Where would you use a friend function vs. a static member function?

问题

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())?

回答1:

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.

回答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.

回答3:

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.

回答4:

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

回答5:

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.

回答6:

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.

回答7:

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.

回答8:

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
};
```

回答9:

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.

回答10:

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.

回答11:

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.

回答12:

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

回答13:

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!!

回答14:

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....

回答15:

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.

来源：https://stackoverflow.com/questions/2315166/where-would-you-use-a-friend-function-vs-a-static-member-function

标签

c++

friend-function

static-functions