I've been wondering, is there any reason for the design decision in C++ to not have a pure abstract class for any of the std library containers?
I appreciate that hash_map came later from the stdext namespace but shares a very similar interface. In the event that I later decide I would like to implement my own map for a particular piece of software, I would have preferred to have some kind of interface to work with.
Example
std::base_map *foo = new std::map<std::string, std::string>;
delete foo;
foo = new stdext::hash_map<std::string, std::string>;
Obviously the above example is not possible, as far as I am aware, however this is similar for list and other std lib containers.
I appreciate that this is not C# or Java, but there is obviously no constraints in C++ to stop this design, so why was it designed like this so that there is no coupling between similar containers.
Because virtual functions add overhead.
Because the containers don't all have the same interface, there are common functions but also important differences regarding iterator invalidation and memory allocation (and so exception behaviour) which you need to understand, if you were using an abstract base you wouldn't know the specifics of how the concrete container would behave.
If you want to write code that is agnostic about the type of container it is passed then in C++ you write a template instead of relying on abstract interfaces, i.e. use static polymorphism not dynamic polymorphism. That avoids the overhead of dynamic dispatch and also allows specialization based on concrete type, because the concrete type is known at compile time.
Finally, it wouldn't have any advantage IMHO. It's better the way it is. As you say, this isn't C# or Java, thankfully.
(P.S. the stdext namespace is not part of C++, it appears to be Microsoft's namespace for non-standard types, a better example would use std::tr1::unordered_map or std::unordered_map instead of stdext::hash_map)
Related
For fun I have been working on my own programming language that compiles down to C++. While most things are fairly straightforward to print, I have been having trouble compiling my golang style interfaces to c++. In golang you don't need to explicitly declare that a particular struct implements an interface, it happens automatically if the struct has all the functions declared in the interface. Originally I was going to compile the interfaces down to a class with all virtual methods like so
class MyInterface {
public:
void DoSomthing() = 0;
}
and all implementing structures would simply extend from the interface like you normally would in c++
class MyClass: public MyInterface {
// ...
}
However this would mean that my compiler would have to loop over every interface defined in the source code (and all dependencies) as well as every struct defined in the source and check if the struct implements the interface using an operation that would take O(N*M) time where N is the number of structs and M is the number of interfaces. I did some searching and stumbled upon some c++ code here: http://wall.org/~lewis/2012/07/23/go-style-interfaces-in-cpp.html that makes golang style interfaces in c++ a reality in which case I could just compile my interfaces to code similar to that (albeit not exactly since I am hesitant to use raw pointers over smart pointers) and not have to worry about explicitly implementing them. However the author states that It should not be done for production code which worries me a little.
This is kinda a loaded question that may be a little subjective, but could anyone with more C++ knowledge tell me if doing it the way suggested in the article is a really bad idea or is it actually not that bad and could be done, or if there is a better way to write c++ code that would allow me to achieve the behavior I want without resorting to the O(N*M) loop?
My initial thought is to make use of the fact that C++ supports multiple inheritance. Decompose your golang interface into single-function interfaces. Hash all interfaces by their unique signature. It now becomes an O(N) operation to find the set of C++ abstract interfaces for your concrete classes.
Similarly, when you consume an object, you find all the consumed interfaces. This is now O(M) by the same logic. The total compiler complexity then becomes O(N)+O(M) instead of O(N*M).
The slight downside is that you're going to have O(N) vtables in C++. Some of those might be merged if certain interfaces are always groupd together.
After reading and watching much about SOLID principles I was very keen to use these principles in my work (mostly C++ development) since I do think they are good principles and that they indeed will bring much benefit to the quality of my code, readability, testability, reuse and maintainability.
But I have real hard time with the 'D' (Dependency inversion).
This principal states that:
A. High-level modules should not depend on low-level modules. Both should depend on abstractions.
B. Abstractions should not depend on details. Details should depend on abstractions.
Let me explain by example:
Lets say I am writing the following interface:
class SOLIDInterface {
//usual stuff with constructor, destructor, don't copy etc
public:
virtual void setSomeString(const std::string &someString) = 0;
};
(for the sake of simplicity please ignore the other things needed for a "correct interface" such as non virutal publics, private virtuals etc, its not part of the problem.)
notice, that setSomeString() is taking an std::string.
But that breaks the above principal since std::string is an implementation.
Java and C# don't have that problem since the language offers interfaces to all the complex common types such as string and containers.
C++ does not offer that.
Now, C++ does offer the possibility to write this interface in such a way that I could write an 'IString' interface that would take any implementation that will support an std::string interface using type erasure
(Very good article: http://www.artima.com/cppsource/type_erasure.html)
So the implementation could use STL (std::string) or Qt (QString), or my own string implementation or something else.
Like it should be.
But this means, that if I (and not only I but all C++ developers) want to write C++ API which obeys SOLID design principles ('D' included), I will have to implement a LOT of code to accommodate all the common non natural types.
Beyond being not realistic in terms of effort, this solution has other problems such as - what if STL changes?(for this example)
And its not really a solution since STL is not implementing IString, rather IString is abstracting STL, so even if I were to create such an interface the principal problem remains.
(I am not even getting into issues such as this adds polymorphic overhead, which for some systems, depending on size and HW requirements may not be acceptable)
So may question is:
Am I missing something here (which I guess the true answer, but what?), is there a way to use Dependency inversion in C++ without writing a whole new interface layer for the common types in a realistic way - or are we doomed to write API which is always dependent on some implementation?
Thanks for your time!
From the first few comments I received so far I think a clarification is needed:
The selection of std::string was just an example.
It could be QString for that matter - I just took STL since it is the standard.
Its not even important that its a string type, it could be any common type.
I have selected the answer by Corristo not because he explicitly answered my question but because the extensive post (coupled with the other answers) allowed me to extract my answer from it implicitly, realizing that the discussion tends to drift from the actual question which is:
Can you implement Dependency inversion in C++ when you use basic complex types like strings and containers and basically any of the STL with an effort that makes sense. (and the last part is a very important element of the question).
Maybe I should have explicitly noted that I am after run-time polymorphism not compile time.
The clear answer is NO, its not possible.
It might have been possible if STL would have exposed abstract interfaces to their implementations (if there are indeed reasons that prevent the STL implementations to derive from these interfaces (say, performance)) then it still could have simply maintained these abstract interfaces to match the implementations).
For types that I have full control over, yes, there is no technical problem implementing the DIP.
But most likely any such interface (of my own) will still use a string or a container, forcing it to use either the STL implementation or another.
All the suggested solutions below are either not polymorphic in runtime, or/and are forcing quiet a some coding around the interface - when you think you have to do this for all these common types the practicality is simply not there.
If you think you know better, and you say it is possible to have what I described above then simply post the code proving it.
I dare you! :-)
Note that C++ is not an object-oriented programming language, but rather lets the programmer choose between many different paradigms. One of the key principles of C++ is that of zero-cost abstractions, which in particular entails to build abstractions in such a way that users don't pay for what they don't use.
The C#/Java style of defining interfaces with virtual methods that are then implemented by derived classes don't fall into that category though, because even if you don't need the polymorphic behavior, were std::string implementing a virtual interface, every call of one of its methods would incur a vtable lookup. This is unacceptable for classes in the C++ standard library supposed to be used in all kinds of settings.
Defining interfaces without inheriting from an abstract interface class
Another problem with the C#/Java approach is that in most cases you don't actually care that something inherits from a particular abstract interface class and only need that the type you pass to a function supports the operations you use. Restricting accepted parameters to those inheriting from a particular interface class thus actually hinders reuse of existing components, and you often end up writing wrappers to make classes of one library conform to the interfaces of another - even when they already have the exact same member functions.
Together with the fact that inheritance-based polymorphism typically also entails heap allocations and reference semantics with all its problems regarding lifetime management, it is best to avoid inheriting from an abstract interface class in C++.
Generic templates for implicit interfaces
In C++ you can get compile-time polymorphism through templates.
In its simplest form, the interface that an object used in a templated function or class need to conform to is not actually specified in C++ code, but implied by what functions are called on them.
This is the approach used in the STL, and it is really flexible. Take std::vector for example. There the requirements on the value type T of objects you store in it are dependent on what operations you perform on the vector. This allows e.g. to store move-only types as long as you don't use any of the operations that need to make a copy. In such a case, defining an interface that the value types needs to conform to would greatly reduce the usefulness of std::vector, because you'd either need to remove methods that require copies or you'd need to exclude move-only types from being stored in it.
That doesn't mean you can't use dependency inversion, though: The common Button-Lamp example for dependency inversion implemented with templates would look like this:
class Lamp {
public:
void activate();
void deactivate();
};
template <typename T>
class Button {
Button(T& switchable)
: _switchable(&switchable) {
}
void toggle() {
if (_buttonIsInOnPosition) {
_switchable->deactivate();
_buttonIsInOnPosition = false;
} else {
_switchable->activate();
_buttonIsInOnPosition = true;
}
}
private:
bool _buttonIsInOnPosition{false};
T* _switchable;
}
int main() {
Lamp l;
Button<Lamp> b(l)
b.toggle();
}
Here Button<T>::toggle implicitly relies on a Switchable interface, requiring T to have member functions T::activate and T::deactivate. Since Lamp happens to implement that interface it can be used with the Button class. Of course, in real code you would also state these requirements on T in the documentation of the Button class so that users don't need to look up the implementation.
Similarly, you could also declare your setSomeString method as
template <typename String>
void setSomeString(String const& string);
and then this will work with all types that implement all the methods you used in the implementation of setSomeString, hence only relying on an abstract - although implicit - interface.
As always, there are some downsides to consider:
In the string example, assuming you only make use of .begin() and .end() member functions returning iterators that return a char when dereferenced (e.g. to copy it into the classes' local, concrete string data member), you can also accidentally pass a std::vector<char> to it, even though it isn't technically a string. If you consider this a problem is arguable, in a way this can also be seen as the epitome of relying only on abstractions.
If you pass an object of a type that doesn't have the required (member) functions, then you can end up with horrible compiler error messages that make it very hard to find the source of the error.
Only in very limited cases it is possible to separate the interface of a templated class or function from its implementation, as is typically done with separate .h and .cpp files. This can thus lead to longer compile times.
Defining interfaces with the Concepts TS
if you really care about types used in templated functions and classes to conform to a fixed interface, regardless of what you actually use, there are ways to restrict the template parameters only to types conforming to a certain interface with std::enable_if, but these are very verbose and unreadable. In order to make this kind of generic programming easier, the Concepts TS allows to actually define interfaces that are checked by the compiler and thus greatly improves diagnostics. With the Concepts TS, the Button-Lamp example from above translates to
template <typename T>
concept bool Switchable = requires(T t) {
t.activate();
t.deactivate();
};
// Lamp as before
template <Switchable T>
class Button {
public:
Button(T&); // implementation as before
void toggle(); // implementation as before
private:
T* _switchable;
bool _buttonIsInOnPosition{false};
};
If you can't use the Concepts TS (it is only implemented in GCC right now), the closest you can get is the Boost.ConceptCheck library.
Type erasure for runtime polymorphism
There is one case where compile-time polymorphism doesn't suffice, and that is when the types you pass to or get from a particular function aren't fully determined at compile-time but depend on runtime parameters (e.g. from a config file, command-line arguments passed to the executable or even the value of a parameter passed to the function itself).
If you need to store objects (even in a variable) of a type dependent on runtime parameters, the traditional approach is to store pointers to a common base class instead and to use dynamic dispatch via virtual member functions to get the behavior you need. But this still suffers from the problem described before: You can't use types that effectively do what you need but were defined in an external library, and thus don't inherit from the base class you defined. So you have to write a wrapper class.
Or you do what you described in your question and create a type-erasure class.
An example from the standard library is std::function. You declare only the interface of the function and it can store arbitrary function pointers and callables that have that interface. In general, writing a type erasure class can be quite tedious, so I refrain from giving an example of a type-erasing Switchable here, but I can highly recommend Sean Parent's talk Inheritance is the base class of evil, where he demonstrates the technique for "Drawable" objects and explores what you can build on top of it in just over 20 minutes.
There are libraries that help writing type-erasure classes though, e.g. Louis Dionne's experimental dyno, where you define the interface via what he calls "concept maps" directly in C++ code, or Zach Laine's emtypen which uses a python tool to create the type erasure classes from a C++ header file you provide. The latter also comes with a CppCon talk describing the features as well as the general idea and how to use it.
Conclusion
Inheriting from a common base class just to define interfaces, while easy, leads to many problems that can be avoided using different approaches:
(Constrained) templates allow for compile-time polymorphism, which is sufficient for the majority of cases, but can lead to hard-to-understand compiler errors when used with types that don't conform to the interface.
If you need runtime polymorphism (which actually is rather rare in my experience), you can use type-erasure classes.
So even though the classes in the STL and other C++ libraries rarely derive from an abstract interface, you can still apply dependency inversion with one of the two methods described above if you really want to.
But as always, use good judgment on a case-by-case basis whether you really need the abstraction or if it is better to simply use a concrete type. The string example you brought up is one where I'd go with concrete types, simply because the different string classes don't share a common interface (e.g. std::string has .find(), but QStrings version of the same function is called .contains()). It might be just as much effort to write wrapper classes for both as it is to write a conversion function and to use that at well-defined boundaries within the project.
Ahh, but C++ lets you write code that is independent of a particular implementation without actually using inheritance.
std::string itself is a good example... it's actually a typedef for std::basic_string<char, std::char_traits<char>, std::allocator<char>>. Which allows you to create strings using other allocators if you choose (or mock the allocator object in order to measure number of calls, if you like). There just isn't any explicit interface like an IAllocator, because C++ templates use duck-typing.
A future version of C++ will support explicit description of the interface a template parameter must adhere to -- this feature is called concepts -- but just using duck-typing enables decoupling without requiring redundant interface definitions.
And because C++ performs optimization after instantiation of templates, there's no polymorphic overhead.
Now, when you do have virtual functions, you'll need to commit to a particular type, because the virtual-table layout doesn't accommodate use of templates each of which generates an arbitrary number of instances each of which require separate dispatch. But when using templates, you'll won't need virtual functions nearly as much as e.g. Java does, so in practice this isn't a big problem.
Coming from Delphi, I'm used to using class references (metaclasses) like this:
type
TClass = class of TForm;
var
x: TClass;
f: TForm;
begin
x := TForm;
f := x.Create();
f.ShowModal();
f.Free;
end;
Actually, every class X derived from TObject have a method called ClassType that returns a TClass that can be used to create instances of X.
Is there anything like that in C++?
Metaclasses do not exist in C++. Part of why is because metaclasses require virtual constructors and most-derived-to-base creation order, which are two things C++ does not have, but Delphi does.
However, in C++Builder specifically, there is limited support for Delphi metaclasses. The C++ compiler has a __classid() and __typeinfo() extension for retrieving a Delphi-compatible TMetaClass* pointer for any class derived from TObject. That pointer can be passed as-is to Delphi code (you can use Delphi .pas files in a C++Builder project).
The TApplication::CreateForm() method is implemented in Delphi and has a TMetaClass* parameter in C++ (despite its name, it can actually instantiate any class that derives from TComponent, if you do not mind the TApplication object being assigned as the Owner), for example:
TForm *f;
Application->CreateForm(__classid(TForm), &f);
f->ShowModal();
delete f;
Or you can write your own custom Delphi code if you need more control over the constructor call:
unit CreateAFormUnit;
interface
uses
Classes, Forms;
function CreateAForm(AClass: TFormClass; AOwner: TComponent): TForm;
implementation
function CreateAForm(AClass: TFormClass; AOwner: TComponent): TForm;
begin
Result := AClass.Create(AOwner);
end;
end.
#include "CreateAFormUnit.hpp"
TForm *f = CreateAForm(__classid(TForm), SomeOwner);
f->ShowModal();
delete f;
Apparently modern Delphi supports metaclasses in much the same way as original Smalltalk.
There is nothing like that in C++.
One main problem with emulating that feature in C++, having run-time dynamic assignment of values that represent type, and being able to create instances from such values, is that in C++ it's necessary to statically know the constructors of a type in order to instantiate.
Probably you can achieve much of the same high-level goal by using C++ static polymorphism, which includes function overloading and the template mechanism, instead of extreme runtime polymorphism with metaclasses.
However, one way to emulate the effect with C++, is to use cloneable exemplar-objects, and/or almost the same idea, polymorphic object factory objects. The former is quite unusual, the latter can be encountered now and then (mostly the difference is where the parameterization occurs: with the examplar-object it's that object's state, while with the object factory it's arguments to the creation function). Personally I would stay away from that, because C++ is designed for static typing, and this idea is about cajoling C++ into emulating a language with very different characteristics and programming style etc.
Type information does not exist at runtime with C++. (Except when enabling RTTI but it is still different than what you need)
A common idiom is to create a virtual clone() method that obviously clones the object which is usually in some prototypical state. It is similar to a constructor, but the concrete type is resolved at runtime.
class Object
{
public:
virtual Object* clone() const = 0;
};
If you don't mind spending some time examining foreign sources, you can take a look at how a project does it: https://github.com/rheit/zdoom/blob/master/src/dobjtype.h (note: this is a quite big and evolving source port of Doom, so be advised even just reading will take quite some time). Look at PClass and related types. I don't know what is done here exactly, but from my limited knowledge they construct a structure with necessary metatable for each class and use some preprocessor magic in form of defines for readability (or something else). Their approach allows seamlessly create usual C++ classes, but adds support for PClass::FindClass("SomeClass") to get the class reference and use that as needed, for example to create an instance of the class. It also can check inheritance, create new classes on the fly and replace classes by others, i. e. you can replace CDoesntWorksUnderWinXP by CWorksEverywhere (as an example, they use it differently of course). I had a quick research back then, their approach isn't exceptional, it was explained on some sites but since I had only so much interest I don't remember details.
Microsoft's GDI+ defines many empty classes to be treated as handles internally. For example, (source GdiPlusGpStubs.h)
//Approach 1
class GpGraphics {};
class GpBrush {};
class GpTexture : public GpBrush {};
class GpSolidFill : public GpBrush {};
class GpLineGradient : public GpBrush {};
class GpPathGradient : public GpBrush {};
class GpHatch : public GpBrush {};
class GpPen {};
class GpCustomLineCap {};
There are other two ways to define handles. They're,
//Approach 2
class BOOK; //no need to define it!
typedef BOOK *PBOOK;
typedef PBOOK HBOOK; //handle to be used internally
//Approach 3
typedef void* PVOID;
typedef PVOID HBOOK; //handle to be used internally
I just want to know the advantages and disadvantages of each of these approaches.
One advantage with Microsoft's approach is that, they can define type-safe hierarchy of handles using empty classes, which (I think) is not possible with the other two approaches, though I wonder what advantages this hierarchy would bring to the implementation? Anyway, what else?
EDIT:
One advantage with the second approach (i.e using incomplete classes) is that we can prevent clients from dereferencing the handles (that means, this approach appears to support encapsulation strongly, I suppose). The code would not even compile if one attempts to dereference handles. What else?
The same advantage one has with third approach as well, that you cannot dereference the handles.
Approach #1 is some mid-way between C style and C++ interface. Instead of member functions you have to pass the handle as argument. The advantage of exposed polymorphism is that you can reduce the amount of functions in interface and the types are checked compile time. Usually most experts prefer pimpl idiom (sometimes called compilation firewall) to such interface. You can not use approach #1 to interface with C so better go full C++.
Approach #2 is C style encapsulation and information hiding. The pointer may be (and often is) a pointer to real thing, so it is not over-engineered. User of library may not dereference that pointer. Disadvantage is that it does not expose any polymorphism. Advantage is that you may use it when interfacing with modules written in C.
Approach #3 is over-abstracted C-style encapsulation. The pointer may be really not a pointer at all since user of library should not cast, deallocate or dereference it. Advantage is that it may so carry exception or error values, disadvantage is that most of it has to be checked run time.
I agree with DeadMG that language-neutral object-oriented interfaces are very easy and elegant to use from C++, but these also involve more run-time checks than compile time checks and are overkill when i don't need to interface with other languages. So i personally prefer Approach #2 if it needs to interface with C or Pimpl idiom when it is C++ only.
2 and 3 are slightly less typesafe as they allow to use handles instead of void*
void bluescreeen(HBOOK hb){
memset(hb,0,100000); // no compile errors
}
Approach 3 is not very good at all, as it allows the mixing and matching of handle types that don't actually make sense, any function that takes a HANDLE can take any HANDLE, even if it's compile-time determinable that that is the wrong type.
The downside of Approach 1 is that you have to do a bunch of casting on the other end to their actual types.
Approach 2 isn't that bad, except you can't do any kind of inheritance with it without having to externally query every time.
However, all of this is entirely moot ever since compilers discovered how to implement efficient virtual functions. The approach taken by DirectX and COM is the best- it's very flexible, powerful, and completely type-safe.
It even allows for some truly insane things, like you can inherit from DirectX interfaces and extend it that way. One of the best advantages of this is Direct2D and Direct3D11. They're not actually compatible (which is truly, horrendously stupid), but you can define a proxy type that inherits from ID3D10Device1 and forwards to the ID3D11Device and solve the problem like that. That kind of thing would never even think about being possible with any of the above approaches.
Oh, and last thing: You really, really shouldn't name your types in allcaps.
A big reason why I use OOP is to create code that is easily reusable. For that purpose Java style interfaces are perfect. However, when dealing with C++ I really can't achieve any sort of functionality like interfaces... at least not with ease.
I know about pure virtual base classes, but what really ticks me off is that they force me into really awkward code with pointers. E.g. map<int, Node*> nodes; (where Node is the virtual base class).
This is sometimes ok, but sometimes pointers to base classes are just not a possible solution. E.g. if you want to return an object packaged as an interface you would have to return a base-class-casted pointer to the object.. but that object is on the stack and won't be there after the pointer is returned. Of course you could start using the heap extensively to avoid this but that's adding so much more work than there should be (avoiding memory leaks).
Is there any way to achieve interface-like functionality in C++ without have to awkwardly deal with pointers and the heap?? (Honestly for all that trouble and awkardness id rather just stick with C.)
You can use boost::shared_ptr<T> to avoid the raw pointers. As a side note, the reason why you don't see a pointer in the Java syntax has nothing to do with how C++ implements interfaces vs. how Java implements interfaces, but rather it is the result of the fact that all objects in Java are implicit pointers (the * is hidden).
Template MetaProgramming is a pretty cool thing. The basic idea? "Compile time polymorphism and implicit interfaces", Effective C++. Basically you can get the interfaces you want via templated classes. A VERY simple example:
template <class T>
bool foo( const T& _object )
{
if ( _object != _someStupidObject && _object > 0 )
return true;
return false;
}
So in the above code what can we say about the object T? Well it must be compatible with '_someStupidObject' OR it must be convertible to a type which is compatible. It must be comparable with an integral value, or again convertible to a type which is. So we have now defined an interface for the class T. The book "Effective C++" offers a much better and more detailed explanation. Hopefully the above code gives you some idea of the "interface" capability of templates. Also have a look at pretty much any of the boost libraries they are almost all chalk full of templatization.
Considering C++ doesn't require generic parameter constraints like C#, then if you can get away with it you can use boost::concept_check. Of course, this only works in limited situations, but if you can use it as your solution then you'll certainly have faster code with smaller objects (less vtable overhead).
Dynamic dispatch that uses vtables (for example, pure virtual bases) will make your objects grow in size as they implement more interfaces. Managed languages do not suffer from this problem (this is a .NET link, but Java is similar).
I think the answer to your question is no - there is no easier way. If you want pure interfaces (well, as pure as you can get in C++), you're going to have to put up with all the heap management (or try using a garbage collector. There are other questions on that topic, but my opinion on the subject is that if you want a garbage collector, use a language designed with one. Like Java).
One big way to ease your heap management pain somewhat is auto pointers. Boost has a nice automatic pointer that does a lot of heap management work for you. The std::auto_ptr works, but it's quite quirky in my opinion.
You might also evaluate whether you really need those pure interfaces or not. Sometimes you do, but sometimes (like some of the code I work with), the pure interfaces are only ever instantiated by one class, and thus just become extra work, with no benefit to the end product.
While auto_ptr has some weird rules of use that you must know*, it exists to make this kind of thing work easily.
auto_ptr<Base> getMeAThing() {
return new Derived();
}
void something() {
auto_ptr<Base> myThing = getMeAThing();
myThing->foo(); // Calls Derived::foo, if virtual
// The Derived object will be deleted on exit to this function.
}
*Never put auto_ptrs in containers, for one. Understand what they do on assignment is another.
This is actually one of the cases in which C++ shines. The fact that C++ provides templates and functions that are not bound to a class makes reuse much easier than in pure object oriented languages. The reality though is that you will have to adjust they manner in which you write your code in order to make use of these benefits. People that come from pure OO languages often have difficulty with this, but in C++ an objects interface includes not member functions. In fact it is considered to be good practice in C++ to use non-member functions to implement an objects interface whenever possible. Once you get the hang of using template nonmember functions to implement interfaces, well it is a somewhat life changing experience. \