Simple question, here: what is the difference between a static member function, i.e. a function that can be called without requiring an object to access it (simply using the class identifier), and a non-member function? Here, I am asking both conceptually and functionally.
Are non-member functions conceptually static?
static member functions can access private and protected sections of a class. Non-member functions cannot do that as default. They can do that only if a class grants them friendship.
Another point to consider is that name of the static member functions are in the scope of the class. Multiple classes can have static member functions with the same name without worrying about names conflicting.
I would like to append the answer of #R Sahu that overloaded operators may not be static functions of a class.:)
Also static functions themselves can be protected and private. So they can be inaccesible outside the class where they are declared or its derived classes.
The other advantage of a static member function is that it is the only way if you want to call it in a thread in Windows API. CreateThread requires a function either to be in the global space, or, if it is a member function it has to be static. And there is no way around it, at least to my knowledge.
Related
I actually have a code (which I can't change) needs a static function as "callback function".
When I change the function to static, I can't access "this" and my class members then. Also I can't pass "this" as parameter to function, again because I can't change the original SDK and definition files, so what can I do?
My code is in a class, one of class functions calls a function which needs a "static callback function" as parameter.
That "callback function" needs also to access "this" and other class members to process.
What can I do?
I'm using QT and VS 2012 as compiler.
Thanks
A static function is in common with a global function, u can't image that u can use "this" in a global function, and not to mention class members...
The only difference is a static function has a "namespace" as known as the calss name.
So, no matter what are you using or which compliler it is, you cannot use "this" or class members in a static function, it's just language features.
And you should modify your code as Mike says.
If you use modern versions of C++ (11 or older) you may use std::bind. Or use boost:bind.
I know that we should declare a function as static in a class if its a utility function or if we have to use in a singleton class to access private static member.
But apart from that does a static function provide any sort of compiler optimization too since it doesn't pass the "this" pointer? Why not just use the utility function through an already instantiated object of class? Or is it just a best practice to make utility functions as statics?
Thanks in advance.
The "non-passing-this" optimization can probably be done by the compiler once you turn the optimizations on. As I see it, a static function has rather idiomatic uses:
Implementing modules. There is a bit of overlap here with namespaces, and I would rather use namespaces.
Factories: you can make the constructor protected/private and then have several static functions (with different, explicit names) creating instances.
For function pointers: static functions do not require the slightly more complicated syntax of pointer to member function. That can be a plus when interacting with libraries written for C.
To keep yourself from using this and have the compiler to enforce it. It makes sense sometimes. For example, if you have a commutative operation that takes two instances, a static function that takes the two instances would emphasize (in my opinion) that the operation is commutative. Of course in many cases you would rather overload an operator.
In general a static function will ease namespace and "friend" clutter by prefixing an otherwise ordinary function with the name of a class, presumably because both are tightly related.
Static exists to associate a method with a class, rather than either:
Associating it with an instance of that class (like writing a normal, non-static member function).
Keeping it in the global namespace or whatever namespace you would otherwise be in (like declaring a function just in the file, not in a class).
Static says that 'conceptually this is something tied to/associated with this class, but it does not depend on any instance of that class'.
In more formal terms: a static member function is the same as a function declared outside of a class in all ways other than that it is part of that class's namespace and in that it has access rights to that class's private/protected data members.
Going back to your question:
There is no optimization gain here.
Utility function has nothing to do with it. It's whether or not it makes sense to scope the function in the class itself (rather than an instance of it).
It does not 'pass the this pointer' because there is no instance to speak of. You can call a static member function without ever invoking that class's constructor.
I have one little question for you :), I understand that every method "secretly" gets "this" pointer of some class that they are inside but why that wont happen to "friend" functions ?
Is it because they are NOT methods of class?
Can anyone explain whole machinery, i am very interested in how "this" really works!
thanks in advance! :)
friend functions and classes are just used for access control checked by the compiler.
friend functions are just standard functions, so there won't be any differences regarding calling conventions.
friend functions are not member of any class, so no this pointer is passed (as done with static member functions)
Non-static member function of a class will get a hidden this pointer (depending on the ABI this is often the first argument), static member functions don't get the this pointer because they don't act on instance data.
How exactly the this pointer is passed to a member function heavily depends on the used ABI, which depends on the architecture and operating system. Either it will be pushed on the stack, or it will be passed through a well known register.
Please consider reading "Where is the 'this' pointer stored in computer memory?".
"Friendship" and "Membership" are two different things. A function can be a member function or not, and independantly be a friend function or not.
You can declare a member function to be a friend function of another class, i.e.
class B{
friend void A::func(B);
//stuff
};
Here, the member function func from class A is declared as friend and can access B's private, and it will have a this pointer, that points to the object of class A on which func has been called.
The this pointer is an implicit parameter of non-static member functions, which is described in section 9.3.2 of the C++ Standard. How it is passed to the function depends on your compiler/architecture, i.e. it is implementation defined (so you might want to read your favorite compiler's documentation to learn about how it manages this pointers).
What is the difference between member and non-member functions in C++?
There are several differences between a member function (which I will now call method) and a free function (which I will now call function).
First, let's just state that they are not so different. Object code can generally be compiled down to C (or assembly) which are procedural languages with no notion of methods. Both methods and functions are then called like subroutines.
Now that this is out of the way, let's look at the differences. They can be classified in two categories: conceptual and syntactic.
Syntactically
The syntax is the obvious part of any language, so it's the easiest to get out of the way.
First note: there are two different kinds of methods in C++ (and a number of other languages), the static methods and regular methods.
Both kinds of methods have full access to the class internals (protected and private sections) as well (of course) as access to the class public interface.
static methods are equivalent to friend functions (apart from some scoping differences).
Within a regular method, a special keyword (this in C++) allows access to the current object on which the method has been invoked (via the ., ->, .* or ->* operators).
A regular method may be const and/or volatile qualified, enabling it for (respectively) const and/or volatile qualified objects. For example, a non-const method cannot be called on a const object. This can be seen as qualifying this within the method, ie void Foo::bar() const has a this of type Foo const*.
A regular method may be marked as virtual. Virtuality enables runtime polymorphism by enabling overriding. I won't extend on this mechanism here, let's just note that functions cannot be virtual.
One often ignored point, is that methods (both static and regular) are scoped within the class. This is important for name lookup (of other methods or attributes/variables) as it means that the elements of the class have priority during lookup from a method on the elements declared outside of the class.
Since the qualification using this-> before attribute or methods is not mandatory, this comes handy in regular methods, though it may introduce subtle bugs. In static methods, it avoids qualifying by the class name the static attributes and methods one whishes to access.
Now that the main syntactic differences have been asserted, let's check the conceptual differences.
Conceptually
OOP is generally about tying together state and behavior (of this state). This is done by creating classes which group attributes (state) and behavior (methods) and (in theory) stating that only the methods can act on the state. Therefore, in OOP, the methods are responsible for implementing the behavior of the class.
The methods participate to the encapsulation of state (freeing clients from the implementation detail) and to the preservation of the class invariants (statements about the class state that hold true from its birth to its death, whatever you do with it).
C++
In C++, as we have seen previously, this is done by using different access levels (public, protected and private) and granting access to the non-public levels to a restricted portion of the code. Generally attributes will be private and thus only accessible to the class methods (and maybe some friends, for syntax quirks).
Note: I urge you not to use protected attributes, it's hard to track down their modifications and since the set of derived classes is unbounded... their implementation cannot be changed easily afterward.
However, beware that C++ discourages from bloating the interface with lots of methods.
The trouble is that because methods are responsible of maintaining invariants, the more there are and the more the responsability is spread, making it more difficult to track down bugs and ensure correctness. Also, since methods depends on the class internals, it makes change more costly.
Instead, in C++, it is generally advised to write a minimal set of methods and delegating the rest of the behavior to non-friend functions (as long as it doesn't increase the cost too much).
See Sutter's take on std::string in Monolith Unstrung.
The delegation to non-friend methods was emphasized by Sutter in its Interface Principle in which he states that functions that are delivered with the class (in the same file/same namespace) and use the class, are logically part of the class interface. He restates in Exceptional C++.
This answer is becoming rather long-winded, yet I suspect that I have overlooked differences that other would find critical... oh well.
A (non-static) member function has an implicit this argument, a non-member doesn't.
Syntactically, you pass that implicit argument on the left of the . or -> operator like.so() or like->so(), instead of as a function argument so( like ).
Likewise, when declaring a member function, you need to do so in the class of which it is a member:
class Class {
public:
void a_public_member_function();
};
Non-member functions are instead declared outside any class (C++ calls this "at namespace scope").
(Non-static) member functions can also be virtual, but non-member functions (and static member functions) cannot.
A non-static member function is invoked on objects of the class it belongs to. It has implicitly access to the this pointer representing the current object. Through this pointer, it may access other members easily and with full access privileges (i.e. access private members).
A non-member function has no implicit this. In the sample below, bar is a member function while freebar is not. Both do more or less the same, but note how bar gets an implicit object pointer via this (also only bar has privileged access to foo's members, freebar only has access to public members).
class foo {
public:
void bar() {
this->x = 0; // equivalent to x = 0;
}
int x;
};
void freebar(foo* thefoo) {
thefoo->x = 1;
}
// ...
foo f;
f.bar();
// f.x is now 0
freebar(&f);
// f.x is now 1
Semantically a member function is more than just a function with an implicit this parameter. It is meant to define the behaviour of an object (i.e. a car object would have drive(), stop() as member functions).
Note that there are also static member functions which have full privileges but don't get an implicit this nor are they invoked through an instance of the class (but rather through the full name of the class).
In the following code, f() is a member function of class Sample, and g() is a non-member function:
class Sample
{
void f();
};
void g();
Its very simple. Since f() is a member of the class Sample, so its called member function (of class Sample). And since g() is not member of any class, so its called non-member function.
A member function is invoked on an object and has access to the fields of the class.
Member functions can be polymorphic (via the virtual keyword) which is essential for OOP.
Member functions get called on instances and have a this pointer available; non-members don't.
Are there performance considerations when moving member functions of a class that are used only by one other member function of that class and making them local functions at the start of the function they are being used by instead?
http://www.flipcode.com/archives/Local_Functions_In_C.shtml
No, there's not. It's just a normal function call.
It's worth mentioning that you can't use local structs or classes as template arguments, so if you were thinking of using them for STL comparators then forget it :)