I've got a namespace. The class is in a namespace.
namespace SALES
{
class Sales
{
...
I've created functions.
void SALES::Sales::set()
{
...
Now I'm creating an object
SALES::Sales interactive ();
but I can't get to methods.
Point my error out, please.
interactive.set(); // doesn't work
SALES::interactive.set(); // doesn't work
interactive.SALES::set(); // doesn't work
The way to access non-static members of a function is by using the member of object operator. For example,
class SomeObject {
public:
SomeObject() {}
void SayHello() { ... }
int age = 10;
};
// When calling members.
SomeObject object;
object.SayHello();
int age = object.age;
And when instantiating, use SALES::Sales interactive; or SALES::Sales interactive{}; instead. If not when you try to do it like this: SALES::Sales interactive ();, C++ will try to resolve it as a function. Now in your case, the first; interactive.set(); needs to work.
If it still doesn't work, it might have to do with the accessibility of members. Classes by default make their members private; only the friend classes and itself can access those objects. So make sure that the members are public.
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**/
}
Can we use public data member or member function in function which is declared in program but that are not member of that class ?
If the non-member function in question has an object on which to invoke the member functions, then yes - that's the idea with public members. For example:
class X
{
public:
void f() { }
int n_;
};
int main()
{
X x; // an actual object/variable of type X
x.f(); // can access public members
x.n_ = 3;
}
this is possible by using friend function concept. any function that is not member function of that class then we can declare this function with friend keyword as friend function. now by using this friend function we can access all private, protected, public data members with the help of object of that class.
Public data members of a class can be accessed by any function.
Public member functions of a class can be called by any function.
That is sort of the purpose of making class members public.
Naturally, there are other conditions (e.g. a function that calls a non-static public member function generally requires access to an instance of the class i.e. an object). Such conditions, if not met, will typically prevent code compiling or result in undefined behaviour - but that is unrelated to the question of whether a member is public or not.
Say you are given the following UML class diagram:
Can a variable of type Mystery invoke the function DoSomething()?
I understand that an object (say Mystery X;) could call GetA() to access the private variable int a
and to access the public variable int b all you need is X.b
but how could this object, X, access the private function DoSomething()
if it's even possible?
I had difficulty understanding exactly what you are asking, but I think I've figured it out.
If you are asking if, given the following declaration:
class Mystery
{
/*...*/
private:
void DoSomething();
};
you can do something like this:
Mystery m;
m.DoSomething();
...then the answer is no. You cannot call private member functions (or refer to private member variables) from outside the context of the class. Only another member function of Mystery can call the privates. For example:
void Mystery::Foo()
{
DoSomething(); // this would be possible if Foo() is a member of Mystery
}
EDIT:
Not only can you not call private members from outside the class, you also can't call them from subclasses. For example, this is not valid:
class Base
{
private:
void Foo() {};
};
class Child : public Base
{
public:
void Bar()
{
Foo(); // ERROR: Can't call private method of base class
}
};
There is one (edit - for completeness sake, there is more than one, see the comments) weird way that a private member function could be called outside your class, but it requires a little bit of a contrived example, where you can return a pointer to this function:
class Mystery;
typedef void (Mystery::*fptr)();
class Mystery{
void DoSomething() {};
public:
static fptr GetPrivateDoSomething()
{
return &DoSomething;
}
};
int main(int argc, char *argv[])
{
Mystery m;
fptr ds = Mystery::GetPrivateDoSomething();
(m.*ds)();
return 0;
}
That being said, don't do this. Private methods are private for a reason - they embody hidden aspects of the design, and were never meant to be a part of the exposed interface. That means that they could be subject to a change in behavior, or even complete removal. Additionally, doing these sorts of shenanigans can lead to code that is highly and awkwardly coupled, which is undesirable.
THAT being said, I have indeed used this and it works quite well, although it was in an entirely different context where it helped to reduce coupling (it was in a highly-modular event registration scheme, if you were wondering).
Any method inside the class is allowed to access the private variables and methods. It works exactly like calling any other method, its just that the compiler will give you an error if you try to do it from outside the class.
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.