Related
Let's say I have, or am going to write, a set of related functions. Let's say they're math-related. Organizationally, should I:
Write these functions and put them in my MyMath namespace and refer to them via MyMath::XYZ()
Create a class called MyMath and make these methods static and refer to the similarly MyMath::XYZ()
Why would I choose one over the other as a means of organizing my software?
By default, use namespaced functions.
Classes are to build objects, not to replace namespaces.
In Object Oriented code
Scott Meyers wrote a whole Item for his Effective C++ book on this topic, "Prefer non-member non-friend functions to member functions". I found an online reference to this principle in an article from Herb Sutter: http://www.gotw.ca/gotw/084.htm
The important thing to know is that: In C++, functions that are in the same namespace as a class is, and that have that class as a parameter, belong to that class' interface (because ADL will search those functions when resolving function calls).
For example:
let's say you have a namespace N
let's say you have a class C, declared in namespace N (in other words, its full name is N::C)
let's say you have a function F, declared in namespace N (in other words, its full name is N::F)
let's say that function F has, among its parameters, a parameter of type C
... Then N::F is part of N::C's public interface.
Namespaced functions, unless declared "friend," have no access to the class's internals, whereas static methods have the right to access the class's internals.
This means, for example, that when maintaining your class, if you need to change your class' internals, you will need to search for side effects in all its methods, including the static ones.
Extension I
Adding code to a class' interface.
In C#, you can add methods to a class even if you have no access to it. But in C++, this is impossible.
But, still in C++, you can still add a namespaced function, even to a class someone wrote for you.
See from the other side, this is important when designing your code, because by putting your functions in a namespace, you will authorize your users to increase/complete the class' interface.
Extension II
A side-effect of the previous point, it is impossible to declare static methods in multiple headers. Every method must be declared in the same class.
For namespaces, functions from the same namespace can be declared in multiple headers (the almost-standard swap function is the best example of that).
Extension III
The basic coolness of a namespace is that in some code, you can avoid mentioning it, if you use the keyword using:
#include <string>
#include <vector>
// Etc.
{
using namespace std ;
// Now, everything from std is accessible without qualification
string s ; // Ok
vector v ; // Ok
}
string ss ; // COMPILATION ERROR
vector vv ; // COMPILATION ERROR
And you can even limit the "pollution" to one class:
#include <string>
#include <vector>
{
using std::string ;
string s ; // Ok
vector v ; // COMPILATION ERROR
}
string ss ; // COMPILATION ERROR
vector vv ; // COMPILATION ERROR
This "pattern" is mandatory for the proper use of the almost-standard swap idiom.
And this is impossible to do with static methods in classes.
So, C++ namespaces have their own semantics.
But it goes further, as you can combine namespaces in a way similar to inheritance.
For example, if you have a namespace A with a function AAA, a namespace B with a function BBB, you can declare a namespace C, and bring AAA and BBB in this namespace with the keyword using.
You can even bring the full content of a namespace inside another, with using namespace, as shown with namespace D!
namespace A
{
void AAA();
void AAA2();
}
namespace B
{
void BBB();
}
namespace C
{
using A::AAA;
using B::BBB;
}
namespace D
{
using namespace A;
using namespace B;
}
void foo()
{
C::AAA();
// C::AAA2(); // ERROR, won't compile
C::BBB();
}
void bar()
{
D::AAA();
D::AAA2();
D::BBB();
}
Conclusion
Namespaces are for namespaces.
Classes are for classes.
C++ was designed so each concept is different, and is used differently, in different cases, as a solution to different problems.
Don't use classes when you need namespaces.
And in your case, you need namespaces.
There are a lot of people who would disagree with me, but this is how I see it:
A class is essentially a definition of a certain kind of object. Static methods should define operations that are intimately tied to that object definition.
If you are just going to have a group of related functions not associated with an underlying object or definition of a kind of object, then I would say go with a namespace only. Just for me, conceptually, this is a lot more sensible.
For instance, in your case, ask yourself, "What is a MyMath?" If MyMath does not define a kind of object, then I would say: don't make it a class.
But like I said, I know there are plenty of folks who would (even vehemently) disagree with me on this (in particular, Java and C# developers).
If you need static data, use static methods.
If they're template functions and you'd like to be able to specify a set of template parameters for all functions together then use static methods in a template class.
Otherwise, use namespaced functions.
In response to the comments: yes, static methods and static data tend to be over-used. That's why I offered only two, related scenarios where I think they can be helpful. In the OP's specific example (a set of math routines), if he wanted the ability to specify parameters - say, a core data type and output precision - that would be applied to all routines, he might do something like:
template<typename T, int decimalPlaces>
class MyMath
{
// routines operate on datatype T, preserving at least decimalPlaces precision
};
// math routines for manufacturing calculations
typedef MyMath<double, 4> CAMMath;
// math routines for on-screen displays
typedef MyMath<float, 2> PreviewMath;
If you don't need that, then by all means use a namespace.
You should use a namespace, because a namespace has the many advantages over a class:
You don't have to define everything in the same header
You don't need to expose all your implementation in the header
You can't using a class member; you can using a namespace member
You can't using class, though using namespace is not all that often a good idea
Using a class implies that there is some object to be created when there really is none
Static members are, in my opinion, very very overused. They aren't a real necessity in most cases. Static members functions are probably better off as file-scope functions, and static data members are just global objects with a better, undeserved reputation.
I would prefer namespaces, that way you can have private data in an anonymous namespace in the implementation file (so it doesn't have to show up in the header at all as opposed to private members). Another benefit is that by using your namespace the clients of the methods can opt out of specifying MyMath::
I want to summarize and add to other answers. Also, my perspective is in the world of header-only.
Namespaces
Pros:
simple solution for naming hierarchies
they carry no semantics, so it is simpler to read
can live in different files (headers)
can be extended
ADL
shortcut can be defined (using).
Plays well with operator overload
Can be used for branding (you can design your code and put a namespace over it without much though)
Cons:
everything is public
private things need unnamed namespace so it is not explicit
ADL (yes, some people despise ADL)
can be extended (this can be a bad thing, specially in combination with ADL, semantics of existing code can change by extending the namespace)
functions need to be defined (or declared) in order of use
Classes with static methods
Pros:
can have private components (function, variables) and they are explicitly marked.
classes can be friended
can be type-parametrized (templates)
can be template parameters themselves
can be instantiated
can be passed to functions (static functions behave like non-static method by default).
it is easier to find patterns and go from groups of independent functions and convert them to a proper class (eventually with non static members)
dependencies among classes is well defined
functions (the static method) can be defined in any order
Cons:
No ADL
cannot be extended
needs the keyword static everywhere (opportunity to make fun of the language)
an overkill to solve the naming problem alone. Difficult to read in that case.
the functions (static methods) always need qualification (myclassspace::fun). There is no way to declare shortcuts (using).
almost useless for operator overload, needs complicated friend mechanism for that.
can not be used for branding.
you need to remember end it with ; :)
In summary, classes with static methods are better units of code and allow more meta programming, and except for ADL and some syntactic quirks, can replicate all the features of namespaces, but they can be an overkill sometimes.
Companies, such as Bloomberg, prefer classes over namespaces.
If you don’t like ADL or operator overload, classes with static methods is the way to go.
IMO, it would be nice if namespace and classes are integrated to become two sides of the same coin.
For example identify a namespace in the language as a class were the methods are static by default.
And then be able to use them as template parameters.
I wouldn't be sure what to do with ADL (may be it could be restricted to symbolic operators functions alone, e.g. operatorX, which was the original motivation for operator overload and ADL in the first place)
Why would I choose one over the other as a means of organizing my software?
If you use namespaces, you will frequently hit a language defect that functions which call each other must be listed in a specific order, because C++ can't see definitions further down in the file.
If you use classes, this defect does not occur.
It can be easier and cleaner to wrap implementation functions in a class than to maintain declarations for them all or put them in an unnatural order to make it compile.
One more reason to use class - Option to make use of access specifiers. You can then possibly break your public static method into smaller private methods. Public method can call multiple private methods.
Both namespace and class method have their uses. Namespace have the ability to be spread across files however that is a weakness if you need to enforce all related code to go in one file. As mentioned above class also allows you to create private static members in the class. You can have it in the anonymous namespace of the implementation file however it is still a bigger scope than having them inside the class.
Let's say I have, or am going to write, a set of related functions. Let's say they're math-related. Organizationally, should I:
Write these functions and put them in my MyMath namespace and refer to them via MyMath::XYZ()
Create a class called MyMath and make these methods static and refer to the similarly MyMath::XYZ()
Why would I choose one over the other as a means of organizing my software?
By default, use namespaced functions.
Classes are to build objects, not to replace namespaces.
In Object Oriented code
Scott Meyers wrote a whole Item for his Effective C++ book on this topic, "Prefer non-member non-friend functions to member functions". I found an online reference to this principle in an article from Herb Sutter: http://www.gotw.ca/gotw/084.htm
The important thing to know is that: In C++, functions that are in the same namespace as a class is, and that have that class as a parameter, belong to that class' interface (because ADL will search those functions when resolving function calls).
For example:
let's say you have a namespace N
let's say you have a class C, declared in namespace N (in other words, its full name is N::C)
let's say you have a function F, declared in namespace N (in other words, its full name is N::F)
let's say that function F has, among its parameters, a parameter of type C
... Then N::F is part of N::C's public interface.
Namespaced functions, unless declared "friend," have no access to the class's internals, whereas static methods have the right to access the class's internals.
This means, for example, that when maintaining your class, if you need to change your class' internals, you will need to search for side effects in all its methods, including the static ones.
Extension I
Adding code to a class' interface.
In C#, you can add methods to a class even if you have no access to it. But in C++, this is impossible.
But, still in C++, you can still add a namespaced function, even to a class someone wrote for you.
See from the other side, this is important when designing your code, because by putting your functions in a namespace, you will authorize your users to increase/complete the class' interface.
Extension II
A side-effect of the previous point, it is impossible to declare static methods in multiple headers. Every method must be declared in the same class.
For namespaces, functions from the same namespace can be declared in multiple headers (the almost-standard swap function is the best example of that).
Extension III
The basic coolness of a namespace is that in some code, you can avoid mentioning it, if you use the keyword using:
#include <string>
#include <vector>
// Etc.
{
using namespace std ;
// Now, everything from std is accessible without qualification
string s ; // Ok
vector v ; // Ok
}
string ss ; // COMPILATION ERROR
vector vv ; // COMPILATION ERROR
And you can even limit the "pollution" to one class:
#include <string>
#include <vector>
{
using std::string ;
string s ; // Ok
vector v ; // COMPILATION ERROR
}
string ss ; // COMPILATION ERROR
vector vv ; // COMPILATION ERROR
This "pattern" is mandatory for the proper use of the almost-standard swap idiom.
And this is impossible to do with static methods in classes.
So, C++ namespaces have their own semantics.
But it goes further, as you can combine namespaces in a way similar to inheritance.
For example, if you have a namespace A with a function AAA, a namespace B with a function BBB, you can declare a namespace C, and bring AAA and BBB in this namespace with the keyword using.
You can even bring the full content of a namespace inside another, with using namespace, as shown with namespace D!
namespace A
{
void AAA();
void AAA2();
}
namespace B
{
void BBB();
}
namespace C
{
using A::AAA;
using B::BBB;
}
namespace D
{
using namespace A;
using namespace B;
}
void foo()
{
C::AAA();
// C::AAA2(); // ERROR, won't compile
C::BBB();
}
void bar()
{
D::AAA();
D::AAA2();
D::BBB();
}
Conclusion
Namespaces are for namespaces.
Classes are for classes.
C++ was designed so each concept is different, and is used differently, in different cases, as a solution to different problems.
Don't use classes when you need namespaces.
And in your case, you need namespaces.
There are a lot of people who would disagree with me, but this is how I see it:
A class is essentially a definition of a certain kind of object. Static methods should define operations that are intimately tied to that object definition.
If you are just going to have a group of related functions not associated with an underlying object or definition of a kind of object, then I would say go with a namespace only. Just for me, conceptually, this is a lot more sensible.
For instance, in your case, ask yourself, "What is a MyMath?" If MyMath does not define a kind of object, then I would say: don't make it a class.
But like I said, I know there are plenty of folks who would (even vehemently) disagree with me on this (in particular, Java and C# developers).
If you need static data, use static methods.
If they're template functions and you'd like to be able to specify a set of template parameters for all functions together then use static methods in a template class.
Otherwise, use namespaced functions.
In response to the comments: yes, static methods and static data tend to be over-used. That's why I offered only two, related scenarios where I think they can be helpful. In the OP's specific example (a set of math routines), if he wanted the ability to specify parameters - say, a core data type and output precision - that would be applied to all routines, he might do something like:
template<typename T, int decimalPlaces>
class MyMath
{
// routines operate on datatype T, preserving at least decimalPlaces precision
};
// math routines for manufacturing calculations
typedef MyMath<double, 4> CAMMath;
// math routines for on-screen displays
typedef MyMath<float, 2> PreviewMath;
If you don't need that, then by all means use a namespace.
You should use a namespace, because a namespace has the many advantages over a class:
You don't have to define everything in the same header
You don't need to expose all your implementation in the header
You can't using a class member; you can using a namespace member
You can't using class, though using namespace is not all that often a good idea
Using a class implies that there is some object to be created when there really is none
Static members are, in my opinion, very very overused. They aren't a real necessity in most cases. Static members functions are probably better off as file-scope functions, and static data members are just global objects with a better, undeserved reputation.
I would prefer namespaces, that way you can have private data in an anonymous namespace in the implementation file (so it doesn't have to show up in the header at all as opposed to private members). Another benefit is that by using your namespace the clients of the methods can opt out of specifying MyMath::
I want to summarize and add to other answers. Also, my perspective is in the world of header-only.
Namespaces
Pros:
simple solution for naming hierarchies
they carry no semantics, so it is simpler to read
can live in different files (headers)
can be extended
ADL
shortcut can be defined (using).
Plays well with operator overload
Can be used for branding (you can design your code and put a namespace over it without much though)
Cons:
everything is public
private things need unnamed namespace so it is not explicit
ADL (yes, some people despise ADL)
can be extended (this can be a bad thing, specially in combination with ADL, semantics of existing code can change by extending the namespace)
functions need to be defined (or declared) in order of use
Classes with static methods
Pros:
can have private components (function, variables) and they are explicitly marked.
classes can be friended
can be type-parametrized (templates)
can be template parameters themselves
can be instantiated
can be passed to functions (static functions behave like non-static method by default).
it is easier to find patterns and go from groups of independent functions and convert them to a proper class (eventually with non static members)
dependencies among classes is well defined
functions (the static method) can be defined in any order
Cons:
No ADL
cannot be extended
needs the keyword static everywhere (opportunity to make fun of the language)
an overkill to solve the naming problem alone. Difficult to read in that case.
the functions (static methods) always need qualification (myclassspace::fun). There is no way to declare shortcuts (using).
almost useless for operator overload, needs complicated friend mechanism for that.
can not be used for branding.
you need to remember end it with ; :)
In summary, classes with static methods are better units of code and allow more meta programming, and except for ADL and some syntactic quirks, can replicate all the features of namespaces, but they can be an overkill sometimes.
Companies, such as Bloomberg, prefer classes over namespaces.
If you don’t like ADL or operator overload, classes with static methods is the way to go.
IMO, it would be nice if namespace and classes are integrated to become two sides of the same coin.
For example identify a namespace in the language as a class were the methods are static by default.
And then be able to use them as template parameters.
I wouldn't be sure what to do with ADL (may be it could be restricted to symbolic operators functions alone, e.g. operatorX, which was the original motivation for operator overload and ADL in the first place)
Why would I choose one over the other as a means of organizing my software?
If you use namespaces, you will frequently hit a language defect that functions which call each other must be listed in a specific order, because C++ can't see definitions further down in the file.
If you use classes, this defect does not occur.
It can be easier and cleaner to wrap implementation functions in a class than to maintain declarations for them all or put them in an unnatural order to make it compile.
One more reason to use class - Option to make use of access specifiers. You can then possibly break your public static method into smaller private methods. Public method can call multiple private methods.
Both namespace and class method have their uses. Namespace have the ability to be spread across files however that is a weakness if you need to enforce all related code to go in one file. As mentioned above class also allows you to create private static members in the class. You can have it in the anonymous namespace of the implementation file however it is still a bigger scope than having them inside the class.
Is it me or the implementation of object oriented paradigm in C++ is missing the concept of class namespace?
Here is an example of what I mean:
Document { Header {} Body {} Footer {} }
Document is an object that can have a header, body, and footer.
A straight forward way to address such object and its elements from external namespace is
Document
Document::Header
Document::Body
Document::Footer
Is there a way to achieve such naming structure in C++ without imposing a restriction on the definition of Document class?
1) namespace Document { ... }
This case requires an object class inside its own namespace and using Document::Document seems redundant and unintended.
2) namespace Document { class ... } typedef Document::Document document;
This case gives document and Document::{part}, which in case sensitive language may seem weird and unrelated.
3) class Document { class ... };
This case requires including Document header in definition of every nested class and doesn't allow to make the final object be a derivative of its parts, since they are defined within its own scope.
4) class document { class ... }; class Document : public document {}
This case is close to what is intended but costs an extra class and inheritance.
!) Ideally, what I'd like to have is
namespace class Document {
class Header;
class Body;
class Footer;
class Document; // linked with the parent namespace as with its own name space
}
Document doc; // ok, resolves Document as a class
Document::{Part} docPart; // ok, resolves Document as namespace
.) Is there any other reasonable way to achieve what is intended without unreasonable extra cost?
I'm also not exactly sure why such trivial things are not a standard way to do it. :/ Any specific reason?
--- clarification ---
To address some of the raised questions,
"What is it useful for?" 1) Plain language 2) Transparent mapping from a language construct to an object's abstract model and vice versa.
"Why would one want to derive an object from its parts?" Not every entity introduced by an object has to be its part. It can be its essence, for example. E.g.:
Document { Skeleton{} Parts { Header {} Body {} Footer {} } }
--- abstract framework ---
Think of an object as a module whose definition may use external symbols and introduce some of its own along with the definition of its own logical entity, to which they should remain related since it introduces them.
--- point ---
The whole module is a definition of an object. It would be nice to be able to use it as such without any additional linguistic ridicules.
=== resolution ===
Thank you for your feedback.
Until there is a way in C++ to link a namespace name to a class, I guess I'll use the
ObjectName { ... Object {} } -> ObjectName::Object, ObjectName::Part
construct in such cases. It may be not as short as I'd like to, but at least transparent enough, with no extra cost, and can be used with forward declarations.
Your third option - using nested class declarations / definitions, will get you the ideal scope resolution as you want (though it will not involve a single namespace).
class Document
{
public:
class Header
{
// Header class declaration here
};
class Body
{
// Body class declaration here
};
class Footer
{
// Footer class declaration here
};
// Document declaration here.
};
Your concerns with that option are:
Requires including Document header in definition of every nested class
Yes - the nested classes are inextricably linked to the surrounding class, their implementations will depend on the definition of the surrounding class because it contains the definitions of the nested classes. This is unavoidable.
Dpesn't allow to make the final object be a derivative of its parts, since they are defined within its own scope.
What you are trying to do seems to make very little logical sense - you are trying to define something based on its own parts, which is a circular definition. What are you trying to achieve by deriving Document from Header, Body or Footer?
The final line of your question suggests that you find your desired functionality "trivial", but in my opinion and experience it is far from trivial. You seem to be conflating a namespace and a class because of syntactical similarities, but they are entirely different concepts. You have to separate the two in your mind, they have very little overlap besides some scoping effects and scope resolution syntax. Your "Document" must be either a namespace and a class. Pick one ;) (technically it's actually possible to have both a namespace Document and a class Document, but this is likely to be a source of confusion.
)
Your first case does exactly what you say you want. There's nothing redundant about Document::Document -- it refers to the Document class in the Document namespace. There might very well be a Document class in the XML namespace, and another one in the MyCompany namespace.
Your second case looks like an attempt to essentially defeat the purpose of using namespaces. If you don't want to use a namespace, don't -- just use the global (unspecified) namespace and risk collisions. If you only want to avoid the Document:: part in the code that's related to your Document class, add a using namespace Document directive in that code.
You just enclose the part classes within the Document class.
class Document {
public:
class Header;
class Body;
class Footer;
};
You can validly use this:
Document myDoc; // Type Document
Document::Header myHeader;
3) class Document { class ... };
This case requires including Document header in definition of every nested class and doesn't allow to make the final object be a derivative of its parts, since they are defined within its own scope.
No, this design is what makes the most sense. I don't understand what you think the difference would be between class and namespace class.
class Document
{
public:
class Header
{
};
Header m_header;
class Body
{
};
Body m_Body;
};
What's wrong with this design? You access the types via Document::Header. You access via instances like myDocument.m_header.
The only inherent oddness is that you can't name a type and member variable the same, but there are plenty of ways around that and it's a superficial restriction really.
Just because a Document object contains a Header object doesn’t mean that the Header class should be contained in the Document class. At least, that’s not usually done.
I would nest classes only in rare instances; namely, when the nested class is an implementation detail of the outer class, and isn’t exposed to the outside world (but even then it’s common to forego nesting).
This, by the way, is independent of C++: classes in general are rarely nested except to hide implementation details. Frameworks to model object relations as in your case wouldn’t normally use nesting. Instead, you might have something like this:
namespace Html {
class Document;
class Header;
class Body;
// …
}
C++ in particular uses flat namespace hierarchies but the above would equally apply to C# or Java.
Finally, to explicitly address your introductory question:
Is it me or the implementation of object oriented paradigm in C++ is missing the concept of class namespace?
C+ has this concept to the same extent as other modern OO languages: a class forms a namespace for the purpose of name lookup so you can achieve what you want. But for the reason mentioned above I don’t think it’s a particularly desirable goal.
It's not totally clear to me how much you want to expose the nested
classes, but if they are part of Document, they probably should be
nested classes. Your example of what you mean, at the start of your
code, is exactly how one would do this in C++:
class Document
{
public: // or not?
class Header
{
};
class Body
{
};
class Footer
{
};
};
About the only objection I can see is that implementation files only
concerned with Document::Header must include the entire Document
class definition, but I don't think this is a major problem; if
Document::Header really isn't independent, then it seems reasonable to
require this. With regards to your second objection to this solution:
you never want to make an object derive from its parts: a Document
hasA Header; it isn't an isA relationship.
If it does make sense for Header et al to be used separately, then the
best solution is to define them outside of the Document class, either
giving them more descriptive names (e.g. DocumentHeader) or wrapping
them in a special namespace (e.g. namespace DocumentParts). Depending
on their relationship with Document, it might make sense to use
typedef in Document so that they can be referred to as either
DocumentParts::Header or Document::Header.
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.
Let's say I have, or am going to write, a set of related functions. Let's say they're math-related. Organizationally, should I:
Write these functions and put them in my MyMath namespace and refer to them via MyMath::XYZ()
Create a class called MyMath and make these methods static and refer to the similarly MyMath::XYZ()
Why would I choose one over the other as a means of organizing my software?
By default, use namespaced functions.
Classes are to build objects, not to replace namespaces.
In Object Oriented code
Scott Meyers wrote a whole Item for his Effective C++ book on this topic, "Prefer non-member non-friend functions to member functions". I found an online reference to this principle in an article from Herb Sutter: http://www.gotw.ca/gotw/084.htm
The important thing to know is that: In C++, functions that are in the same namespace as a class is, and that have that class as a parameter, belong to that class' interface (because ADL will search those functions when resolving function calls).
For example:
let's say you have a namespace N
let's say you have a class C, declared in namespace N (in other words, its full name is N::C)
let's say you have a function F, declared in namespace N (in other words, its full name is N::F)
let's say that function F has, among its parameters, a parameter of type C
... Then N::F is part of N::C's public interface.
Namespaced functions, unless declared "friend," have no access to the class's internals, whereas static methods have the right to access the class's internals.
This means, for example, that when maintaining your class, if you need to change your class' internals, you will need to search for side effects in all its methods, including the static ones.
Extension I
Adding code to a class' interface.
In C#, you can add methods to a class even if you have no access to it. But in C++, this is impossible.
But, still in C++, you can still add a namespaced function, even to a class someone wrote for you.
See from the other side, this is important when designing your code, because by putting your functions in a namespace, you will authorize your users to increase/complete the class' interface.
Extension II
A side-effect of the previous point, it is impossible to declare static methods in multiple headers. Every method must be declared in the same class.
For namespaces, functions from the same namespace can be declared in multiple headers (the almost-standard swap function is the best example of that).
Extension III
The basic coolness of a namespace is that in some code, you can avoid mentioning it, if you use the keyword using:
#include <string>
#include <vector>
// Etc.
{
using namespace std ;
// Now, everything from std is accessible without qualification
string s ; // Ok
vector v ; // Ok
}
string ss ; // COMPILATION ERROR
vector vv ; // COMPILATION ERROR
And you can even limit the "pollution" to one class:
#include <string>
#include <vector>
{
using std::string ;
string s ; // Ok
vector v ; // COMPILATION ERROR
}
string ss ; // COMPILATION ERROR
vector vv ; // COMPILATION ERROR
This "pattern" is mandatory for the proper use of the almost-standard swap idiom.
And this is impossible to do with static methods in classes.
So, C++ namespaces have their own semantics.
But it goes further, as you can combine namespaces in a way similar to inheritance.
For example, if you have a namespace A with a function AAA, a namespace B with a function BBB, you can declare a namespace C, and bring AAA and BBB in this namespace with the keyword using.
You can even bring the full content of a namespace inside another, with using namespace, as shown with namespace D!
namespace A
{
void AAA();
void AAA2();
}
namespace B
{
void BBB();
}
namespace C
{
using A::AAA;
using B::BBB;
}
namespace D
{
using namespace A;
using namespace B;
}
void foo()
{
C::AAA();
// C::AAA2(); // ERROR, won't compile
C::BBB();
}
void bar()
{
D::AAA();
D::AAA2();
D::BBB();
}
Conclusion
Namespaces are for namespaces.
Classes are for classes.
C++ was designed so each concept is different, and is used differently, in different cases, as a solution to different problems.
Don't use classes when you need namespaces.
And in your case, you need namespaces.
There are a lot of people who would disagree with me, but this is how I see it:
A class is essentially a definition of a certain kind of object. Static methods should define operations that are intimately tied to that object definition.
If you are just going to have a group of related functions not associated with an underlying object or definition of a kind of object, then I would say go with a namespace only. Just for me, conceptually, this is a lot more sensible.
For instance, in your case, ask yourself, "What is a MyMath?" If MyMath does not define a kind of object, then I would say: don't make it a class.
But like I said, I know there are plenty of folks who would (even vehemently) disagree with me on this (in particular, Java and C# developers).
If you need static data, use static methods.
If they're template functions and you'd like to be able to specify a set of template parameters for all functions together then use static methods in a template class.
Otherwise, use namespaced functions.
In response to the comments: yes, static methods and static data tend to be over-used. That's why I offered only two, related scenarios where I think they can be helpful. In the OP's specific example (a set of math routines), if he wanted the ability to specify parameters - say, a core data type and output precision - that would be applied to all routines, he might do something like:
template<typename T, int decimalPlaces>
class MyMath
{
// routines operate on datatype T, preserving at least decimalPlaces precision
};
// math routines for manufacturing calculations
typedef MyMath<double, 4> CAMMath;
// math routines for on-screen displays
typedef MyMath<float, 2> PreviewMath;
If you don't need that, then by all means use a namespace.
You should use a namespace, because a namespace has the many advantages over a class:
You don't have to define everything in the same header
You don't need to expose all your implementation in the header
You can't using a class member; you can using a namespace member
You can't using class, though using namespace is not all that often a good idea
Using a class implies that there is some object to be created when there really is none
Static members are, in my opinion, very very overused. They aren't a real necessity in most cases. Static members functions are probably better off as file-scope functions, and static data members are just global objects with a better, undeserved reputation.
I want to summarize and add to other answers. Also, my perspective is in the world of header-only.
Namespaces
Pros:
simple solution for naming hierarchies
they carry no semantics, so it is simpler to read
can live in different files (headers)
can be extended
ADL
shortcut can be defined (using).
Plays well with operator overload
Can be used for branding (you can design your code and put a namespace over it without much though)
Cons:
everything is public
private things need unnamed namespace so it is not explicit
ADL (yes, some people despise ADL)
can be extended (this can be a bad thing, specially in combination with ADL, semantics of existing code can change by extending the namespace)
functions need to be defined (or declared) in order of use
Classes with static methods
Pros:
can have private components (function, variables) and they are explicitly marked.
classes can be friended
can be type-parametrized (templates)
can be template parameters themselves
can be instantiated
can be passed to functions (static functions behave like non-static method by default).
it is easier to find patterns and go from groups of independent functions and convert them to a proper class (eventually with non static members)
dependencies among classes is well defined
functions (the static method) can be defined in any order
Cons:
No ADL
cannot be extended
needs the keyword static everywhere (opportunity to make fun of the language)
an overkill to solve the naming problem alone. Difficult to read in that case.
the functions (static methods) always need qualification (myclassspace::fun). There is no way to declare shortcuts (using).
almost useless for operator overload, needs complicated friend mechanism for that.
can not be used for branding.
you need to remember end it with ; :)
In summary, classes with static methods are better units of code and allow more meta programming, and except for ADL and some syntactic quirks, can replicate all the features of namespaces, but they can be an overkill sometimes.
Companies, such as Bloomberg, prefer classes over namespaces.
If you don’t like ADL or operator overload, classes with static methods is the way to go.
IMO, it would be nice if namespace and classes are integrated to become two sides of the same coin.
For example identify a namespace in the language as a class were the methods are static by default.
And then be able to use them as template parameters.
I wouldn't be sure what to do with ADL (may be it could be restricted to symbolic operators functions alone, e.g. operatorX, which was the original motivation for operator overload and ADL in the first place)
I would prefer namespaces, that way you can have private data in an anonymous namespace in the implementation file (so it doesn't have to show up in the header at all as opposed to private members). Another benefit is that by using your namespace the clients of the methods can opt out of specifying MyMath::
Why would I choose one over the other as a means of organizing my software?
If you use namespaces, you will frequently hit a language defect that functions which call each other must be listed in a specific order, because C++ can't see definitions further down in the file.
If you use classes, this defect does not occur.
It can be easier and cleaner to wrap implementation functions in a class than to maintain declarations for them all or put them in an unnatural order to make it compile.
One more reason to use class - Option to make use of access specifiers. You can then possibly break your public static method into smaller private methods. Public method can call multiple private methods.
Both namespace and class method have their uses. Namespace have the ability to be spread across files however that is a weakness if you need to enforce all related code to go in one file. As mentioned above class also allows you to create private static members in the class. You can have it in the anonymous namespace of the implementation file however it is still a bigger scope than having them inside the class.