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Musing on a Sunday...
Deriving from a class brings all names from the base class into the scope of the derived class. However, it also adds the base class non-static data members to every instance of the derived class.
Is there a way to achieve the former without the latter? I'm asking in the interest of concise notation.
Obviously, when the base class doesn't have any data members, I get what I want. There are quite a few empty classes or class templates in the standard library defined to do just that - inject names summarily into a class scope through inheritance. There's even the empty base class optimization to make this as cheap as possible.
But if I wanted to do the same with a non-empty base class, I would be tempted to employ something like:
struct Bar {
using struct Foo;
};
But, alas, that's not supported by C++. My question is, is there another way to achieve the same which I overlooked?
To provide a more complete example:
struct Foo {
enum { some_constant=42 };
// data members follow here ...
};
struct Bar {
using class Foo; // this doesn't compile
int f();
};
int Bar::f() {
return some_constant; // I want to use the constant directly, without Foo::
}
One clumsy way could be to split the definitions in Foo into two classes, one with the constants (which would be an empty class I could derive from without penalty) and the other with the data members, but that looks rather like an inelegant hack to me.
If there isn't a clean way to achieve this, maybe someone can provide a rationale for why it doesn't exist, or perhaps shouldn't exist.
Deriving from a class brings all names from the base class into the scope of the derived class.
Let me stop you there. Yes, it is true that inheriting from a base class causes the (non-private) names in the base class to be accessible from the derived class definition. However, that's not why you inherit from a base class; that's merely the mechanism by which inheritance achieves its goal.
To publicly inherit from a base class is to make a statement about the relationship between the derived and base classes. You're saying that every instance of the derived class should behave like the base class in virtually all ways. Even virtual function overriding still carries with it the expectation that the derived class implementations of these methods are conceptually doing the same job, just in a way appropriate for that derived class.
This is true even of mixin-style base classes, where the base class is used to define common functionality that is imported into a particular derived class. In such interfaces, there is little expectation of a user explicitly talking to base class definitions. But this provision of common functionality is ultimately still based on the semantic idea of a derived class being a base class. And that's very important for many of them to do their job.
Consider what is probably the most prominent mixin in the C++ standard library: std::enable_shared_from_this<T>. It has non-static data members, without which it couldn't actually provide the functionality it does (well, it could, but you would have to provide some interface in your derived class to store them, so it may as well do it).
This is true of private inheritance, though there is some modification. While to the outside world, the derived class is just a derived class, to the class definition itself, it still remains a base class. It remains wholly a base class, along with all the baggage that comes along with it.
Do not mistake the mechanism for the meaning. Mechanisms are important; don't get me wrong. But those mechanisms exist to facilitate meaning.
Having a class contain everything of some other class except the non-static data members is, semantically, nonsense. It doesn't mean anything about the relationship between the types. And you've essentially admitted that the main reason you want this is so that you don't have to scope-qualify the names defined in the "base" class.
This is a mechanical reason, not a semantic one. You shouldn't employ a semantic tool like inheritance to escape the mechanical consequences of how you have chosen to design your types.
In reference to your specific example you could make the constants you want to access static, which will allow you to access them from the second class by fully qualifying with the "base" class
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What are the differences between struct and class in C++?
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In what scenarios is it better to use a struct vs a class in C++?
The differences between a class and a struct in C++ are:
struct members and base classes/structs are public by default.
class members and base classes/structs are private by default.
Both classes and structs can have a mixture of public, protected and private members, can use inheritance, and can have member functions.
I would recommend you:
use struct for plain-old-data structures without any class-like features;
use class when you make use of features such as private or protected members, non-default constructors and operators, etc.
As everyone else notes there are really only two actual language differences:
struct defaults to public access and class defaults to private access.
When inheriting, struct defaults to public inheritance and class defaults to private inheritance. (Ironically, as with so many things in C++, the default is backwards: public inheritance is by far the more common choice, but people rarely declare structs just to save on typing the "public" keyword.
But the real difference in practice is between a class/struct that declares a constructor/destructor and one that doesn't. There are certain guarantees to a "plain-old-data" POD type, that no longer apply once you take over the class's construction. To keep this distinction clear, many people deliberately only use structs for POD types, and, if they are going to add any methods at all, use classes. The difference between the two fragments below is otherwise meaningless:
class X
{
public:
// ...
};
struct X
{
// ...
};
(Incidentally, here's a thread with some good explanations about what "POD type" actually means: What are POD types in C++?)
There are lots of misconceptions in the existing answers.
Both class and struct declare a class.
Yes, you may have to rearrange your access modifying keywords inside the class definition, depending on which keyword you used to declare the class.
But, beyond syntax, the only reason to choose one over the other is convention/style/preference.
Some people like to stick with the struct keyword for classes without member functions, because the resulting definition "looks like" a simple structure from C.
Similarly, some people like to use the class keyword for classes with member functions and private data, because it says "class" on it and therefore looks like examples from their favourite book on object-oriented programming.
The reality is that this completely up to you and your team, and it'll make literally no difference whatsoever to your program.
The following two classes are absolutely equivalent in every way except their name:
struct Foo
{
int x;
};
class Bar
{
public:
int x;
};
You can even switch keywords when redeclaring:
class Foo;
struct Bar;
(although this breaks Visual Studio builds due to non-conformance, so that compiler will emit a warning when you do this.)
and the following expressions both evaluate to true:
std::is_class<Foo>::value
std::is_class<Bar>::value
Do note, though, that you can't switch the keywords when redefining; this is only because (per the one-definition rule) duplicate class definitions across translation units must "consist of the same sequence of tokens". This means you can't even exchange const int member; with int const member;, and has nothing to do with the semantics of class or struct.
The only time I use a struct instead of a class is when declaring a functor right before using it in a function call and want to minimize syntax for the sake of clarity. e.g.:
struct Compare { bool operator() { ... } };
std::sort(collection.begin(), collection.end(), Compare());
From the C++ FAQ Lite:
The members and base classes of a struct are public by default, while in class, they default to private. Note: you should make your base classes explicitly public, private, or protected, rather than relying on the defaults.
struct and class are otherwise functionally equivalent.
OK, enough of that squeaky clean techno talk. Emotionally, most developers make a strong distinction between a class and a struct. A struct simply feels like an open pile of bits with very little in the way of encapsulation or functionality. A class feels like a living and responsible member of society with intelligent services, a strong encapsulation barrier, and a well defined interface. Since that's the connotation most people already have, you should probably use the struct keyword if you have a class that has very few methods and has public data (such things do exist in well designed systems!), but otherwise you should probably use the class keyword.
You can use "struct" in C++ if you are writing a library whose internals are C++ but the API can be called by either C or C++ code. You simply make a single header that contains structs and global API functions that you expose to both C and C++ code as this:
// C access Header to a C++ library
#ifdef __cpp
extern "C" {
#endif
// Put your C struct's here
struct foo
{
...
};
// NOTE: the typedef is used because C does not automatically generate
// a typedef with the same name as a struct like C++.
typedef struct foo foo;
// Put your C API functions here
void bar(foo *fun);
#ifdef __cpp
}
#endif
Then you can write a function bar() in a C++ file using C++ code and make it callable from C and the two worlds can share data through the declared struct's. There are other caveats of course when mixing C and C++ but this is a simplified example.
One place where a struct has been helpful for me is when I have a system that's receiving fixed format messages (over say, a serial port) from another system. You can cast the stream of bytes into a struct that defines your fields, and then easily access the fields.
typedef struct
{
int messageId;
int messageCounter;
int messageData;
} tMessageType;
void processMessage(unsigned char *rawMessage)
{
tMessageType *messageFields = (tMessageType *)rawMessage;
printf("MessageId is %d\n", messageFields->messageId);
}
Obviously, this is the same thing you would do in C, but I find that the overhead of having to decode the message into a class is usually not worth it.
As every one says, the only real difference is the default access. But I particularly use struct when I don't want any sort of encapsulation with a simple data class, even if I implement some helper methods. For instance, when I need something like this:
struct myvec {
int x;
int y;
int z;
int length() {return x+y+z;}
};
To answer my own question (shamelessly), As already mentioned, access privileges are the only difference between them in C++.
I tend to use a struct for data-storage only. I'll allow it to get a few helper functions if it makes working with the data easier. However as soon as the data requires flow control (i.e. getters/setters that maintain or protect an internal state) or starts acquring any major functionality (basically more object-like), it will get 'upgraded' to a class to better communicate intent.
For C++, there really isn't much of a difference between structs and classes. The main functional difference is that members of a struct are public by default, while they are private by default in classes. Otherwise, as far as the language is concerned, they are equivalent.
That said, I tend to use structs in C++ like I do in C#, similar to what Brian has said. Structs are simple data containers, while classes are used for objects that need to act on the data in addition to just holding on to it.
Structs (PODs, more generally) are handy when you're providing a C-compatible interface with a C++ implementation, since they're portable across language borders and linker formats.
If that's not a concern to you, then I suppose the use of the "struct" instead of "class" is a good communicator of intent (as #ZeroSignal said above). Structs also have more predictable copying semantics, so they're useful for data you intend to write to external media or send across the wire.
Structs are also handy for various metaprogramming tasks, like traits templates that just expose a bunch of dependent typedefs:
template <typename T> struct type_traits {
typedef T type;
typedef T::iterator_type iterator_type;
...
};
...But that's really just taking advantage of struct's default protection level being public...
As others have pointed out
both are equivalent apart from default visibility
there may be reasons to be forced to use the one or the other for whatever reason
There's a clear recommendation about when to use which from Stroustrup/Sutter:
Use class if the class has an invariant; use struct if the data members can vary independently
However, keep in mind that it is not wise to forward declare sth. as a class (class X;) and define it as struct (struct X { ... }).
It may work on some linkers (e.g., g++) and may fail on others (e.g., MSVC), so you will find yourself in developer hell.
Both struct and class are the same under the hood though with different defaults as to visibility, struct default is public and class default is private. You can change either one to be the other with the appropriate use of private and public. They both allow inheritance, methods, constructors, destructors, and all the rest of the goodies of an object oriented language.
However one huge difference between the two is that struct as a keyword is supported in C whereas class is not. This means that one can use a struct in an include file that can be #include into either C++ or C so long as the struct is a plain C style struct and everything else in the include file is compatible with C, i.e. no C++ specific keywords such as private, public, no methods, no inheritance, etc. etc. etc.
A C style struct can be used with other interfaces which support using C style struct to carry data back and forth over the interface.
A C style struct is a kind of template (not a C++ template but rather a pattern or stencil) that describes the layout of a memory area. Over the years interfaces usable from C and with C plug-ins (here's looking at you Java and Python and Visual Basic) have been created some of which work with C style struct.
An advantage of struct over class is that it save one line of code, if adhering to "first public members, then private". In this light, I find the keyword class useless.
Here is another reason for using only struct and never class. Some code style guidelines for C++ suggest using small letters for function macros, the rationale being that when the macro is converted to an inline function, the name shouldn't need to be changed. Same here. You have your nice C-style struct and one day, you find out you need to add a constructor, or some convenience method. Do you change it to a class? Everywhere?
Distinguishing between structs and classes is just too much hassle getting into the way of doing what we should be doing - programming. Like so many of C++'s problems it arises out of the strong desire for backwards compatibility.
They are pretty much the same thing. Thanks to the magic of C++, a struct can hold functions, use inheritance, created using "new" and so on just like a class
The only functional difference is that a class begins with private access rights, while a struct begins with public. This is the maintain backwards compatibility with C.
In practice, I've always used structs as data holders and classes as objects.
Class.
Class members are private by default.
class test_one {
int main_one();
};
Is equivalent to
class test_one {
private:
int main_one();
};
So if you try
int two = one.main_one();
We will get an error: main_one is private because its not accessible. We can
solve it by initializing it by specifying its a public ie
class test_one {
public:
int main_one();
};
Struct.
A struct is a class where members are public by default.
struct test_one {
int main_one;
};
Means main_one is private ie
class test_one {
public:
int main_one;
};
I use structs for data structures where the members can take any value, it's
easier that way.
After years of programming in C++, my main language, I come to the dead conclusion that this is another one of C++ dumb feature.
There is no real difference between the two, and no reason why I should spend extra time deciding whether I should define my entity as a struct or a class.
To answer this question, feel free to always define your entity as a struct. Members will be public by default which is the norm. But even more importantly, inheritance will be public by default. Protected inheritance, and even worse, private inheritance, are the exceptions.
I have never had a case where private inheritance was the right thing to do. Yes I tried to invent problems to use private inheritance but it didn't work. And Java, the role model of Object Oriented programming defaults to public inheritance if you don't use the accessor keywords. And by the way, Java doesn't allow accessor keywords on inherited classes, they can only be publicly inherited. So you can see, the cpp team really fell down here.
Another frustrating thing about this, is that if you define as a class and declare as a struct you get compilation warning. As though this is something that impacted the performance or accuracy of your program. One answer also noted that MSVC may propogate a compiler error instead.
Those persons that use classes when it is raining and structs when it is shining are doing so based on what they have been taught. It's not something they discovered to be true. Java does not have a pair of names for classes, and only have the class keyword. If you want a data structure, simply make all your members public and don't add functions. This works in Java and I don't see any problem. What's the problem? You need 4 or 5 characters of BOM code to determine how to interpret the context of a class entity.
they're the same thing with different defaults (private by default for class, and public by default for struct), so in theory they're totally interchangeable.
so, if I just want to package some info to move around, I use a struct, even if i put a few methods there (but not many). If it's a mostly-opaque thing, where the main use would be via methods, and not directly to the data members, i use a full class.
Structs by default have public access and classes by default have private access.
Personally I use structs for Data Transfer Objects or as Value Objects. When used as such I declare all members as const to prevent modification by other code.
Just to address this from a C++20 Standardese perspective (working from N4860)...
A class is a type. The keywords "class" and "struct" (and "union") are - in the C++ grammar - class-keys, and the only functional significance of the choice of class or struct is:
The class-key determines whether ... access is public or private by default (11.9).
Data member default accessibility
That the class keyword results in private-by-default members, and `struct keyword results in public-by-default members, is documented by the examples in 11.9.1:
class X {
int a; // X::a is private by default: class used
...vs...
struct S {
int a; // S::a is public by default: struct used
Base class default accessibility
1.9 also says:
In the absence of an access-specifier for a base class, public is assumed when the derived class is defined with the class-key struct and private is assumed when the class is defined with the class-key class.
Circumstances where consistent use of struct or class is required...
There's a requirement:
In a redeclaration, partial specialization, explicit specialization or explicit instantiation of a class template, the class-key shall agree in kind with the original class template declaration (9.2.8.3).
...in any elaborated-type-specifier, the enum keyword shall be used to refer to an enumeration (9.7.1), the union class-key shall be used to refer to a union (11.5), and either the class or struct class-key shall be
used to refer to a non-union class (11.1).
The following example (of when consistency is not required) is provided:
struct S { } s;
class S* p = &s; // OK
Still, some compilers may warn about this.
Interestingly, while the types you create with struct, class and union are all termed "classes", we have...
A standard-layout struct is a standard layout class defined with the class-key struct or the class-key class.
...so in Standardese, when there's talk of a standard-layout struct it's using "struct" to imply "not a union"s.
I'm curious if there are similar use of "struct" in other terminology, but it's too big a job to do an exhaustive search of the Standard. Comments about that welcome.
Technically both are the same in C++ - for instance it's possible for a struct to have overloaded operators etc.
However :
I use structs when I wish to pass information of multiple types simultaneously
I use classes when the I'm dealing with a "functional" object.
Hope it helps.
#include <string>
#include <map>
using namespace std;
struct student
{
int age;
string name;
map<string, int> grades
};
class ClassRoom
{
typedef map<string, student> student_map;
public :
student getStudentByName(string name) const
{ student_map::const_iterator m_it = students.find(name); return m_it->second; }
private :
student_map students;
};
For instance, I'm returning a struct student in the get...() methods over here - enjoy.
When would you choose to use struct
and when to use class in C++?
I use struct when I define functors and POD. Otherwise I use class.
// '()' is public by default!
struct mycompare : public std::binary_function<int, int, bool>
{
bool operator()(int first, int second)
{ return first < second; }
};
class mycompare : public std::binary_function<int, int, bool>
{
public:
bool operator()(int first, int second)
{ return first < second; }
};
I use structs when I need to create POD type or functor.
All class members are private by default and all struct members are public by default.
Class has default private bases and Struct has default public bases. Struct in case of C cannot have member functions where as in case of C++ we can have member functions being added to the struct. Other than these differences, I don't find anything surprising about them.
I use struct only when I need to hold some data without any member functions associated to it (to operate on the member data) and to access the data variables directly.
Eg: Reading/Writing data from files and socket streams etc. Passing function arguments in a structure where the function arguments are too many and function syntax looks too lengthy.
Technically there is no big difference between class and struture except default accessibility.
More over it depends on programming style how you use it.
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I thought that Structs was intended as a Data Structure (like a multi-data type array of information) and classes was inteded for Code Packaging (like collections of subroutines & functions)..
:(
I never use "struct" in C++.
I can't ever imagine a scenario where you would use a struct when you want private members, unless you're willfully trying to be confusing.
It seems that using structs is more of a syntactic indication of how the data will be used, but I'd rather just make a class and try to make that explicit in the name of the class, or through comments.
E.g.
class PublicInputData {
//data members
};
I am not very familiar with C++ , and while I am trying some test programms I came to a question regarding the best if I may say so way to define some primitive elements in C++ code.
Let's take a class that describes rectangles. It would create them, draw them , rotate, resize, etc... now in most cases we have to deal with points on the canvas.
The rectangle its self is described by 2 points: Upper Left and Lower Right corner.
Also in order to Rotate it, you need an angle, and a point(anchor point).
Or maybe to move it you need a new anchor point for the given rectangle. I guess I made my point in using points .
So what is more efficient ? to define this primitive point as a class or as a struct?
class cPoint
{
public:
int X;
int Y;
};
or
typedef struct
{
int X;
int Y;
}sPoint;
Niether are more efficient. On a technical level, there is no difference between a class and a struct aside from default visibility of members (public in struct, private in class) and default inheritance model (public in struct, private in class).
They typedef struct {} name model is not idiomatic in C++. In fact, it's an abomination -- a holdover from C. Don't use this model. Use this struct name {}; instead. Using the typedef struct {} name; model doesn't gain you anything in C++ (it was needed in C), and may cost you sojmething in terms of maintainability. For instance, it might be harder to grep for typedef struct declarations. But since it doesn't gain you anything by doing this, there's no compelling reason not to simply do struct name {}; in C++.
Aside from technical issues, the differences between struct and class are semantic ones. It is traditional and expected that objects declared as structs are simple objects which consist of only public: data members (so-called PODs). If it has private or protected data, is expected to be derived from, or has any methods, it is declared as a class.
This guideline is open to interpretation, and is just that -- a guideline. There is nothing to prevent you from declaring an abstract base class as a struct, for example. However you may want to consider following this guideline in order to follow the Principle of Least Surprise, making your code easier to understand and maintain.
Both are nearly equivalent. More precisely, struct { is the same as class {public:
An optimizing compiler would probably generate exactly the same code. Use MELT or simply pass -fdump-tree-all (beware, that option produces hundreds of dump files) to g++ (assuming you use a recent GCC compiler) -preferably with some optimization like -O - to find out (or look at the produced assembler code with g++ -O -fverbose-asm -S ...)
typedef struct is actually the C way to do this. In C++ the two versions would look very similar: Your class as written, and the struct as follows:
struct sPoint
{
int X;
int Y;
};
The two forms are functionally identical but you can provide your future maintainers with significant information by picking and sticking to some convention about how they're used. For example one approach is that if you intend to make the data elements private and give it useful methods (for example if you use inline accessors you can insert print calls every time the methods are used) then by all means make it a class. If you intend to have the data be public and access them as members then make it a struct.
There's no performance difference between a class and a struct
A class defaults to private access, whilst a struct defaults to public access. If interoperability with C is an issue for you then you'll have to use struct, and obviously it can't have any member functions.
As an aside, there's no std::is_struct in the standard library. Instead the std::is_class method returns true if the type is a class or a structure.
Simply put, the first way is more C++, and the second way is more C. Both work, while the first way is more 'standard' now.
A struct in C++ is like a class that would have public members by default*
There is no other formal difference, though your code would probably look confusing if you started using structs as classes, especially the inheritance mechanisms where data privacy is a major benefit.
If you are about to declare private/protected members, there is really little point in using a struct, though your code will still be 100% legal.
*including inherited members, since the zealots and nitpickers around seem to think the point is of a capital importance and only ignorant heatens would fail to mention it.
Except for the fact that this fine doctrine point is defined (or rather hinted, since the inference that base classes are simply defining inherited members is left to the sagacity of the reader) in another verse of the Stoustrup Holy Bible, there is really nothing to fuss about IMHO.
To properly declare the struct in your example, use
struct sPoint {
int X;
int Y;
};
In general, structs and classes in C++ are identical, except that data is public in a struct by default. The other difference is that the struct keyword cannot be used as the type in a template, although a struct can be used as the parameter.
There is a more thorough discussion here: C++ - struct vs. class
technically, struct{} and class{} are the same.
they differ on semantic level, with different member visibility.
struct{...} is equivalent to class{public:...}
And, it is also legal to declare a class using struct keyword. (add member functions, access specifier to struct{})
Generally, using struct for Plain-Old-Data (POD) type, class for Object-Oriented type to improve readability.
typedef struct{} should only be used to hide implementation detail(eg: supply a close-source library to users)
From my opinion, in your case, using struct is better, because Point's member need to be modified directly by other code.
I'm creating a class called ImageLoader that will be used to load various image formats. For various image formats there are certain structs used. For example with bmp files you have a BITMAPFILEHEADER struct and two others.
What I want to know is, when I'm putting my class definition in the header file, do I make the struct typedefs part of the class definition, or should they be separate, outside the class definition?
I'm not sure because if I was just declaring a struct variable, that would obviously happen in the class, but because I'm defining a type, I'm not sure if it's considered good design to define a type inside a class.
My general rule is that if it will only be used in conjunction with that class, then declare it inside (it implies ownership); otherwise declare it separately.
You get better encapsulation if you leave out everything from the header that you possibly can. Even if some methods of your class need parameters or return types of the struct, you might get away with a forward declaration.
The only time you need to put it in the header is when it's part of the public interface.
As to whether it goes in the class or not, consider whether it's useful on its own or if it is totally subservient to the class. If it can stand alone it should probably go in its own header.
I would not say that declaring a type in a class is an indicator of bad design. Assuming that your mention of "design" means something along the lines of "readability", I would be a stickler for consistency and insist that the same relationships be expressed in the same way.
Otherwise, you are not going to be struck down by a divine wrath for nesting types (Considering SGI is not a smoldering crater). This is pretty context-centric, so there are no hard and fast rules outside of what you define based on your requirements.
If client accessibility is not an issue, I declare most everything in an appropriate scope in headers and just document what my code means. Again, this is if I do not have strict usage/readability guidelines to enforce. I'd go with Mark's suggestion if I did.
Two cents: You could try enumerating image types and using one public struct for config data so that you could justify pulling everything else behind closed doors.
BITMAPFILEHEADER is a structure defined in the Win32 Platform SDK. I'm not sure I've understood your request about it and your class...
In general, if you are defining structures that are not exposed to the client of your class, I'd define them in the private part of your class, or in a sub-namespace Details in your header file, e.g.:
namespace YourCoolLibrary
{
namespace Details
{
struct SomeInternalStructure
{
...
};
} // namespace Details
class YourCoolClass
{
...
private:
Details::SomeInternalStructure m_something;
};
} // namespace YourCoolLibrary
There are even many more choices. If you put it in the class you have to choose if it's public, protected, or private making the class definition visible for every user of the class, only derived classes or no other classes.
If you do not need the details of the class definition I'd put only forward declarations into ImageLoader to keep it simpler. The full definition of the inner class then goes into the implementation file.
My rule of thumb is to make names as local as possible, so if I use it accidentally at the wrong place the compiler will complain.
If the typedef logically belongs into the class you are creating, put it inside; if it would make sense on a global level, leave it outside.
What are the pros and cons of using nested public C++ classes and enumerations? For example, suppose you have a class called printer, and this class also stores information on output trays, you could have:
class printer
{
public:
std::string name_;
enum TYPE
{
TYPE_LOCAL,
TYPE_NETWORK,
};
class output_tray
{
...
};
...
};
printer prn;
printer::TYPE type;
printer::output_tray tray;
Alternatively:
class printer
{
public:
std::string name_;
...
};
enum PRINTER_TYPE
{
PRINTER_TYPE_LOCAL,
PRINTER_TYPE_NETWORK,
};
class output_tray
{
...
};
printer prn;
PRINTER_TYPE type;
output_tray tray;
I can see the benefits of nesting private enums/classes, but when it comes to public ones, the office is split - it seems to be more of a style choice.
So, which do you prefer and why?
Nested classes
There are several side effects to classes nested inside classes that I usually consider flaws (if not pure antipatterns).
Let's imagine the following code :
class A
{
public :
class B { /* etc. */ } ;
// etc.
} ;
Or even:
class A
{
public :
class B ;
// etc.
} ;
class A::B
{
public :
// etc.
} ;
So:
Privilegied Access: A::B has privilegied access to all members of A (methods, variables, symbols, etc.), which weakens encapsulation
A's scope is candidate for symbol lookup: code from inside B will see all symbols from A as possible candidates for a symbol lookup, which can confuse the code
forward-declaration: There is no way to forward-declare A::B without giving a full declaration of A
Extensibility: It is impossible to add another class A::C unless you are owner of A
Code verbosity: putting classes into classes only makes headers larger. You can still separate this into multiple declarations, but there's no way to use namespace-like aliases, imports or usings.
As a conclusion, unless exceptions (e.g. the nested class is an intimate part of the nesting class... And even then...), I see no point in nested classes in normal code, as the flaws outweights by magnitudes the perceived advantages.
Furthermore, it smells as a clumsy attempt to simulate namespacing without using C++ namespaces.
On the pro-side, you isolate this code, and if private, make it unusable but from the "outside" class...
Nested enums
Pros: Everything.
Con: Nothing.
The fact is enum items will pollute the global scope:
// collision
enum Value { empty = 7, undefined, defined } ;
enum Glass { empty = 42, half, full } ;
// empty is from Value or Glass?
Ony by putting each enum in a different namespace/class will enable you to avoid this collision:
namespace Value { enum type { empty = 7, undefined, defined } ; }
namespace Glass { enum type { empty = 42, half, full } ; }
// Value::type e = Value::empty ;
// Glass::type f = Glass::empty ;
Note that C++0x defined the class enum:
enum class Value { empty, undefined, defined } ;
enum class Glass { empty, half, full } ;
// Value e = Value::empty ;
// Glass f = Glass::empty ;
exactly for this kind of problems.
One con that can become a big deal for large projects is that it is impossible to make a forward declaration for nested classes or enums.
If you're never going to be using the dependent class for anything but working with the independent class's implementations, nested classes are fine, in my opinion.
It's when you want to be using the "internal" class as an object in its own right that things can start getting a little manky and you have to start writing extractor/inserter routines. Not a pretty situation.
It seems like you should be using namespaces instead of classes to group like things that are related to each other in this way. One con that I could see in doing nested classes is you end up with a really large source file that could be hard to grok when you are searching for a section.
There are no pros and cons per se of using nested public C++ classes. There are only facts. Those facts are mandated by the C++ standard. Whether a fact about nested public C++ classes is a pro or a con depends on the particular problem that you are trying to solve. The example you have given does not allow a judgement about whether nested classes are appropriate or not.
One fact about nested classes is, that they have privileged access to all members of the class that they belong to. This is a con, if the nested classes does not need such access. But if the nested class does not need such access, then it should not have been declared as a nested class. There are situations, when a class A wants to grant privileged access to certain other classes B. There are three solutions to this problem
Make B a friend of A
Make B a nested class of A
Make the methods and attributes, that B needs, public members of A.
In this situation, it's #3 that violates encapsulation, because A has control over his friends and over his nested classes, but not over classes that call his public methods or access his public attributes.
Another fact about nested classes is, that it is impossible to add another class A::C as a nested class of A unless you are owner of A. However, this is perfectly reasonable, because nested classes have privileged access. If it were possible to add A::C as a nested class of A, then A::C could trick A into granting access to privileged information; and that yould violate encapsulation. It's basically the same as with the friend declaration: the friend declaration does not grant you any special privileges, that your friend is hiding from others; it allows your friends to access information that you are hiding from your non-friends. In C++, calling someone a friend is an altruistic act, not an egoistic one. The same holds for allowing a class to be a nested class.
Som other facts about nested public classes:
A's scope is candidate for symbol lookup of B: If you don't want this, make B a friend of A instead of a nested class. However, there are cases where you want exactly this kind of symbol lookup.
A::B cannot be forward-declared: A and A::B are tightly coupled. Being able to use A::B without knowing A would only hide this fact.
To summarize this: if the tool does not fit your needs, don't blame the tool; blame yourself for using the tool; others might have different problems, for which the tool is perfect.
paercebal said everything I would say about nested enums.
WRT nested classes, my common and almost sole use case for them is when I have a class which is manipulating a specific type of resource, and I need a data class which represents something specific to that resource. In your case, output_tray might be a good example, but I don't generally use nested classes if the class is going to have any methods which are going to be called from outside the containing class, or is more than primarily a data class. I generally also don't nest data classes unless the contained class is not ever directly referenced outside the containing class.
So, for example, if I had a printer_manipulator class, it might have a contained class for printer manipulation errors, but printer itself would be a non-contained class.
Hope this helps. :)
Remember that you can always promote a nested class to a top-level one later, but you may not be able to do the opposite without breaking existing code. Therefore, my advice would be make it a nested class first, and if it starts to become a problem, make it a top-level class in the next version.
For me a big con to having it outside is that it becomes part of the global namespace. If the enum or related class only really applies to the class that it's in, then it makes sense. So in the printer case, everything that includes the printer will know about having full access to the enum PRINTER_TYPE, where it doesn't really need to know about it. I can't say i've ever used an internal class, but for an enum, this seems more logical to keep it inside. As another poster has pointed out, it's also a good idea to to use namespaces to group similar items, since clogging the global namespace can really be a bad thing. I have previously worked on projects which are massive and just bringing up an auto complete list on the global namespace takes 20 minutes. In my opinion nested enums and namespaced classes/structs are probably the cleanest approach.
I agree with the posts advocating for embedding your enum in a class but there are cases where it makes more sense to not do that (but please, at least put it in a namespace). If multiple classes are utilizing an enum defined within a different class, then those classes are directly dependent on that other concrete class (that owns the enum). That surely represents a design flaw since that class will be responsible for that enum as well as other responsibilities.
So, yeah, embed the enum in a class if other code only uses that enum to interface directly with that concrete class. Otherwise, find a better place to keep the enum such as a namespace.
If you put the enum into a class or a namespace, intellisense will be able to give you guidance when you're trying to remember the enum names. A small thing for sure, but sometimes the small things matter.
Visual Studio 2008 does not seem to be able to provide intellisense for nested classes, so I have switched to the PIMPL idiom in most cases where I used to have a nested class. I always put enums either in the class if it is used only by that class, or outside the class in the same namespace as the class when more than one class uses the enum.
I can see a con for nested classes, that one may better use generic programming.
If the little class is defined outside the big one, you can make the big class a class template and use any "little" class you may need in the future with the big class.
Generic programming is a powerful tool, and, IMHO, we should keep it in mind when developing extensible programs. Strange, that no one has mentioned this point.
Only problem with nested classes that I bumped into yet was that C++ does not let us refer to the object of the enclosing class, in the nested class functions. We cannot say "Enclosing::this"
(But maybe there's a way?)