the A::f() syntax, it would still be impossible to write a template function such as g<>() above, because namespace names are not valid template arguments.
REDUNDANT DECLARATIONS:
The second reason is that if we were to put the free function f() in a namespace, we would not be allowed to inline its definition directly in the class definition without introducing any other declaration for f():
struct A
{
static void f() { cout << x; } // OK
private:
friend void N::f() { cout << x; } // ERROR
static int x;
};
In order to fix the above, we would to preceed the definition of class A with the following declaration:
namespace N
{
void f(); // Declaration of f() inside namespace N
}
struct A
{
...
private:
friend void N::f() { cout << x; } // OK
...
};
This, however, defeats our intention of having f() declared and defined in just one place.
Moreover, if we wanted to declare and define f() separately while keeping f() in a namespace, we would still have to introduce a declaration for f() before the class definition for A: failing to do so would cause the compiler to complain about the fact that f() had to be declared inside namespace N before the name N::f could be used legally.
Thus, we would now have f() mentioned in three separate places rather than just two (declaration and definition):
The declaration inside namespace N before A's definition;
The friend declaration inside A's definition;
The definition of f() inside namespace N.
The reason why the declaration and definition of f() inside N cannot be joined (in general) is that f() is supposed to access the internals of A and, therefore, A's definition must be seen when f() is defined. Yet, as previously said, f()'s declaration inside N must be seen before the corresponding friend declaration inside of A is made. This effectively forces us to split the declaration and the definition of f().
SEMANTIC CONSIDERATIONS:
While the above two points are universally valid, there are reasons why one might prefer declaring f() as static over making it a friend of A or vice versa which are driven by the universe of discourse.
To clarify, it is important to stress the fact that a member function of a class, whether it is static or non-static, is logically part of that class. It contributes to its definition and thus provides a conceptual characterization of it.
On the other hand, a friend function, in spite of being granted access to the internal members of the class it is friend of, is still an algorithm which is logically external to the definition of the class.
A function can be friend of more than one class, but it can be member of just one.
Thus, in a particular application domain, the designer may want to keep into consideration the semantics of both the function and the class when deciding whether to make the former a friend or a member of the latter (this applies not only to static functions, but to non-static functions as well, where other language constraints may intervene).
Does the function logically contribute to characterize a class and/or its behavior, or is it rather an external algorithm? This question can't be answered without knowledge of the particular application domain.
TASTE:
I believe that any argument other the ones just given stems purely from a matter of taste: both the free friend and the static member approach, in fact, allow to clearly state what the interface of a class is into one single spot (the class's definition), so design-wise they are equivalent (modulo the above observations, of course).
The remaining differences are stylistic: whether we want to write the static keyword or the friend keyword when declaring a function, and whether we want to write the A:: class scope qualifier when defining the class rather than the N:: namespace scope qualifier. Thus, I will not comment further on this.
The difference is clearly expressing the intent of the relationship between the class and the function.
You use friend when you want to intentionally indicate a strong coupling and special relationship between two unrelated classes or between a class and a function.
You use static member function when the function is logically a part of the class to which it is a member.
Friend functions (and classes) can access the private and protected members of your class.
There's rarely a good case for using a friend function or class. Avoid them in general.
Static functions may only access static data (that is, class-scoped data). They may be called without creating an instance of your class. Static functions are great for circumstances you want all of the instances of your class to behave the same way. You can use them:
as callback functions
to manipulate class-scoped members
to retrieve constant data that you don't want to enumerate in your header file
Static functions are used when you want a function that is the same for every instance of a class. Such functions do not have access to "this" pointer and thus cannot access any non static fields. They are used often when you want a function that can be used without instantiating the class.
Friend functions are functions which are not in the class and you want to give them access to private members of your class.
And this(static vs. friend) is not a matter of using one vs the other since they are not opposites.
The standard requires that operator = () [] and -> must be members, and class-specific
operators new, new[], delete and delete[] must be static members. If the situation
arises where we don't need the object of the class to invoke a function, then make
the function static. For all other functions:
if a function requires the operators = () [] and -> for stream I/O,
or if it needs type conversions on its leftmost argument,
or if it can be implemented using the class' public interface alone,
make it nonmember ( and friend if needed in the first two cases)
if it needs to behave virtually,
add a virtual member function to provide the virtual behaviour
and implement in terms of that
else
make it a member.
Static function can only access members of one class. Friend function has access to several classes, as explained by the following code:
class B;
class A { int a; friend void f(A &a, B &b); };
class B { int b; friend void f(A &a, B &b); };
void f(A &a, B &b) { std::cout << a.a << b.b; }
f() can access data of both A and B class.
One reason to prefer a friend over static member is when the function needs to be written in assembly (or some other language).
For instance, we can always have an extern "C" friend function declared in our .cpp file
class Thread;
extern "C" int ContextSwitch(Thread & a, Thread & b);
class Thread
{
public:
friend int ContextSwitch(Thread & a, Thread & b);
static int StContextSwitch(Thread & a, Thread & b);
};
And later defined in assembly:
.global ContextSwitch
ContextSwitch: // ...
retq
Technically speaking, we could use a static member function to do this, but defining it in assembly won't be easy due to name mangling (http://en.wikipedia.org/wiki/Name_mangling)
Another situation is when you need to overload operators. Overloading operators can be done only through friends or non-static members. If the first argument of the operator is not an instance of the same class, then non-static member would also not work; friend would be the only option:
class Matrix
{
friend Matrix operator * (double scaleFactor, Matrix & m);
// We can't use static member or non-static member to do this
};
A static function is a function that does not have access to this.
A friend function is a function that can access private members of the class.
You would use a static function if the function has no need to read or modify the state of a specific instance of the class (meaning you don't need to modify the object in memory), or if you need to use a function pointer to a member function of a class. In this second instance, if you need to modify the state of the resident object, you would need to pass this in and use the local copy. In the first instance, such a situation may happen where the logic to perform a certain task is not reliant on an object's state, yet your logical grouping and encapsulation would have it be a member of a specific class.
You use a friend function or class when you have created code that is not a member of your class and should not be a member of your class, yet has a legitimate purpose for circumventing the private/protected encapsulation mechanisms. One purpose of this may be that you have two classes that have need of some common data yet to code the logic twice would be bad. Really, I have only used this functionality in maybe 1% of the classes I've ever coded. It is rarely needed.
A friend function can not be inherited while a static function can be. So when an aim can be achieved with both static function and friend function, think that whether you want to inherit it or not.
Static function can be used in many different ways.
For example as simple factory function:
class Abstract {
private:
// no explicit construction allowed
Abstract();
~Abstract();
public:
static Abstract* Construct() { return new Abstract; }
static void Destroy(Abstract* a) { delete a; }
};
...
A* a_instance = A::Conctruct();
...
A::Destroy(a_instance);
This is very simplified example but I hope it explains what I meant.
Or as thread function working with Your class:
class A {
public:
static void worker(void* p) {
A* a = dynamic_cast<A*>(p);
do something wit a;
}
}
A a_instance;
pthread_start(&thread_id, &A::worker, &a_instance);
....
Friend is completely different story and they usage is exactly as described by thebretness
Friend functions can access the private and protected members of other classes.
Means they can be used to access all the data weather it is private or public.
So friend functions are used to access that data which static methods can not.
Those methods are made static which are called so many times that declaring a different location inside every object, for them becomes too costly(In terms of memory).
This can be made clear with the help of example:
Let the class's name is fact and its data member is n(which represents integer whose factorial is concern)
then in this case declaring find_factorial() as static would be wise decision!!
They are used as callback functions
to manipulate class-scoped members
to retrieve constant data that you don't want to enumerate in your header file
Now we are clear with following questions..
When a friend function is used? When a static function is used?
Now If both are viable options to solve a problem,
We can weight up their suitability in terms of accessibility(accessibility of Private data) and memory efficiency.
By default no one can be preferred as there are many situation when we need better memory management and sometimes we are are concerned with the scope of data.
For example:
foo::create() will be preferred over create_foo() when we have to call create() method after every small instance of time and we are not interested on scope of data(Private data)
And if we are interested to get the private information of more than one class(s) then create_foo() will be preferred over foo::create().
I hope this would help you!!
Here is what I think it is:
Friend function- when you need access to a different class member, but the classes are not related. Static function- when you no not need access to the 'this' pointer. But, I have a feeling there is more to it....
Static data members always share the memory.
only static function can used static data members.
static member function can be called with class name.
They must be defined outside of the class when we create a object of static member or member function in the class. It will automatically initialize the value.
It always used keyword static.
Static members can share by all the objects.
Type and scope of data members and member function is outside of the class.
A static member variable must be defined outside of the class.
Related
Let's say we have a class with a single private variable and a max method that takes a single parameter of the same type of the class:
class A
{
private:
int Number;
public:
A() : Number(0) {}
A(int val) : Number(val) {}
int max(A b)
{
if( Number > b.Number )
return Number;
return b.Number;
}
};
What is strange to me is that the parameter b in the max method has access to the private variable Number. However in the main function we do not have access to the parameter Number (as expected since it is declared private)
int main()
{
A a;
a.Number = 0; // ERROR
return 0;
}
So my question is why does the function in the class have access to the variable Number when it is declared private.
It's just the rule, that's all.
It's designed to make member functions, especially functions like assignment operators, copy constructors, and overloaded operators, simpler to write. If you couldn't access the members of the "other" object directly then you'd need a plethora of friend declarations or ugly "getters"; the latter tend to offer little more than a circumvention of encapsulation.
Perhaps the way you propose could have been the default, and to bring in the private and protected members and functions could have required the declaration
friend class;
But C++ was not designed this way, and that proposal would now be an hideous breaking change. Something to muse on though to recover your expected behaviour could be a declaration
friend class = delete;
which would, as far as I can tell, not be a breaking change. Why not propose something of this form to the ISO C++ Committee?
The principle idea behind encapsulation is to hide the internals of a class from external classes.
When you consider that the function you're writing is internal to the class A, it makes sense for the function to be aware of A's internals - regardless of the context.
If I will define a private static field in class. Given that it's a private field, can't I initialize it outside the class?
class X{
static int private_static_field;
public:
static void setPrivateStaticField(int val);
};
void X::setPrivateStaticField(int val) {
private_static_field = val;
}
int X::private_static_field(0); // something like that it's ok? if yes, I must write this line? why? can I write it in main?
It's look that it's ok (according to the compiler), but if so, I don't understand the concept of private - How it's ok if it's outside the class?
In addition, given the class above, and the next code:
int main() {
X x1();
x1.setPrivateStaticField(3);
return 0;
}
What is the meaning of x1.setPrivateStaticField(3); , after all, this function is static and hence it's not related to some object.
Hence, I don't understand how it's ok to call setPrivateStaticField with object (x1) ?
(I thought that just X::setPrivateStaticField(3); will be ok and that x1.setPrivateStaticField(3); will be error)
I don't understand the concept of private - How it's ok if it's outside the class?
There is no contradiction here. Prior to C++ 17 static member variables required a definition that is placed separately from the class declaration.
Despite the fact that the text of the definition is placed outside the class, the member variable remains part of the class where it is declared, and retains its accessibility according to its declaration inside the class. This includes private accessibility.
What is the meaning of x1.setPrivateStaticField(3); , after all, this function is static and hence it's not related to some object.
Although C++ compiler lets you call static member functions on the object, it is cleaner to call them using scope resolution operator :: and the class name:
X::setPrivateStaticField(3);
Allowing or disallowing class method calls on an instance is up to the designers of the language. C++ designers decided to allow it; designers of other programming languages disallow it, or require compilers to issue a warning.
Within a class definition static data members are declared but not defined. So they even can have an incomplete type.
So this record
int X::private_static_field(0);
is a definition of the static data member declared in the class definition like
class X{
static int private_static_field;
// ...
This record
x1.setPrivateStaticField(3);
means an access to a class member. Static members are also class members. Using this access method the compiler will know to search the name setPrivateStaticField in the class definition because the name x1 defines an object of the class X.
Another way to access a static member is to use the following record
X::setPrivateStaticField
My code:-
#include<iostream>
using namespace std;
class a{
private:
int x;
public:
a(int data)
{
x=data;
}
friend void printPrivateMember(a);
};
void printPrivateMember(a obj)
{
cout<<obj.x; //I can access private data member by an object inside this function.
}
int main()
{
a obj1(5);
printPrivateMember(obj1);
cout<<obj1.x; //this gives error
return 0;
}
I wanted to know as to how can I access a PRIVATE data type by an object in the friend function but cannot do so in main.
When I read about access specifier . It was specified that private can be accessed by only member functions (I don't have a problem with the friend function) and not by the object of that class. I wanted to know as to what difference is there because of which I can access private member by an object in one case and cannot do so in another. The same is applicable for copy constructor.
That's exactly what friend functions do: any friend function of a class can access it's private members. Since your printPrivateMember is declared as a friend of a, it can access it's private x member. Since main is not a friend function, it can't.
Forestalling a question about declaring main as friend, this question covers it.
Because friends could do that.
$11/1 Member access control [class.access]
(emphasis mine)
1 A member of a class can be
(1.1) — private; that is, its name can be
used only by members and friends of the class in which it is
declared.
(1.2) — protected; that is, its name can be used only by
members and friends of the class in which it is declared, by classes
derived from that class, and by their friends (see 11.4).
(1.3) —
public; that is, its name can be used anywhere without access
restriction.
As you correctly observed, only member functions (including constructors and destructors) and friend functions and classes may access you're privates. That's the purpose of friends: they provide an exception (not std::exception) to the encapsulation mechanism.
Now you may think about whether this breaks encapsulation or actually stabilizes it.
if you want to access private member, you'd better use a public function like:
class a {
private:
int m;
public:
int getM() {
return m;
}
};
Your use of the phrase not by the object of that class makes me think that you are unclear on the access rules. The access rules don't apply to the objects but who can access member variables and member functions of the objects.
A member variable of a class can be accessed in a function -- which can be a member function of the class, a member function of another class, or a global function.
It can also be accessed in the global space, e.g. to initialize a global variable.
A friend declaration in a class changes the default access rules that are in place by use of private, protected, and public access specifiers.
A function declared a friend of a class can access all the members of all instances of the class.
The answer by songyuanyao cites the section of the standard that provides more details on the subject.
This function should be public, so that you can access it through main().
void print(){
/**print or return your private variable here**/
}
I think i'm aware of accessibilty but I'm not sure if I understand visibility very clearly
For example:
class X
{
int x;
};
Here, 'x' is only visible in class and but accessible outside of class. If I'm correct, Can someone explain the text in that answer about how visibility is not controlled etc..?
(C++03/11.0) It should be noted that it is access to members and base classes that
is controlled, not their visibility. Names of members are still
visible, and implicit conversions to base classes are still
considered, when those members and base classes are inaccessible. The
interpretation of a given construct is established without regard to
access control. If the interpretation established makes use of
inaccessible member names or base classes, the construct is
ill-formed.
Perhaps this example helps:
class Bob
{
private:
int foo(int, int);
};
class David : Bob
{
void goo() {
int a = foo(1, 2); // #1
}
};
class Dani : Bob
{
void foo();
void goo() {
int a = foo(1, 2); // #2
}
};
On line #1, the name foo is visible, but the function which it names is not accessible (on account of being private to Bob). This is a compilation error, but the compiler knows that there is a potential function Bob::foo that would match, but isn't accessible.
On line #2, the name foo only refers to Dani::foo, while Bob::foo is not visible (because it is hidden), and so there is simply no matching function for the call foo(1, 2). This is also a compilation error, but this time the error is that there is no matching function at all for the call.
C++ has some esoteric feature concerning private class member names visibility and accessibility. By definition, a private class member name is only accessible by the class members and friends. However the rule of visibility can confuse many. They can be summarized as follows.
A private member's name is only accessible to other members and friends.
A private member is visible to all code that sees the class's definition. This means that its parameter types must be declared even if they can never be needed in this translation unit...
Overload resolution happens before accessibility checking.
In C++ today ("C++03" and earlier variants), the notions of accessibility and visibility are
independent. Members of classes and namespaces are visible whenever they are "in
scope" and there is no mechanism to reduce this visibility from the point of declaration.
Accessibility is only a parameter for class members and is orthogonal to the notion of
visibility. This latter observation is frequently surprising to novice C++ programmers. See this PDF.
Consider the following example.
#include < complex>
class Calc
{
public:
double Twice( double d );
private:
int Twice( int i );
std::complex Twice( std::complex c );
};
int main()
{
Calc c;
return c.Twice( 21 ); // error, Twice is inaccessible
}
When the compiler has to resolve the call to a function, it does three main things, in order:
Before doing anything else, the compiler searches for a scope that
has at least one entity named Twice and makes a list of candidates.
In this case, name lookup first looks in the scope of Calc to see if
there is at least one function named Twice; if there isn't, base
classes and enclosing namespaces will be considered in turn, one at a
time, until a scope having at least one candidate is found. In this
case, though, the very first scope the compiler looks in already has
an entity named Twice — in fact, it has three of them, and so that
trio becomes the set of candidates. (For more information about name
lookup in C++, with discussion about how it affects the way you
should package your classes and their interfaces
Next, the compiler performs overload resolution to pick the unique
best match out of the list of candidates. In this case, the argument
is 21, which is an int, and the available overloads take a double, an
int, and a complex. Clearly the int parameter is the best match for
the int argument (it's an exact match and no conversions are
required), and so Twice(int) is selected.
Finally, the compiler performs accessibility checking to determine
whether the selected function can be called.
Note that accessibility (defined by modifiers in C++) and visibility are independent. Visibility is based on the scoping rules of C++. A class member can be visible and inaccessible at the same time.
Static members as an example are visible globally through out the running of your application but accessible only with regard to the modifier applied to them.
As a note: when you declare a class, the scope is private by default (opposed to a struct where members are public by default.)
The variable member 'x' is only accessible by your class and its friends. No one else can ever access 'x' directly (it can indirectly if you have a function returning a reference to it, which is a really bad idea.)
The text you quoted talks about visibility to the compiler, so X::x exists, no matter what. It won't disappear just because it's private. The visibility is used to find the member you are referencing and the first that matches is returned. At that point the compiler checks the accessibility, if accessible, you're all good. If not it is ill-formed.
Note that I mentioned friends. That keyword makes all variable members accessible. When the compiler deals with a friends, it completely ignores all the protected and private keywords.
In most cases, that's a very easy process. It goes in order. Period.
Where it becomes more complicated is when you start using virtual functions: these can be made public, protected, and private and that can change depending on the class declaration... (A derives from B and makes a protected virtual function public; it's generally not a good idea, but C++ doesn't prevent you from doing so.) Of course this only applies to functions, not variable members, so that's a different subject.
That accessability and visibility are independet confuses especially in such situations:
class A
{
public:
void Foo(int i){
}
};
class B : public A
{
private:
void Foo(){
}
};
int main(){
B b{};
b.Foo(12);
}
Programmers from other languages would expect that A::Foo(int) would be callable because it is public. The point here is, that the private B::Foo hides the inherited proc.
This can be solved with a using declaration using A::Foo. But it becomes really hard in this sitation:
class A
{
public:
void Foo(int i){
}
};
class B : public A
{
public:
using A::Foo;
private:
void Foo(){
}
};
class C : public B
{
public:
using B::Foo;
private:
void Foo(char c){
}
}
int main(){
B b{};
b.Foo(12);
}
Using requires that there is NO private function. AFAIK the best way to solve it is to use some prefixes / suffixes for private or protected functions (like do_XXX() or do_XXX_internal).
In the STL private members are usually prefixed by a single underscore (these are reserved identifiers).
Why is friendship not at least optionally inheritable in C++? I understand transitivity and reflexivity being forbidden for obvious reasons (I say this only to head off simple FAQ quote answers), but the lack of something along the lines of virtual friend class Foo; puzzles me. Does anyone know the historical background behind this decision? Was friendship really just a limited hack that has since found its way into a few obscure respectable uses?
Edit for clarification: I'm talking about the following scenario, not where children of A are exposed to either B or to both B and its children. I can also imagine optionally granting access to overrides of friend functions, etc.
class A {
int x;
friend class B;
};
class B {
// OK as per friend declaration above.
void foo(A& a, int n) { a.x = n; }
};
class D : public B { /* can't get in A w/o 'friend class D' declaration. */ };
Accepted answer: as Loki states, the effect can be simulated more or less by making protected proxy functions in friended base classes, so there is no strict need for granting friendship to a class or virtual method heirarchy. I dislike the need for boilerplate proxies (which the friended base effectively becomes), but I suppose that this was deemed preferable over a language mechanism that would more likely be misused most of the time. I think it's probably time I bought and read Stroupstrup's The Design and Evolution of C++, which I've seen enough people here recommend, to get better insight to these types of questions ...
Because I may write Foo and its friend Bar (thus there is a trust relationship).
But do I trust the people who write classes that are derived from Bar?
Not really. So they should not inherit friendship.
Any change in the internal representation of a class will require a modification to anything that is dependent on that representation. Thus all members of a class and also all friends of the class will require modification.
Therefore if the internal representation of Foo is modified then Bar must also be modified (because friendship tightly binds Bar to Foo). If friendship was inherited then all class derived from Bar would also be tightly bound to Foo and thus require modification if Foo's internal representation is changed. But I have no knowledge of derived types (nor should I. They may even be developed by different companies etc). Thus I would be unable to change Foo as doing so would introduce breaking changes into the code base (as I could not modify all class derived from Bar).
Thus if friendship was inherited you are inadvertently introducing a restriction on the ability to modify a class. This is undesirable as you basically render useless the concept of a public API.
Note: A child of Bar can access Foo by using Bar, just make the method in Bar protected. Then the child of Bar can access a Foo by calling through its parent class.
Is this what you want?
class A
{
int x;
friend class B;
};
class B
{
protected:
// Now children of B can access foo
void foo(A& a, int n) { a.x = n; }
};
class D : public B
{
public:
foo(A& a, int n)
{
B::foo(a, n + 5);
}
};
A friended class may expose its friend through accessor functions, and then grant access through those.
class stingy {
int pennies;
friend class hot_girl;
};
class hot_girl {
public:
stingy *bf;
int &get_cash( stingy &x = *bf ) { return x.pennies; }
};
class moocher {
public: // moocher can access stingy's pennies despite not being a friend
int &get_cash( hot_girl &x ) { return x.get_cash(); }
};
This allows finer control than optional transitivity. For example, get_cash may be protected or may enforce a protocol of runtime-limited access.
C++ Standard, section 11.4/8
Friendship is neither inherited nor transitive.
If friendship would be inherited, then a class that wasn't meant to be a friend would suddenly have access to your class internals and that violates encapsulation.
Because it's just unnecessary.
The usage of the friend keyword is itself suspicious. In term of coupling it's the worst relationship (way ahead of inheritance and composition).
Any change to the internals of a class have a risk to impact the friends of this class... do you really want an unknown number of friends ? You would not even be able to list them if those who inherit from them could be friends also, and you would run in the risk of breaking your clients code each time, surely this is not desirable.
I freely admit that for homework/pet projects dependency is often a far away consideration. On small size projects it doesn't matter. But as soon as several persons work on the same project and this grows into the dozens of thousands of lines you need to limit the impact of changes.
This bring a very simple rule:
Changing the internals of a class should only affect the class itself
Of course, you'll probably affect its friends, but there are two cases here:
friend free function: probably more of a member function anyway (I am think std::ostream& operator<<(...) here, which is not a member purely by accident of the language rules
friend class ? you don't need friend classes on real classes.
I would recommend the use of the simple method:
class Example;
class ExampleKey { friend class Example; ExampleKey(); };
class Restricted
{
public:
void forExampleOnly(int,int,ExampleKey const&);
};
This simple Key pattern allows you to declare a friend (in a way) without actually giving it access to your internals, thus isolating it from changes. Furthermore it allows this friend to lend its key to trustees (like children) if required.
A guess: If a class declares some other class/function as a friend, it's because that second entity needs privileged access to the first. What use is there in granting the second entity privileged access to an arbitrary number of classes derived from the first?
A derived class can inherit only something, which is 'member' of the base. A friend declaration is not a member of the befriending class.
$11.4/1- "...The name of a friend is
not in the scope of the class, and the
friend is not called with the member
access operators (5.2.5) unless it is
a member of another class."
$11.4 - "Also, because the base-clause
of the friend class is not part of its
member declarations, the base-clause
of the friend class cannot access the
names of the private and protected
members from the class granting
friendship."
and further
$10.3/7- "[Note: the virtual specifier
implies membership, so a virtual
function cannot be a nonmember (7.1.2)
function. Nor can a virtual function
be a static member, since a virtual
function call relies on a specific
object for determining which function
to invoke. A virtual function declared
in one class can be declared a friend
in another class. ]"
Since the 'friend' is not a member of the base class in the first place, how can it be inherited by the derived class?
Friend function in a class assigns the extern property to the function. i.e. extern means that the function has been declared and defined somewhere out of the class.
Hence it means friend function is not a member of a class. So the inheritance only allows you to inherit the properties of a class not external things. And also if inheritance is allowed for friend functions, then a third party class inheriting.
Friend is good in inheritance like style interface for container
But for me, as the first say, C++ lack the propagatable inheritance
class Thing;
//an interface for Thing container's
struct IThing {
friend Thing;
protected:
int IThing_getData() = 0;
};
//container for thing's
struct MyContainer : public IThing {
protected: //here is reserved access to Thing
int IThing_getData() override {...}
};
struct Thing {
void setYourContainer(IThing* aContainerOfThings) {
//access to unique function in protected area
aContainerOfThings->IThing_getData(); //authorized access
}
};
struct ChildThing : public Thing {
void doTest() {
//here the lack of granularity, you cannot access to the container.
//to use the container, you must implement all
//function in the Thing class
aContainerOfThings->IThing_getData(); //forbidden access
}
};
For me the problem of C++ is the lack of very good granularity to
control all access from anywhere for anything :
friend Thing can become friend Thing.* to grant access to all child of Thing
And more, friend [named area] Thing.* to grant access for a precise
are in the Container class via special named area for the friend.
Ok stop the dream. But now, you know an interesting usage of friend.
In another order, you can also found interesting to known all class are friendly with self. In other word, a class instance can call all
members of another instance of same name without restriction:
class Object {
private:
void test() {}
protected:
void callAnotherTest(Object* anotherObject) {
//private, but yes you can call test() from
//another object instance
anotherObject)->test();
}
};
Simple logic : 'I have a friend Jane. Just because we became friends yesterday does not make all of her friends mine.'
I still need to approve those individual friendships, and the level of trust would be accordingly.