Can two classes friend each other? - c++

I haven't yet worked out a specific case. But I am about to embark on writing some code that I feel will end up needing this; and so I wanted to know if:
Two classes can friend each other; so that they can freely access the private
and protected members of the other (I believe the answer is yes, and ofcourse I
can simply try it out!). Any detailed references or other question links with answers
are also very welcome. I am aware of forward declarations and include guard compiler
pre-directives and their use. My questions are rather more related to the semantics
of the C++ language in terms of what it can offer with regard to this possibility
of mutual friendship and how to use it properly.
Is this generally recommended? Do people employ this kind of design on a regular basis?
Under what circumstances would this be a recommended design (preferably with some
examples).

You can have mutual friendship:
class A {
friend class B;
};
class B {
friend class A;
};
Whether or not this makes sense depends entirely on the problem you are trying to solve. It definitely could make sense in certain circumstances.
The only example from my current project that utilizes mutual friendship is a container implementation: the container class is a friend of its iterator class and vice versa.

Related

Why define Parent class as friend class?

I am looking at other's code and find one part I can't understand.
class a {
public:
function xxx () {.....}
}
class b : public a {
public:
xxxx
protected:
constructor()....
friend class a ; <= here why it is needed ????
}
As I understand, since b had already inherited from a, it should be able to use the function of a directly. What's the purpose of this "friend" declaration used for?
The friend allows a to use b's methods, not the other way around, which isn't implicit.
The design looks fishy though, a base class shouldn't care about derived classes.
friend class a; grants class a the right to access non-public members of b. So in this small example, an instance of a can call b::constructor(). Without friendship, it wouldn't be possible.
As to why, there is not enough information to answer that, other than there must be a need for instances of a to call b::constructor() (assuming that to be anything other than the syntax error it currently is).
As I understand, since b had already inherited from a, it should be able to use the function of a directly.
Yes. The friend specification though allows access the other way around (instances of a will be able to access private data and functions of b).
What's the purpose of this "friend" declaration used for?
The example above doesn't suggest any. The only situation where it may make sense is with using CRTP in certain situations (i.e. a is a template of b) but even so, if you see such a requirement ("must add friend declaration in b") it is possible that the design you're looking at is flawed.
Can you post a concrete example?
Depending on your projects/requirements, your class designs change. I have no comment on your class hierarchy but true your question is all about theories of friend usage. If you don't use friend, you will not be able to call B members from A. It is there for...cross-mating :D
It almost certainly means that there is a serious design problem. One of the basic rules of thumb for inheritance is that base classes should not need any information about derived classes. Making a a friend of b makes it possible for member functions of a to get at the internals of b objects.

friend class/function in c++ [duplicate]

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Closed 10 years ago.
Possible Duplicate:
When should you use 'friend' in C++?
I see a lot of people recommending a function/class to be made a friend of another class here in SO though there are other alternatives. Shouldn't friend be sparingly used in C++? I feel other options must be considered before deciding on using the friend feature. Opinions/suggestions are welcome.
Some say friend is a violation of encapsulation. In my opinion, it is the exact contrary. Without friend, we would end up with either:
huge monolithic classes, that does plenty of things they were not meant to, just to avoid letting outer classes access their internals
badly encapsulated classes: to separate concerns and write classes with one and only one responsability, we would need to provide public access to internal data needed by the related set of classes.
friend is a good way to overcome this issue, since it lets you free to separate responsabilities in several classes, while at the same time letting you restrict access to implementation details to the few functions or types which need them.
I agree. The friend keyword should be used sparingly.
It may be useful if you have a set of classes that interact together when you want to expose a clean api from those classes to users, but when the classes can interact with each other using a richer interface.
Eg:
class Folder
{
public:
int GetFileCount();
private:
void IncrementFileCount(); // users of this class have no right to use this
friend class File;
};
class File
{
public:
File(string name, Folder& containingFolder)
{
containingFolder.IncrementFileCount(); // except for indirectly when they create a file
}
};
Without specific examples this is hard to decide. While friend isn't strictly necessary it does have its uses. If, as you claim, there are better alternatives then obviously use them, by simple definition of the word “better”. Or maybe the decision which solution is better isn't that clean-cut after all.
Personally, I prefer to avoid it when possible but I prefer to use it over method duplication: for example, I do not like to write a print method just to avoid making operator << a friend because I don't see the benefit of the duplicate method.
For all those who thinks friend violates the encapsulation, here is what Bjarne Stroustup has to say .
But I personally don't use friend unless it is inevitable. Scenarios like implementation of Iterator patterns there is no other choice.
Friend is friend if used properly otherwise he is enemy!
Friend functions are advantageous in cases where you would want to call a 3rd party library function which needs access to members of your class, consider for example:
class A {
private:
int x,y;
double result;
public:
friend void *power(void *x);
}
You can now call the pow function found in math.h using this friend function.Your power function can now be defined as:
void *power(void *X)
{
A *a;
a = static_cast< A *> (X);
a->result = pow(a->x,a->y);
return NULL;
}
Although there are easier ways of calling the pow function.This example is only meant to illustrate the importance of friend function in calling library functions.
Hope it is useful.

When to use friend class in C++ [duplicate]

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Closed 10 years ago.
Possible Duplicate:
When should you use 'friend' in C++?
I was brushing up on my C++ (I'm a Java developer) and I came across the friend class keyword which I had forgotten about for a while. Is this one of those features that's just part of the kitchen sink, or is there a good reason for doing this rather than just a vanilla getter? I understand the difference in that it limits who can access the data, but I can't think of a scenario when this would be necessary.
Note: I've seen a similar question, but specifically I'm asking, is this just an advanced feature that adds no real value except to confuse people looking at you're code until they realize what you're doing?
I agree with the comments that say the friend keyword can improve encapsulation if used wisely. I'd just add that the most common (legitimate!) use for friend classes may be testing. You may want a tester class to have a greater degree of access than other client classes would have. A tester class could have a good reason to look at internal details that are deliberately hidden from other classes.
In my experience, the cases when friend (or mutable, which is a little similar) to actually enhance encapsulation of data are rare compared with how often it's used to break encapsulation.
It's rarely useful to me but when I do use it it's for cases in which I've had to split a class that was formerly a single class into two separate classes that need to access some common data/functionality.
Edit to respond to Outlaw Programmer's comment: We absolutely agree on this. One other option apart from friend'ing classes after splitting them is to make public accessors, which sometimes break encapsulation! I think that some people think that friendly classes somehow breaks encapsulation because they've seen it used improperly a lot, and many people probably never see code where it's been used correctly, because it's a rare thing. I like your way of stating it though - friendliness is a good middle ground between not allowing you to split up your class and making EVERYTHING accessible to the public.
Edit to respond to David Thornley: I agree that the flexibility that C++ allows you to do things like this is a result of the design decisions that went into C++. I think that's what it makes it even more important to understand what things are generally good and bad style in flexible languages. Java's perspective is that you should never have friend classes so that these aren't provided, but as C++ programmers it's our responsibility as a community to define appropriate use of these very flexible but sometimes misused language constructs.
Edit to respond to Tom: Mutable doesn't necessarily break encapsulation, but many of the uses of the mutable keyword that I've seen in real-life situations break encapsulation, because it's much more common to see people breaking encapsulation with mutable than to actually find and understand a proper use of mutable in the first place.
When you wish that one class (Factory) be responsible for creating instances of another class (Type). You can make the constructor of the Type private and thus make sure that only the Factory can create Type objects. It is useful when you wish to delegate the checks to some other class which could serve as a validator.
Just one usage scenario.
P.S. Really missing the "friend" keyword in C#...
A concrete instance would be a class factory, where you want one class to only be created through another factory class, so you make the constructors private, and the factory class a friend of the produced class.
It's kinda' like a 2" 12-point 3/4"-drive socket - not terribly common, but when you need it, you're awfully glad you have it.
Helps with Memento design pattern
The FAQ's section about friends: here
The FQA's section about friends: here
Two different points of view about friend.
I look at the friend construct as one of those features of the language that should be used in rare occasions, but that doesn't make it useless. There are several patterns that call for making friend classes, many of them already on this site in that "Related" bar on the right. ====>
Friendship is used when you have multiple classes and/or functions that work together to provide the same abstraction or interface. The classic example is implementing some kind of numerical class, and all the non-member operator functions (*, -, +, <<, etc) are given friendship so that they can work on the private data of the numerical class.
Such use cases are somewhat rare, but they do exist, and friend is very useful.
Here is one example, of several, I'm sure, where a friend class can be legitimately used without disregarding the reasons for encapsulation.
MyClass inherits from GeneralClass. MyClass has gotten big, so you created HelperClass to encapsulate some of the function of MyClass. However, HelperClass needs access to some protected functions in GeneralClass to properly perform it's function, so you make HelperClass a friend to MyClass.
This is better than exposing the protected functions, because they don't need to be available to everybody, but it helps keep your code organized in an OOP way to keep MyClass from getting too complex. It makes sense, because although HelperClass isn't concretely related to MyClass by inheritance, it does have some sort of logical connection to it, embodied in the code, and in design, as "friend".
I always ( and only ) use friend for unit testing private methods. The only other way I can imagine to do this would be to load up the public interface with a whole lot of testing methods, which is just too messy and so I prefer to hide the test methods in a seperate test class.
Something like this:
class cMyClassTest;
class cMyClass
{
public:
.....
private:
friend cMyClassTest;
int calc(); // tricky algorithm, test carefully
};
class cMyClassTest
{
public:
int test_calc()
{
cMyClass test;
....
int result = test.calc();
if( result == 42 )
return 1;
return 0;
}
};
friend class mean we all know that is acesss the value of variable from other class so it is mainly used for use the values so we no need to return the value of other class to main function then main to needed class member function but it having the problem that is a class is friend for other class then friend class should be in below of that class

In what scenarios should one declare a member function a friend?

In what kind of scenarios would we declare a member function as a 'friend function' ?..What exact purpose does 'friend function' which defies one of central concept of 'Encapsulation' of OOP serve?
You would use a friend function for the same sort of reasons that you would use a friend class, but on a member function (rather than entire class) basis. Some good explanations are in this thread.
While friend functions and classes do violate encapsulation, they can be useful in some scenarios. For example, you may want to allow a test harness to access class internals to allow you to do whitebox testing. Rather than opening up the entire class to the test harness, you could open up a particular function which accesses the internals required by the test harness. While this still violates encapsulation, it's less risky than opening up the entire class.
Also see this article for some more information about friend classes and functions.
Friend functions and classes do not violate encapsulation when you are trying to build an abstraction or interface that must physically span multiple C++ classes or functions! That is why friend was invented.
Those types of cases don't come up often, but sometimes you are forced to implement an abstraction or interface with disparate classes and functions. The classic example is implementing some type of complex number class. The non-member operator functions are given friendship to the main complex number class.
I also recall doing this when programming with CORBA in C++. CORBA forced me to have separate classes to implement CORBA servants. But for a particular part of our software, I needed to marry these together as one interface. Friendship allowed these two classes to work together to provide a seamless service to one part of our software.
Having the ability to mark a particular member function on another class as a friend to your class may seem even stranger, but it is just a way of tightly controlling the friendship. Instead of allowing the entire other class "in" as your friend, you only allow one of its member functions access. Again, this isn't common, but very useful when you need it.
See C++ FAQ Lite:
Sometimes friends are syntactically better (e.g., in class Fred, friend functions allow the Fred parameter to be second, while members require it to be first). Another good use of friend functions are the binary infix arithmetic operators. E.g., aComplex + aComplex should be defined as a friend rather than a member if you want to allow aFloat + aComplex as well (member functions don't allow promotion of the left hand argument, since that would change the class of the object that is the recipient of the member function invocation).
Sometimes public/private/protected protection level is not quite enough for real world situations. So thus we give a small get-out clause that helps without having to make methods publicly accessible.
I personally use this the same way that Java uses the 'Package' protection level.
If I have a class in the same package that needs access I will consider using friend. If it is a class in another package then I will wonder why on earth is this other class needing access and look at my design.
One point that I find relevant: member classes have access to the private parts of the containing class. This may sometimes be a better alternative to "friend".
class A
{
private:
int b;
public:
class MemberNotFriend {
public:
static void test() {
A a;
a.b = 0;
}
};
};
void test()
{
A::MemberNotFriend::test();
}
Here is a simple, concrete example of how I am using a friend function:
I have a game where each sprite object stores its info like X,Y position as private members.
However, I wish to separate the game objects from the rendering: a game object does not need the exact details of how it is rendered. A game object only stores game state, and this game state may be rendered in a number of different ways.
Thus the game object class has a friend function: render(). The render() function is implemented outside the game object class, but it can access the X,Y position position membefrs as needed to render the game object.

How should I order the members of a C++ class?

Is it better to have all the private members, then all the protected ones, then all the public ones? Or the reverse? Or should there be multiple private, protected and public labels so that the operations can be kept separate from the constructors and so on? What issues should I take into account when making this decision?
I put the public interface first, but I didn't always do this. I used to do things backwards to this, with private, then protected, then public. Looking back, it didn't make a lot of sense.
As a developer of a class, you'll likely be well acquainted with its "innards" but users of the class don't much care, or at least they shouldn't. They're mostly interested in what the class can do for them, right?
So I put the public first, and organize it typically by function/utility. I don't want them to have to wade through my interface to find all the methods related to X, I want them to see all that stuff together in an organized manner.
I never use multiple public/protected/private sections - too confusing to follow in my opinion.
Google favors this order: "Typedefs and Enums, Constants, Constructors, Destructor, Methods, including static methods, Data Members, including static data members."
Matthew Wilson (Safari subscription required) recommends the following order: "Construction, Operations, Attributes, Iteration, State, Implementation, Members, and my favorite, Not to be implemented."
They offer good reasons, and this kind of approach seems to be fairly standard, but whatever you do, be consistent about it.
Coding style is a source for surprisingly heated conversation, with that in mind I risk providing a different opinion:
Code should be written so it is most readable for humans. I complete agree with this statement that was given here several times.
The deviation is which roll are we taking about.
To help the user of the class understand how to use it, one should write and maintain proper documentation. A user should never be needing to read the source code to be able to use the class. If this is done (either manually or using in-source documentation tools) then the order in which public and private class members are defined in the source does not matter for the user.
However, for someone who needs to understand the code, during code review, pull request, or maintenance, the order matters a great deal - the rule is simple:
items should be defined before they are used
This is neither a compiler rule not is it a strictly public v.s. private rule, but common sense - human readability rule. We read code sequentially, and if we need "juggle" back and forth every time we see a class member used, but don't know its type for example, it adversely affects the readability of the code.
Making a division strictly on private v.s. public violates this rule because private class members will appear after they have been used in any public method.
It's my opinion, and I would wager a guess that most people would agree, that public methods should go first. One of the core principles of OO is that you shouldn't have to care about implementation. Just looking at the public methods should tell you everything you need to know to use the class.
As always, write your code for humans first. Consider the person who will be using your class and place the most important members/enums/typedefs/whatever to them at the top.
Usually this means that public members are at the top since that's what most consumers of your class are most interested in. Protected comes next followed by privates. Usually.
There are some exceptions.
Occasionally initialisation order is important and sometimes a private will need to be declared before a public. Sometimes it's more important for a class to be inherited and extended in which case the protected members may be placed higher up. And when hacking unit tests onto legacy code sometimes it's just easier to expose public methods - if I have to commit this near-sin I'll place these at the bottom of the class definition.
But they're relatively rare situations.
I find that most of the time "public, protected, private" is the most useful to consumers of your class. It's a decent basic rule to stick by.
But it's less about ordering by access and more about ordering by interest to the consumer.
I usually define first the interface (to be read), that is public, then protected, then private stuff. Now, in many cases I go a step forward and (if I can handle it) use the PIMPL pattern, fully hiding all the private stuff from the interface of the real class.
class Example1 {
public:
void publicOperation();
private:
void privateOperation1_();
void privateOperation2_();
Type1 data1_;
Type2 data2_;
};
// example 2 header:
class Example2 {
class Impl;
public:
void publicOperation();
private:
std::auto_ptr<Example2Impl> impl_;
};
// example2 cpp:
class Example2::Impl
{
public:
void privateOperation1();
void privateOperation2();
private: // or public if Example2 needs access, or private + friendship:
Type1 data1_;
Type2 data2_;
};
You can notice that I postfix private (and also protected) members with an underscore. The PIMPL version has an internal class for which the outside world does not even see the operations. This keeps the class interface completely clean: only real interface is exposed. No need to argue about order.
There is an associated cost during the class construction as a dynamically allocated object must be built. Also this works really well for classes that are not meant to be extended, but has some short comings with hierarchies. Protected methods must be part of the external class, so you cannot really push them into the internal class.
I tend to follow the POCO C++ Coding Style Guide.
i think it's all about readability.
Some people like to group them in a fixed order, so that whenever you open a class declaration, you quickly know where to look for e.g. the public data members.
In general, I feel that the most important things should come first. For 99.6% of all classes, roughly, that means the public methods, and especially the constructor. Then comes public data members, if any (remember: encapsulation is a good idea), followed by any protected and/or private methods and data members.
This is stuff that might be covered by the coding standards of large projects, it can be a good idea to check.
In our project, we don't order the members according to access, but by usage. And by that I mean, we order the members as they are used. If a public member uses a private member in the same class, that private member is usually located in front of the public member somewhere, as in the following (simplistic) example:
class Foo
{
private:
int bar;
public:
int GetBar() const
{
return bar;
}
};
Here, the member bar is placed before the member GetBar() because the former is used by the latter. This can result in multiple access sections, as in the following example:
class Foo
{
public:
typedef int bar_type;
private:
bar_type bar;
public:
bar_type GetBar() const
{
return bar;
}
};
The bar_type member is used by the bar member, see?
Why is this? I dunno, it seemed more natural that if you encounter a member somewhere in the implementation and you need more details about that (and IntelliSense is screwed up again) that you can find it somewhere above from where you're working.
In practice, it rarely matters. It's primarily a matter of personal preference.
It's very popular to put public methods first, ostensibly so that users of the class will be able to find them more easily. But headers should never be your primary source of documentation, so basing "best practices" around the idea that users will be looking at your headers seems to miss the mark for me.
It's more likely for people to be in your headers if they're modifying the class, in which case they should care about the private interface.
Whichever you choose, make your headers clean and easy to read. Being able to easily find whatever info I happen to be looking for, whether I'm a user of the class or a maintainer of the class, is the most important thing.
It is really helpful to the folks that will use your class to list the public interface first. It's the part they care about and can use. Protected and private can follow along after.
Within the public interface, it's convenient to group constructors, property accessors and mutators, and operators in distinct groups.
Note that (depending on your compiler and dynamic linker), you can retain compatibility with previous versions of a shared library by only adding to the end of the class (i.e. to the end of the interface), and not removing or changing anything else. (This is true for G++ and libtool, and the three part versioning scheme for GNU/Linux shared libraries reflects this.)
There's also the idea that you should order members of the class to avoid wasted space due to memory alignment; one strategy is to order members from smallest to largest size. I've never done this either in C++ or C though.
Overall, your public interface should come before anything, because that's the main/only thing that users of your classes should be interested in. (Of course, in reality that doesn't always hold, but it's a good start.)
Within that, member types and constants are best first, followed by construction operators, operations, and then member variables.
Put the private fields first.
With modern IDEs, people don't read the class to figure out what it's public interface is.
They just use intellisence (or a class browser) for that.
If someone is reading through the class definition, it's usually because they want to understand how it works.
In that case, knowing the fields helps the most. It tells you what the parts of the object are.
binary compatibility
There are a few concrete reasons for the ordering of class members.
These have to do with binary compatibility.
Binary compatibility mainly affects changes to system DLLs and device drivers.
If you're not interested in these, ignore this answer.
Public members must go before private members.
This is so you can mix and change private members without affecting the location of public data.
New public members must go last.
This again avoids affecting the position of existing public members.
The same ordering applies to vtable members.
Apart from this there's no reason to not to follow your own/your colleagues' preferences.
Depends entirely on your preference. There is no "the right way".
When doing C++ in my own pet projects I personally keep convention that I put access modifier before each member or method declaration.