For elegance, encapsulation and to exploit ADL (Argument Dependent Lookup) is common to define a function inside the namespace of the function argument.
Suppose I have two libraries in different namespace. There are three cases 1) one is part of a library I control and the other is third party (for example Boost), or 2) I control both, or 3) I control none (just writing "glue" code).
I have something like this,
namespace ns_A{
struct A{...}; // something that looks like iostream
}
namespace ns_B{
struct B{...};
}
I want to "stream" B in to A, what is the best option
namespace ???{ // what is more correct ns_A, or ns_B?
A& operator<<(A& a, B const& b){...}
}
or should I put it in both namespaces?
namespace ns_B{
A& operator<<(A& a, B const& b){...}
}
namespace ns_A{
using ns_B::operator<<;
}
Which is the best namespace to define a binary function like this?
(Does C++11's namespace inline change any recommendation?)
(I use the example operator<< because, other things being equal it seems intuitively be better to prefer namespace ns_B.)
EDIT: this is the most complete guide and reference I could find on real use of namespaces
https://www.google.com/amp/s/akrzemi1.wordpress.com/2016/01/16/a-customizable-framework/amp/
In case 1, it is easy: put it in the namespace that you control.
In case 2, it's up to your choice: whatever appears more logical. In your example case, I would prefer ns_B.
The only tricky situation is 3. You shouldn't really add to either namespace. If you want the new 'glue' functionality as part of your own third namespace mine, then naturally put it in there and any use of that functionality within mine will be automatically resolved. Naturally, this will not envoke ADL, but there is no need for it, since all you want is to use the new functionality within mine, not somewhere else.
You can put your operator in either namespace and it will work. As a best practice, put it in the namespace that belongs to your code.
My suggestion: Don't use any of the namespaces. Code in ns_A is not aware, in itself, of the existence of anything in ns_A - it doesn't depend on it; so code regarding both ns_B and ns_A constructs does not belong in ns_A. The same is true for ns_B, by symmetry.
Your operator<< should be in the "least common namespace" among ns_A and ns_B, which is probably no namespace (but if ns_A is ns1::ns2 and ns_B is ns1::ns3 then use ns1).
Forcing code into a namespace it does not clearly belong in is, in my opinion, not elegant and breaks encapsulation, conceptually. As for ADL, I think you should not expect more than what the "least common namespace" of ns_A and ns_B gives you.
Related
I have been writing in c++ for a few months, and i am comfortable enough with it now to begin implementing my own library, consisting of things that i have found myself reusing again and again. One thing that nagged me was the fact that you always had to provide a beginning and end iterator for functions like std::accumulate,std::fill etc...
The option to provide a qualified container was completely absent and it was simply an annoyance to write begin and end over and over. So, I decided to add this functionality to my library, but i came across problem, i couldn't figure out the best approach of doing so. Here were my general solutions:
1. Macros
- A macro that encapsulates an entire function call
ex. QUICK_STL(FCall)
- A macro that takes the container, function name, and optional args
ex. QUICK_STL(C,F,Args...)
2. Wrapper Function/Functor
- A class that takes the container, function name, and optional args
ex. quick_stl(F, C, Args...)
3. Overload Functions
- Overload every function in namespace std OR my library namespace
ex
namespace std { // or my library root namespace 'cherry'
template <typename C, typename T>
decltype(auto) count(const C& container, const T& value);
}
I usually steer clear of macros, but in this case it could certainty save alot
of lines of code from being written. With regards to function overloading, every single function that i want to use i must overload, which wouldn't really scale. The upside to that approach though is that you retain the names of the functions. With perfect forwarding and decltype(auto) overloading becomes alot easier, but still will take time to implement, and would have to be modified if ever another function was added. As to whether or not i should overload the std namespace i am rather skeptical on whether or not it would be appropriate in this case.
What would be the most appropriate way of going about overloading functions in the STD namespace (note these functions will only serve as proxy's to the original functions)?
You need to read this: Why do all functions take only ranges, not containers?
And This: STL algorithms: Why no additional interface for containers (additional to iterator pairs)?
I have been writing in c++ for a few months, and i am comfortable
enough with it now to begin implementing my own library...
Let me look on the brighter side and just say... Some of us have been there before.... :-)
One thing that nagged me was the fact that you always had to provide a
beginning and end iterator for functions like
std::accumulate,std::fill etc...
That's why you have Boost.Ranges and the Eric's proposed ranges that seems like it isn't gonna make it to C++17.
Macros
See Macros
Wrapper Function/Functor
Not too bad...Provided you do it correctly, You can do that, that's what essentially Ranges do for Containers... See the aforementioned implementations
Overload Functions
Overload every function in namespace std ...
Don't do that... The C++ standard doesn't like it.
See what the standard has to say
$17.6.4.2.1 The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within
namespace std unless otherwise specified. A program may add a template
specialization for any standard library template to namespace std only
if the declaration depends on a user-defined type and the
specialization meets the standard library requirements for the
original template and is not explicitly prohibited.
In C++, should we be prepending stuff in the global namespace with ::?
For example, when using WinAPI, which is in C, should I do ::HANDLE instead of HANDLE, and ::LoadLibrary instead of LoadLibrary? What does C++ say about this? Is it generally a good idea, factoring in issues like readability and maintainability?
Names in C++ can be qualified and unqualified. There are different rules for qualified and unqualified name lookup. ::HANDLE is a qualified name, whereas HANDLE is an unqualified name. Consider the following example:
#include <windows.h>
int main()
{
int HANDLE;
HANDLE x; //ERROR HANDLE IS NOT A TYPE
::HANDLE y; //OK, qualified name lookup finds the global HANDLE
}
I think that the desicion of choosing HANDLE vs. ::HANDLE is a matter of coding style. Of course, as my example suggests, there might be situations where qualifying is mandatory. So, you might as well use :: just in case, unless the syntax is somewhat disgusting for you.
As namespaces don't exists in C, don't use ::HANDLE to access HANDLE type.
Using the prepending :: for global namespace is a good idea for readability, you know the type you want to access is from global namespace.
Moreover, if you are in a nested namespace and declare your own HANDLE type (for example), then the compiler will use this one instead of windows.h one!
Thus, always prefer using :: before names when working in nested namespace.
The main point of interest is what the differences are from the point of view of the compiler, as it has already been said, if you include the :: then you are using qualified lookup, rather than unqualified lookup.
The advantage of using qualified lookup is that it will be able to pinpoint a particular symbol always. The disadvantage is that it will always pinpoint that particular symbol --i.e. it will disable Argument Dependent Lookup. ADL is a big and useful part of the language, and by qualifying you effectively disable it, and that is bad.
Consider that you had a function f in the global namespace, and that you added a type T inside namespace N. Not consider that you wanted to add an overload of f that would take a T as argument. Following the interface principle, you can add f to the N namespace, as f is actually an operation performed on T, and it so belongs with the type. In this case, if you had code that called (consider generic code) ::f(obj) on an object of unknown type U the compiler will not be able to pick up ::N::f(obj) as a potential overload as the code is explicitly asking for an overload in the global namespace.
Using unqualified lookup gives you the freedom of defining the functions where they belong, together with the types that are used as arguments. While it is not exactly the same, consider the use of swap, if you qualify std::swap then it will not pick up your hand rolled void swap( T&, T& ) inside your N namespace...
I would only fully qualify identifiers when the compiler would otherwise not pick up the element I want.
It's largely a matter of style; there are no performance or efficiency concerns to speak of. It can be a good practice on large projects and projects intended to be compiled on many different platforms, as under these circumstances collisions between global names and names in a namespace are more likely to occur.
Normally, you do not have to prepend :: for the global namespace. (Only in some really rare circumstances). IMHO it harms readability, but, on the other hand it probably won't break your code
I put all of my code into a namespace, and I tend to prefer the C++ headers over the C headers, so the only symbols left in the global namespace tend to be from the Windows API. I avoid pulling symbols from other namespaces into the current namespace (e.g., I never have using namespace std;), preferring instead to qualify things explicitly. This is in line with Google's C++ style guide.
I've therefore gotten into the habit of qualifying WinAPI function calls with :: for a few reasons:
Consistency. For everything outside the current namespace, I refer to it explicitly (e.g., std::string), so why not refer to the Windows APIs explicitly (e.g., ::LoadLibraryW)? The Windows APIs namespace is the global namespace.
A lot of the WinAPI functions are named generically (e.g., DeleteObject). Unless you're very familiar with the code you're reading, you may not know whether DeleteObject is a call to something in the current namespace or to the Windows API. Thus, I find the :: clarifies.
A lot of Windows frameworks have methods with the same names as the raw calls. For example, ATL::CWindow has a GetClientRect method with a slightly different signature than WinAPI's GetClientRect. In this framework, it's common for your class to be derived from ATL::CWindow, so, in your class's implementation, it's normal to say GetClientRect to invoke the inherited ATL method and ::GetClientRect if you need to call the WinAPI function. It's not strictly necessary, since the compiler will find the right one based on the signature. Nevertheless, I find that the distinction clarifies for the reader.
(I know the question wasn't really about WinAPI, but the example was in terms of WinAPI.)
No, if you do not have a LoadLibrary method in your class you do not need to use the global scope. In fact, you should not use global scope because if you later on add a LoadLibrary to your class your intentions is probably to override the global function...
Most of the research I've done on the use of using declarations, including reading relevant sections of various style guides, indicates that whether or not to use using declarations in C++ source files, as long as they appear after all #includes, is a decision left to the coder. Even the style guides I read, which usually come down on one side or the other of such common disputes for the sake of consistency, are fairly flexible in this regard.
My question is, given this high degree of flexibility, how important is it to use a consistent style? For example, suppose an author wrote something like
using std::vector;
vector<T> v;
std::cout << v[0] << std::endl;
Is the inconsistent application of using on std::vector but not std::cout or std::endl generally considered acceptable, or would it be considered undisciplined?
I think the whole point of using is that you use it inconsistently among names. Names you need very frequently in some block can be declared locally with a using declaration, while others are not. I don't see a problem with that.
Declaring a name to have namespace scope is always much harder to take. I think if the name clearly is known to belong to a particular namespace so that confusing it with other namespaces won't occur, It won't hurt to put a using declaration if it makes your code more readable.
I am now a strong proponent for explicitly stating the namespace (ie no 'using')
Most peoples namespace history goes like this (in non trivial, >100kloc projects)
Innocence -> style 1
using namespace std;
Ouch -> style 2
using std::string;
using std::vector;
OK, enough already -> style 3
std::string foo = "xxx";
Assuming you don't say using namespace std; anywhere, I don't think most developers care one way or another in other people's code. The only thing that might bother them is the overuse of the std:: qualifier --- that is if you're saying "std::vector" 20 times in the function, maybe it's time for a "using std::vector". Otherwise, no one should care.
Sometimes, in my own code, I'll use the "std::" qualifier specifically to indicate that this is the only place that I'm using that identifer.
I try for not using using (no pun intended).
For saving typing, I like to do typedefs, e.g.:
typedef std::vector< int > IntVector;
typedef std::vector< Foo > FooVector;
This is less an answer than a counterpoint to a few other answers that have advocated always explicitly including the namespace as part of the name. At times, this is a poor idea. In some cases, you want to use a name that's been specialized for the type at hand if it exists, but use a standard-provided alternative otherwise.
As a typical example, let's consider a sort function. If you're sorting some objects of type T, you're going to end up swapping items. You want to use a special swap(T &, T&) if it exists, but template <class T> std::swap otherwise.
If you try to specify the full name of the swap you're going to use explicitly, you have to specify one or the other -- either you specify a specialized version, and instantiating your sort over a type that doesn't define it's own swap will fail, or else you specify std::swap, and ignore any swap that's been provided specifically for the type you're sorting.
using provides a way out of this dilemma though:
using namespace std;
template <class T>
mysort(/* ... */ ) {
// ...
if (less(x[a], x[b])
swap(x[a], x[b]);
// ...
}
Now, if the namespace in which T is found contains a swap(T &, T&), it'll be found via argument dependent lookup, and used above. If it doesn't exist, then std::swap will be found (and used) because the using namespace std; made it visible as well.
As an aside, I think with one minor modification, using namespace x; could be made almost entirely innocuous. As it stands right now, it introduces the names from that namespace into the current scope. If one of those happens to be the same as a name that exists in the current scope, we get a conflict. The problem, of course, is that we may not know everything that namespace contains, so there's almost always at least some potential for a conflict.
The modification would be to treat using namespace x; as if it created a scope surrounding the current scope, and introduced the names from that namespace into that surrounding scope. If one of those happened to be the same as a name introduced in the current scope, there would be no conflict though -- just like any other block scoping, the name in the current scope would hide the same name from the surrounding scope.
I haven't thought this through in a lot of detail, so there would undoubtedly be some corner cases that would require more care to solve, but I think the general idea would probably make a lot of things quite a bit simpler anyway.
I was just reading a little bit on them from http://www.cplusplus.com/doc/tutorial/namespaces/
and it seems like a struct is capable of the same things? Or even a class for that matter. Maybe someone here can better define what a namespace is, and how it differs from a struct/class?
Namespaces and class-types are not capable of the same things. Namespaces are mainly used to group types and functions together to avoid name collisions, while class-types hold data and operations that work on that data.
To just group functions and objects by using a class-types you'd have to make them static:
struct X {
static void f();
};
Without static you'd have to create instances of the class-types to use them. A namespace is much better suited here:
namespace X {
void f();
}
Another important thing are using declarations and directives:
namespace X {
void f();
void g();
}
void h() {
using X::f;
f(); // f() now visible in current scope
using namespace X;
f(); g(); // both visible
}
With class-types there simply is no mechanism that allows that.
What class-types give you over namespaces is that you can have multiple instances with differing state - if you need that use a class-type.
Well, seems everyone's going at it, so I'll add my own arguments.
First things first, namespace and struct are completely different beasts: they have different syntax and different semantics.
The obvious:
a struct introduces a type, you can use as templates argument
a namespace can be spread in several files
Syntactically:
both can be "aliased", namespace with namespace ns = mylong::name::space; and struct with typedef mylong::name::Space lilstruct;
ADL (or Argument Dependent Lookup) is tailored for namespaces
Semantically:
a namespace only defines a scope for the definition of symbols, which allows to group together objects that work together (classes and free-functions) while isolating them for the rest of the world (name clashes). As such it often represents a logical unit of work within the project (for small projects, there is a single namespace).
a struct or class defines a logical binding between data and the methods to act upon it, which is the corner stone of encapsulation. It usually has one clear responsability and a number of invariants.
Note that sometimes a struct or class is just used to bind together objects that work together without having any logic, for example struct Person { std::string name, firstName; };.
That being said: there is no point in C++ for a struct of static methods. It's just a perversion from Java or C# and their "pure" OO approach. C++ supports free functions, so there is no point not using them, especially since it's better for encapsulation (they don't have access to private/protected parts, so you can't mess up an invariant and they don't depend on the representation of the class either).
If it can be done with a namespace, use a namespace.
A struct does much more than defining a scope. It defines a type.
If you don't want people to use the "using" feature of C++ with your class, which can be dangerous and is often ill advised in complex code, then go ahead and use struct with statics.
In other words: if your functions should always be referred to with "group::function", then you can box in your users by declaring as a struct.
In addition, and importantly, you can forward-declare structs in older versions of C++. You cannot do this with namespaces until C++ 11.
Consider:
std::string out zip::pack(const std::string &in)
std::string out zip::unpack(const std::string &in)
In this case, requiring users to specify zip:: makes sense. It's a short, specific and informative. And the names of the underlying functions are ambiguous without it. Use a struct with statics.
Consider:
std::string out CorpDataUtils::zipPack(const std::string &in)
std::string out CorpDataUtils::zipUnpack(const std::string &in)
These should certainly be in a namespace. The namespace name is long, and uninformative, probably more to do with the organization of whoever is maintaining it - which is fine... but really it should be a namespace... not a struct.
In C++ a struct is exactly the same as a class, except structs are public by default. Classes are private. So whenever you want to group free functions together use a namespace. When you want to group data and functions, use a struct/class, and optionally a namespace around it all.
Notice that If you put your functions in a struct then you would have to have an instance of your struct when you want to call those functions, unless they are static.
When creating your own library, it's normally good practice to namespace all your exported functions and classes.
That way, if someone includes your library, they won't be polluting their namespace, and there is less likelyhood of name clashes.
This is a counter example where using a struct instead of a namespace gives some unexpected benefits.
I wanted to generalise a solution to a 2D problem to K dimensions. The 2D solution was enclosed in a namespace.
Templates to the rescue. I started changing the implementation:
struct Point;
to
template< size_t DIMS >
struct Point;
I needed to template most classes, structs, and functions. That was tedious, repetitive, and error prone. And then I had this mischevios idea. I changed
namespace KDimSpace {
to
template< size_t DIMS >
struct KDimSpace {
and that was pretty much it. I could rip off all template< size_t DIMS > junk inside. This is so much easier - the number of dimensions DIMS is declared only once and used consistently by all types and functions.
And then, there is one more thing - rather than hiding internals of the implementation behind ::detail (sub)namespace there is public: and private:!
There are two annoyances:
functions have to be marked as static
it is not possible to define operators (e.g. operator<< for std::ostream) because operators cannot be marked as static (and then ADL might get in the way as well).
Bottom line - C++ could be a better language with fewer primitives not more. I would like namespaces to be as close to structs as classes are.
Something that has been troubling me for a while:
The current wisdom is that types should be kept in a namespace that only
contains functions which are part of the type's non-member interface (see C++ Coding Standards Sutter and Alexandrescu or here) to prevent ADL pulling in unrelated definitions.
Does this imply that all classes must have a namespace of their own? If
we assume that a class may be augmented in the future by the addition of
non-member functions, then it can never be safe to put two types in the
same namespace as either one of them may introduce non-member functions
that could interfere with the other.
The reason I ask is that namespaces are becoming cumbersome for me. I'm
writing a header-only library and I find myself using classes names such as
project::component::class_name::class_name. Their implementations call
helper functions but as these can't be in the same namespace they also have
to be fully qualified!
Edit:
Several answers have suggested that C++ namespaces are simply a mechanism for avoiding name clashes. This is not so. In C++ functions that take a parameter are resolved using Argument Dependent Lookup. This means that when the compiler tries to find a function definition that matches the function name it will look at every function in the same namespace(s) as the type(s) of its parameter(s) when finding candidates.
This can have unintended, unpleasant consequences as detailed in A Modest Proposal: Fixing ADL. Sutter and Alexandrescu's rule states never put a function in the same namespace as a class unless it is meant to be part of the interface of that class. I don't see how I can obey that rule unless I'm prepared to give every class its own namespace.
More suggestions very welcome!
No. I have never heard that convention. Usually each library has its own namespace, and if that library has multiple different modules (e.g. different logical units that differ in functionality), then those might have their own namespace, although one namespace per library is sufficient. Within the library or module namespace, you might use namespace detail or an anonymous namespace to store implementation details. Using one namespace per class is, IMHO, complete overkill. I would definitely shy away from that. At the same time, I would strongly to urge you to have at least one namespace for your library and put everything within that one namespace or a sub-namespace thereof to avoid name clashes with other libraries.
To make this more concrete, allow me to use the venerable Boost C++ Libraries as an example. All of the elements within boost reside in boost::. There are some modules within Boost, such as the interprocess library that have its own namespace such as boost::interprocess::, but for the most part, elements of boost (especially those used very frequently and across modules) simply reside in boost::. If you look within boost, it frequently uses boost::detail or boost::name_of_module::detail for storing implementation details for the given namespace. I suggest you model your namespaces in that way.
No, no and a thousand times no! Namespaces in C++ are not architectural or design elements. They are simply a mechanism for preventing name clashes. If in practice you don't have name clashes, you don't need namespaces.
To avoid ADL, you need only two namespaces: one with all your classes, and the other with all your loose functions. ADL is definitely not a good reason for every class to have its own namespace.
Now, if you want some functions to be found via ADL, you might want to make a namespace for that purpose. But it's still quite unlikely that you'd actually need a separate namespace per class to avoid ADL collisions.
Probably not. See Eric Lippert's post on the subject.
Couple things here:
Eric Lippert is a C# designer, but what he's saying about bad hierarchical design applies here.
A lot of what is being described in that article has to do with naming your class the same thing as a namespace in C#, but many of the same pitfalls apply to C++.
You can save on some of the typedef pain by using typedefs but that's of course only a band-aid.
It's quite an interesting paper, but then given the authors there was a good chance it would be. However, I note that the problem concerns mostly:
typedef, because they only introduce an alias and not a new type
templates
If I do:
namespace foo
{
class Bar;
void copy(const Bar&, Bar&, std::string);
}
And invoke it:
#include <algorithms>
#include "foo/bar.h"
int main(int argc, char* argv[])
{
Bar source; Bar dest;
std::string parameter;
copy(source, dest, parameter);
}
Then it should pick foo::copy. In fact it will consider both foo::copy and std::copy but foo::copy not being template will be given priority.