I have heard using namespace std; is bad practice, and that I should use std::cout and std::cin directly instead.
Why is this? Does it risk declaring variables that share the same name as something in the std namespace?
Consider two libraries called Foo and Bar:
using namespace foo;
using namespace bar;
Everything works fine, and you can call Blah() from Foo and Quux() from Bar without problems. But one day you upgrade to a new version of Foo 2.0, which now offers a function called Quux(). Now you've got a conflict: Both Foo 2.0 and Bar import Quux() into your global namespace. This is going to take some effort to fix, especially if the function parameters happen to match.
If you had used foo::Blah() and bar::Quux(), then the introduction of foo::Quux() would have been a non-event.
It can get worse than what Greg wrote!
Library Foo 2.0 could introduce a function, Quux(), that is an unambiguously better match for some of your calls to Quux() than the bar::Quux() your code called for years. Then your code still compiles, but it silently calls the wrong function and does god-knows-what. That's about as bad as things can get.
Keep in mind that the std namespace has tons of identifiers, many of which are very common ones (think list, sort, string, iterator, etc.) which are very likely to appear in other code, too.
If you consider this unlikely: There was a question asked here on Stack Overflow where pretty much exactly this happened (wrong function called due to omitted std:: prefix) about half a year after I gave this answer. Here is another, more recent example of such a question.
So this is a real problem.
Here's one more data point: Many, many years ago, I also used to find it annoying having to prefix everything from the standard library with std::. Then I worked in a project where it was decided at the start that both using directives and declarations are banned except for function scopes. Guess what? It took most of us very few weeks to get used to writing the prefix, and after a few more weeks most of us even agreed that it actually made the code more readable. There's a reason for that: Whether you like shorter or longer prose is subjective, but the prefixes objectively add clarity to the code. Not only the compiler, but you, too, find it easier to see which identifier is referred to.
In a decade, that project grew to have several million lines of code. Since these discussions come up again and again, I once was curious how often the (allowed) function-scope using actually was used in the project. I grep'd the sources for it and only found one or two dozen places where it was used. To me this indicates that, once tried, developers don't find std:: painful enough to employ using directives even once every 100 kLoC even where it was allowed to be used.
Bottom line: Explicitly prefixing everything doesn't do any harm, takes very little getting used to, and has objective advantages. In particular, it makes the code easier to interpret by the compiler and by human readers — and that should probably be the main goal when writing code.
The problem with putting using namespace in the header files of your classes is that it forces anyone who wants to use your classes (by including your header files) to also be 'using' (i.e. seeing everything in) those other namespaces.
However, you may feel free to put a using statement in your (private) *.cpp files.
Beware that some people disagree with my saying "feel free" like this -- because although a using statement in a cpp file is better than in a header (because it doesn't affect people who include your header file), they think it's still not good (because depending on the code it could make the implementation of the class more difficult to maintain). This C++ Super-FAQ entry says,
The using-directive exists for legacy C++ code and to ease the transition to namespaces, but you probably shouldn’t use it on a regular basis, at least not in your new C++ code.
The FAQ suggests two alternatives:
A using-declaration:
using std::cout; // a using-declaration lets you use cout without qualification
cout << "Values:";
Just typing std::
std::cout << "Values:";
I recently ran into a complaint about Visual Studio 2010. It turned out that pretty much all the source files had these two lines:
using namespace std;
using namespace boost;
A lot of Boost features are going into the C++0x standard, and Visual Studio 2010 has a lot of C++0x features, so suddenly these programs were not compiling.
Therefore, avoiding using namespace X; is a form of future-proofing, a way of making sure a change to the libraries and/or header files in use is not going to break a program.
Short version: don't use global using declarations or directives in header files. Feel free to use them in implementation files. Here's what Herb Sutter and Andrei Alexandrescu have to say about this issue in C++ Coding Standards (bolding for emphasis is mine):
Summary
Namespace usings are for your convenience, not for you to inflict on others: Never write a using declaration or a using directive before an #include directive.
Corollary: In header files, don’t write namespace-level using directives or using declarations; instead, explicitly namespace-qualify all names. (The second rule follows from the first, because headers can never know what other header #includes might appear after them.)
Discussion
In short: You can and should use namespace using declarations and directives liberally in your implementation files after #include directives and feel good about it. Despite repeated assertions to the contrary, namespace using declarations and directives are not evil and they do not defeat the purpose of namespaces. Rather, they are what make namespaces usable.
One shouldn't use the using directive at the global scope, especially in headers. However, there are situations where it is appropriate even in a header file:
template <typename FloatType> inline
FloatType compute_something(FloatType x)
{
using namespace std; // No problem since scope is limited
return exp(x) * (sin(x) - cos(x * 2) + sin(x * 3) - cos(x * 4));
}
This is better than explicit qualification (std::sin, std::cos...), because it is shorter and has the ability to work with user defined floating point types (via argument-dependent lookup (ADL)).
Do not use it globally
It is considered "bad" only when used globally. Because:
You clutter the namespace you are programming in.
Readers will have difficulty seeing where a particular identifier comes from, when you use many using namespace xyz;.
Whatever is true for other readers of your source code is even more true for the most frequent reader of it: yourself. Come back in a year or two and take a look...
If you only talk about using namespace std; you might not be aware of all the stuff you grab -- and when you add another #include or move to a new C++ revision you might get name conflicts you were not aware of.
You may use it locally
Go ahead and use it locally (almost) freely. This, of course, prevents you from repetition of std:: -- and repetition is also bad.
An idiom for using it locally
In C++03 there was an idiom -- boilerplate code -- for implementing a swap function for your classes. It was suggested that you actually use a local using namespace std; -- or at least using std::swap;:
class Thing {
int value_;
Child child_;
public:
// ...
friend void swap(Thing &a, Thing &b);
};
void swap(Thing &a, Thing &b) {
using namespace std; // make `std::swap` available
// swap all members
swap(a.value_, b.value_); // `std::stwap(int, int)`
swap(a.child_, b.child_); // `swap(Child&,Child&)` or `std::swap(...)`
}
This does the following magic:
The compiler will choose the std::swap for value_, i.e. void std::swap(int, int).
If you have an overload void swap(Child&, Child&) implemented the compiler will choose it.
If you do not have that overload the compiler will use void std::swap(Child&,Child&) and try its best swapping these.
With C++11 there is no reason to use this pattern any more. The implementation of std::swap was changed to find a potential overload and choose it.
If you import the right header files you suddenly have names like hex, left, plus or count in your global scope. This might be surprising if you are not aware that std:: contains these names. If you also try to use these names locally it can lead to quite some confusion.
If all the standard stuff is in its own namespace you don't have to worry about name collisions with your code or other libraries.
Another reason is surprise.
If I see cout << blah, instead of std::cout << blah I think: What is this cout? Is it the normal cout? Is it something special?
Experienced programmers use whatever solves their problems and avoid whatever creates new problems, and they avoid header-file-level using-directives for this exact reason.
Experienced programmers also try to avoid full qualification of names inside their source files. A minor reason for this is that it's not elegant to write more code when less code is sufficient unless there are good reasons. A major reason for this is turning off argument-dependent lookup (ADL).
What are these good reasons? Sometimes programmers explicitly want to turn off ADL, other times they want to disambiguate.
So the following are OK:
Function-level using-directives and using-declarations inside functions' implementations
Source-file-level using-declarations inside source files
(Sometimes) source-file-level using-directives
I agree that it should not be used globally, but it's not so evil to use locally, like in a namespace. Here's an example from "The C++ Programming Language":
namespace My_lib {
using namespace His_lib; // Everything from His_lib
using namespace Her_lib; // Everything from Her_lib
using His_lib::String; // Resolve potential clash in favor of His_lib
using Her_lib::Vector; // Resolve potential clash in favor of Her_lib
}
In this example, we resolved potential name clashes and ambiguities arising from their composition.
Names explicitly declared there (including names declared by using-declarations like His_lib::String) take priority over names made accessible in another scope by a using-directive (using namespace Her_lib).
I also consider it a bad practice. Why? Just one day I thought that the function of a namespace is to divide stuff, so I shouldn't spoil it with throwing everything into one global bag.
However, if I often use 'cout' and 'cin', I write: using std::cout; using std::cin; in the .cpp file (never in the header file as it propagates with #include). I think that no one sane will ever name a stream cout or cin. ;)
It's nice to see code and know what it does. If I see std::cout I know that's the cout stream of the std library. If I see cout then I don't know. It could be the cout stream of the std library. Or there could be an int cout = 0; ten lines higher in the same function. Or a static variable named cout in that file. It could be anything.
Now take a million line code base, which isn't particularly big, and you're searching for a bug, which means you know there is one line in this one million lines that doesn't do what it is supposed to do. cout << 1; could read a static int named cout, shift it to the left by one bit, and throw away the result. Looking for a bug, I'd have to check that. Can you see how I really really prefer to see std::cout?
It's one of these things that seem a really good idea if you are a teacher and never had to write and maintain any code for a living. I love seeing code where (1) I know what it does; and, (2) I'm confident that the person writing it knew what it does.
It's all about managing complexity. Using the namespace will pull things in that you don't want, and thus possibly make it harder to debug (I say possibly). Using std:: all over the place is harder to read (more text and all that).
Horses for courses - manage your complexity how you best can and feel able.
A concrete example to clarify the concern. Imagine you have a situation where you have two libraries, foo and bar, each with their own namespace:
namespace foo {
void a(float) { /* Does something */ }
}
namespace bar {
...
}
Now let's say you use foo and bar together in your own program as follows:
using namespace foo;
using namespace bar;
void main() {
a(42);
}
At this point everything is fine. When you run your program it 'Does something'. But later you update bar and let's say it has changed to be like:
namespace bar {
void a(float) { /* Does something completely different */ }
}
At this point you'll get a compiler error:
using namespace foo;
using namespace bar;
void main() {
a(42); // error: call to 'a' is ambiguous, should be foo::a(42)
}
So you'll need to do some maintenance to clarify that 'a' meant foo::a. That's undesirable, but fortunately it is pretty easy (just add foo:: in front of all calls to a that the compiler marks as ambiguous).
But imagine an alternative scenario where bar changed instead to look like this instead:
namespace bar {
void a(int) { /* Does something completely different */ }
}
At this point your call to a(42) suddenly binds to bar::a instead of foo::a and instead of doing 'something' it does 'something completely different'. No compiler warning or anything. Your program just silently starts doing something completely different than before.
When you use a namespace you're risking a scenario like this, which is why people are uncomfortable using namespaces. The more things in a namespace, the greater the risk of conflict, so people might be even more uncomfortable using namespace std (due to the number of things in that namespace) than other namespaces.
Ultimately this is a trade-off between writability vs. reliability/maintainability. Readability may factor in also, but I could see arguments for that going either way. Normally I would say reliability and maintainability are more important, but in this case you'll constantly pay the writability cost for an fairly rare reliability/maintainability impact. The 'best' trade-off will determine on your project and your priorities.
Consider
// myHeader.h
#include <sstream>
using namespace std;
// someoneElses.cpp/h
#include "myHeader.h"
class stringstream { // Uh oh
};
Note that this is a simple example. If you have files with 20 includes and other imports, you'll have a ton of dependencies to go through to figure out the problem. The worse thing about it is that you can get unrelated errors in other modules depending on the definitions that conflict.
It's not horrible, but you'll save yourself headaches by not using it in header files or the global namespace. It's probably all right to do it in very limited scopes, but I've never had a problem typing the extra five characters to clarify where my functions are coming from.
You need to be able to read code written by people who have different style and best practices opinions than you.
If you're only using cout, nobody gets confused. But when you have lots of namespaces flying around and you see this class and you aren't exactly sure what it does, having the namespace explicit acts as a comment of sorts. You can see at first glance, "oh, this is a filesystem operation" or "that's doing network stuff".
Using many namespaces at the same time is obviously a recipe for disaster, but using JUST namespace std and only namespace std is not that big of a deal in my opinion because redefinition can only occur by your own code...
So just consider them functions as reserved names like "int" or "class" and that is it.
People should stop being so anal about it. Your teacher was right all along. Just use ONE namespace; that is the whole point of using namespaces the first place. You are not supposed to use more than one at the same time. Unless it is your own. So again, redefinition will not happen.
I agree with the others here, but I would like to address the concerns regarding readability - you can avoid all of that by simply using typedefs at the top of your file, function or class declaration.
I usually use it in my class declaration as methods in a class tend to deal with similar data types (the members) and a typedef is an opportunity to assign a name that is meaningful in the context of the class. This actually aids readability in the definitions of the class methods.
// Header
class File
{
typedef std::vector<std::string> Lines;
Lines ReadLines();
}
and in the implementation:
// .cpp
Lines File::ReadLines()
{
Lines lines;
// Get them...
return lines;
}
as opposed to:
// .cpp
vector<string> File::ReadLines()
{
vector<string> lines;
// Get them...
return lines;
}
or:
// .cpp
std::vector<std::string> File::ReadLines()
{
std::vector<std::string> lines;
// Get them...
return lines;
}
A namespace is a named scope. Namespaces are used to group related declarations and to keep separate
items separate. For example, two separately developed libraries may use the same name to refer to different
items, but a user can still use both:
namespace Mylib{
template<class T> class Stack{ /* ... */ };
// ...
}
namespace Yourlib{
class Stack{ /* ... */ };
// ...
}
void f(int max) {
Mylib::Stack<int> s1(max); // Use my stack
Yourlib::Stack s2(max); // Use your stack
// ...
}
Repeating a namespace name can be a distraction for both readers and writers. Consequently, it is possible
to state that names from a particular namespace are available without explicit qualification. For example:
void f(int max) {
using namespace Mylib; // Make names from Mylib accessible
Stack<int> s1(max); // Use my stack
Yourlib::Stack s2(max); // Use your stack
// ...
}
Namespaces provide a powerful tool for the management of different libraries and of different versions of code. In particular, they offer the programmer alternatives of how explicit to make a reference to a nonlocal name.
Source: An Overview of the C++ Programming Language
by Bjarne Stroustrup
An example where using namespace std throws a compilation error because of the ambiguity of count, which is also a function in algorithm library.
#include <iostream>
#include <algorithm>
using namespace std;
int count = 1;
int main() {
cout << count << endl;
}
It doesn't make your software or project performance worse. The inclusion of the namespace at the beginning of your source code isn't bad. The inclusion of the using namespace std instruction varies according to your needs and the way you are developing the software or project.
The namespace std contains the C++ standard functions and variables. This namespace is useful when you often would use the C++ standard functions.
As is mentioned in this page:
The statement using namespace std is generally considered bad
practice. The alternative to this statement is to specify the
namespace to which the identifier belongs using the scope operator(::)
each time we declare a type.
And see this opinion:
There is no problem using "using namespace std" in your source file
when you make heavy use of the namespace and know for sure that
nothing will collide.
Some people had said that is a bad practice to include the using namespace std in your source files because you're invoking from that namespace all the functions and variables. When you would like to define a new function with the same name as another function contained in the namespace std you would overload the function and it could produce problems due to compile or execute. It will not compile or executing as you expect.
As is mentioned in this page:
Although the statement saves us from typing std:: whenever
we wish to access a class or type defined in the std namespace, it
imports the entirety of the std namespace into the current namespace
of the program. Let us take a few examples to understand why this
might not be such a good thing
...
Now at a later stage of development, we wish to use another version of
cout that is custom implemented in some library called “foo” (for
example)
...
Notice how there is an ambiguity, to which library does cout point to?
The compiler may detect this and not compile the program. In the worst
case, the program may still compile but call the wrong function, since
we never specified to which namespace the identifier belonged.
It's case by case. We want to minimize the "total cost of ownership" of the software over its lifespan. Stating "using namespace std" has some costs, but not using it also has a cost in legibility.
People correctly point out that when using it, when the standard library introduces new symbols and definitions, your code ceases to compile, and you may be forced to rename variables. And yet this is probably good long-term, since future maintainers will be momentarily confused or distracted if you're using a keyword for some surprising purpose.
You don't want to have a template called vector, say, which isn't the vector known by everyone else. And the number of new definitions thus introduced in the C++ library is small enough it may simply not come up. There is a cost to having to do this kind of change, but the cost is not high and is offset by the clarity gained by not using std symbol names for other purposes.
Given the number of classes, variables, and functions, stating std:: on every one might fluff up your code by 50% and make it harder to get your head around. An algorithm or step in a method that could be taken in on one screenful of code now requires scrolling back and forth to follow. This is a real cost. Arguably it may not be a high cost, but people who deny it even exists are inexperienced, dogmatic, or simply wrong.
I'd offer the following rules:
std is different from all other libraries. It is the one library everyone basically needs to know, and in my view is best thought of as part of the language. Generally speaking there is an excellent case for using namespace std even if there isn't for other libraries.
Never force the decision onto the author of a compilation unit (a .cpp file) by putting this using in a header. Always defer the decision to the compilation unit author. Even in a project that has decided to use using namespace std everywhere may fine a few modules that are best handled as exceptions to that rule.
Even though the namespace feature lets you have many modules with symbols defined the same, it's going to be confusing to do so. Keep the names different to the extent possible. Even if not using the namespace feature, if you have a class named foo and std introduces a class named foo, it's probably better long-run to rename your class anyway.
An alternative to using namespaces is to manually namespace symbols by prefixing them. I have two libraries I've used for decades, both starting as C libraries, actually, where every symbol is prefixed with "AK" or "SCWin". Generally speaking, this is like avoiding the "using" construct, but you don't write the twin colons. AK::foo() is instead AKFoo(). It makes code 5-10% denser and less verbose, and the only downside is that you'll be in big trouble if you have to use two such libraries that have the same prefixing. Note the X Window libraries are excellent in this regard, except they forgot to do so with a few #defines: TRUE and FALSE should have been XTRUE and XFALSE, and this set up a namespace clash with Sybase or Oracle that likewise used TRUE and FALSE with different values! (ASCII 0 and 1 in the case of the database!) One special advantage of this is that it applies seemlessly to preprocessor definitions, whereas the C++ using/namespace system doesn't handle them. A nice benefit of this is that it gives an organic slope from being part of a project to eventually being a library. In a large application of mine, all window classes are prefixed Win, all signal-processing modules Mod, and so on. There's little chance of any of these being reused so there's no practical benefit to making each group into a library, but it makes obvious in a few seconds how the project breaks into sub-projects.
I agree with others – it is asking for name clashes, ambiguities and then the fact is it is less explicit. While I can see the use of using, my personal preference is to limit it. I would also strongly consider what some others pointed out:
If you want to find a function name that might be a fairly common name, but you only want to find it in the std namespace (or the reverse – you want to change all calls that are not in namespace std, namespace X, ...), then how do you propose to do this?
You could write a program to do it, but wouldn't it be better to spend time working on your project itself rather than writing a program to maintain your project?
Personally, I actually don't mind the std:: prefix. I like the look more than not having it. I don't know if that is because it is explicit and says to me "this isn't my code... I am using the standard library" or if it is something else, but I think it looks nicer. This might be odd given that I only recently got into C++ (used and still do C and other languages for much longer and C is my favourite language of all time, right above assembly).
There is one other thing although it is somewhat related to the above and what others point out. While this might be bad practise, I sometimes reserve std::name for the standard library version and name for program-specific implementation. Yes, indeed this could bite you and bite you hard, but it all comes down to that I started this project from scratch, and I'm the only programmer for it. Example: I overload std::string and call it string. I have helpful additions. I did it in part because of my C and Unix (+ Linux) tendency towards lower-case names.
Besides that, you can have namespace aliases. Here is an example of where it is useful that might not have been referred to. I use the C++11 standard and specifically with libstdc++. Well, it doesn't have complete std::regex support. Sure, it compiles, but it throws an exception along the lines of it being an error on the programmer's end. But it is lack of implementation.
So here's how I solved it. Install Boost's regex, and link it in. Then, I do the following so that when libstdc++ has it implemented entirely, I need only remove this block and the code remains the same:
namespace std
{
using boost::regex;
using boost::regex_error;
using boost::regex_replace;
using boost::regex_search;
using boost::regex_match;
using boost::smatch;
namespace regex_constants = boost::regex_constants;
}
I won't argue on whether that is a bad idea or not. I will however argue that it keeps it clean for my project and at the same time makes it specific: True, I have to use Boost, but I'm using it like the libstdc++ will eventually have it. Yes, starting your own project and starting with a standard (...) at the very beginning goes a very long way with helping maintenance, development and everything involved with the project!
Just to clarify something: I don't actually think it is a good idea to use a name of a class/whatever in the STL deliberately and more specifically in place of. The string is the exception (ignore the first, above, or second here, pun if you must) for me as I didn't like the idea of 'String'.
As it is, I am still very biased towards C and biased against C++. Sparing details, much of what I work on fits C more (but it was a good exercise and a good way to make myself a. learn another language and b. try not be less biased against object/classes/etc which is maybe better stated as less closed-minded, less arrogant, and more accepting.). But what is useful is what some already suggested: I do indeed use list (it is fairly generic, is it not ?), and sort (same thing) to name two that would cause a name clash if I were to do using namespace std;, and so to that end I prefer being specific, in control and knowing that if I intend it to be the standard use then I will have to specify it. Put simply: no assuming allowed.
And as for making Boost's regex part of std. I do that for future integration and – again, I admit fully this is bias - I don't think it is as ugly as boost::regex:: .... Indeed, that is another thing for me. There are many things in C++ that I still have yet to come to fully accept in looks and methods (another example: variadic templates versus var arguments [though I admit variadic templates are very very useful!]). Even those that I do accept it was difficult, and I still have issues with them.
From my experiences, if you have multiple libraries that uses say, cout, but for a different purpose you may use the wrong cout.
For example, if I type in, using namespace std; and using namespace otherlib; and type just cout (which happens to be in both), rather than std::cout (or 'otherlib::cout'), you might use the wrong one, and get errors. It's much more effective and efficient to use std::cout.
I do not think it is necessarily bad practice under all conditions, but you need to be careful when you use it. If you're writing a library, you probably should use the scope resolution operators with the namespace to keep your library from butting heads with other libraries. For application level code, I don't see anything wrong with it.
With unqualified imported identifiers you need external search tools like grep to find out where identifiers are declared. This makes reasoning about program correctness harder.
This is a bad practice, often known as global namespace pollution. Problems may occur when more than one namespace has the same function name with signature, then it will be ambiguous for the compiler to decide which one to call and this all can be avoided when you are specifying the namespace with your function call like std::cout . Hope this helps. :)
"Why is 'using namespace std;' considered a bad practice in C++?"
I put it the other way around: Why is typing five extra characters considered cumbersome by some?
Consider e.g. writing a piece of numerical software. Why would I even consider polluting my global namespace by cutting general "std::vector" down to "vector" when "vector" is one of the problem domain's most important concepts?
To answer your question I look at it this way practically: a lot of programmers (not all) invoke namespace std. Therefore one should be in the habit of NOT using things that impinge or use the same names as what is in the namespace std. That is a great deal granted, but not so much compared to the number of possible coherent words and pseudonyms that can be come up with strictly speaking.
I mean really... saying "don't rely on this being present" is just setting you up to rely on it NOT being present. You are constantly going to have issues borrowing code snippets and constantly repairing them. Just keep your user-defined and borrowed stuff in limited scope as they should be and be VERY sparing with globals (honestly globals should almost always be a last resort for purposes of "compile now, sanity later"). Truly I think it is bad advice from your teacher because using std will work for both "cout" and "std::cout" but NOT using std will only work for "std::cout". You will not always be fortunate enough to write all your own code.
NOTE: Don't focus too much on efficiency issues until you actually learn a little about how compilers work. With a little experience coding you don't have to learn that much about them before you realize how much they are able to generalize good code into something something simple. Every bit as simple as if you wrote the whole thing in C. Good code is only as complex as it needs to be.
Every once in a while, I stumble across some code that I'm maintaining that challenges the way I think about my code style. Today was one of those days...
I'm aware that about why you would want to use the scope operator to define global scope. In fact, here scope resolution operator without a scope is a great link tell you why.
However, I saw something that made me think today. All the classes in question were wrapped into a namespace for the project (good!), but I did see copious usage of the global scope operator. Namely, it was used for everything from C libraries (with the exception of uint8_t and the like... yes, the programmer used the .h version of this library since apparently the version of g++ they were running still threw warnings about the new C++ standard). Is this useful? I view this as just as a waste of characters (reminds me of using the this pointer... except in the case of the copy constructor and assignment operator where it helps in clarifying which object is which). Am I missing something? Sure, someone can come around and name something along the lines of usleep() or stderr (where I saw the most usage of "::"), but won't they know that doing so will probably break something horribly? At what point do you say "screw it" in terms of the scope operator and just tell yourself that someone who names a function a certain way within your namespace is asking for trouble?
So my question is... what is the "correct" (subjective I understand) way of using the global scope operator in this context? Should everything not included in std or your own namespaces have the global scope explicitly defined? I tend to err on the side of caution and use "std::" to avoid the using directive, but is anything gained from using the global scope operator here? I tend to think for clarity's sake it does lend to the fact that we aren't getting the variable or function in question from the current namespace, but I'm torn between including it and not given today's developments.
As always, thanks for the help and guidance as I look to make my code cleaner, more readable, and (of course) significantly more awesome.
I use it quite infrequently; only when some ambiguity needs resolving for whatever reason. This is pretty subjective, though.
There may be some occasions (say, inside a template) where you're worried about ADL causing ambiguities only in certain cases:
template <typename T>
void foo(T t)
{
::bar(t); // :: just in case there's a `bar` in `T`'s namespace
}
There is almost no correct answer to this as it's almost totally style related, with the exception that if you think you may want to change from where you are going to import some declaration(s)/definition(s), in which case, when you use them, you don't specify any scope and import them using the using directive (to import a subset from the namespace) or the using namespace directive to import the entire set from the namespace.
Mostly the using directive is used as a convenience directive, but it is a powerful way to direct which declarations/definitions are used. My preference is to not specify the scope and import the declarations. Doing this allows for easy changes if ever they are needed while reducing visual noise. Also, specifying the scope would mean I'd be "locked in" from where I am getting the declarations (well, I'd have to do a global search and replace to change it).
If ever there is a conflict (you try an use a declared item with the same name that has been imported from more than one namespace) the compiler will let you know, so there's no real danger.
Readable code is that has the least amount of noise. namespace prefixes normally provide nothing but noise. So baseline is to not have them at all.
Namespaces were introduced to C++ mainly to handle 3rd party stuff out of one's control. To allow libraries drop prefixing, while clients can use terse names by applying using.
Just because you can have the same name in many namespaces does not imply it is a good idea to too. If a project uses some environment, platform API, library set, whatever that puts name in global, those names are better be avoided for other purposes. As with or without prefixes they will bear mental overhead.
Use of :: is rare in well-shaped code, and frequent uses appear in wrapper classes for that same functionality.
Consider the following cases.
Public library.
You are writing an exportable library with public headers. And you absolutely have no clue in what environment your headers will be included. For example, someone may do:
namespace std = my_space::std;
#include "your_header"
And all your definitions will be corrupted, if you simply use: std::list<int>, etc. So, it's a good practice to prepend :: to everything global. This way you can be absolutely sure what you're using. Of course, you can do using (in C++11) or typedef - but it's a wrong way to go in headers.
Collaborative .cc/.cpp files.
Inside your own code that is not exposed to public in any way, but still editable not only by you - it's a good practice to announce, what you're going to use from outside of your namespace. Say, your project allows to use a number of vectors - not only an std::vector. Then, where it's appropriate, you put a using directive:
// some includes
using vector = ::std::vector<int>; // C++11
typedef ::std::vector<int> vector; // C++03
namespace my_namespace {
...
} // namespace my_namespace
It may be put after all includes, or inside specific functions. It's not only gives control over the code, but also makes it readable and shortens expressions.
Another common case - is a mixing of global C functions and a C++ code. It's not a good idea to do any typedefs with function names. In this situation you should prepend C functions with the global scope resolution operator :: - to avoid compilation problems, when someone implements a function with a same signature in the same namespace.
Don't use :: for relative types and namespaces - or you'll lose the benefit of using namespaces at all.
Your own code.
Do what you want and how you want. There is only one good way - the way you fill comfortable with your own code. Really, don't bother about global scope resolution in this situation, if it's not requiered to resolve an ambiguity.
I'm wondering a bit about the namespace and using in C++ basically I would like to know the differences and figure out how to use it in the best way.
As I see it there are (at least) three ways to resolve a class name and I am not sure how to choose among them:
using namespace <namespace>
using <namespace>::<what_to_use>
<namespace>::<what_to_use> <use_it>
I would like to know the advantages especially if there are performance involved in one or the other way, if it's just syntactical and a matter of preference or if there are other things I haven't considered regarding this.
First is an using namespace directive, it brings all the symbol names from the specified namespace in your current namespace, irrespective of whether you need/use them. Certainly undesirable.
Second is using namespace declaration. It only brings the specified symbol name in your current namespace. Advantage is you don't have to type the fully qualified name everytime.
Third is an fully qualified names of the symbol. Disadvantage is that you have to type the fully qualified name everywhere you use the symbol.
Clearly, Second & Third are the more suitable ones. There is no performance difference in either of them. The only difference is the amount of characters you type in. Simply, choose either depending on what your coding standard specifies.
EDIT:
As #Jerry points out, using declaration in combination with ADL(Argument dependent lookup) can lead to undesirable effects.
You can find a detailed explanation in one of my answers:
Detailed explanation on how Koenig lookup works with namespaces and why its a good thing?
under the section,
Why the criticism of Koenig Algorithm?
There is one (admittedly, somewhat uncommon) situation in which the form you use really can make a difference, and the form you want to use is using namespace foo, and it's most commonly applied to the std namespace (i.e., where you write using namespace std;.
The most obvious example is that you're writing a sort for a user-defined type. It's possible that this will be applied to a type for which the user has also defined their own swap.
You're stuck with a situation where you want to use their swap if they've defined one, but use std::swap if they haven't defined one. If you use std::swap directly in your code, then you'll end up using std::swap even if the type has a swap of its own defined. Conversely, your code will fail to compile if you directly specify a swap specifically for the type, and none has been supplied.
To get around this, you do something like:
using namespace std;
template <class Iter>
void my_sort(Iter first, Iter last) {
// ...
if (*last < *first)
swap(*first, *last);
}
This will find the swap specifically for the type being compared (i.e., a swap defined in the same namespace as that type), if there is one (via argument dependent lookup), and std::swap if none is defined for the type (via the using namespace std;).
This can have an impact on performance -- if they've written a swap specifically for their type, you can generally expect that it's because by doing so, they can provide better performance. That means that explicitly specifying std::swap may work, but will probably lead to inferior performance.
Otherwise, it's almost entirely a matter of convenience and readability -- I mostly prefer giving the full names (e.g., std::swap) except in a situation like above, where (at the time that I'm writing the code) either of at least two possibilities might be preferred, and I want to give the compiler sufficient leeway to pick the right one.
The other time I find using declarations/directives useful is when namespaces get really deeply nested. Boost (for one obvious example) has some names that would be way too long for convenient use if you used the fully qualified name every time. This was especially true for the (now, thankfully, mostly obsolete) Boost Lambda library, where you used placeholders like _1, that would have ended up as something like boost::lambda::placeholders::_1 (but I'm going from memory, so that's probably at least partly wrong) if you insisted on using a fully qualified name. That would have defeated a large part of the purpose of using the lambda library in the first place.
There's no performance gain or penalty whatsoever. All calls and variables are resolved at compile-time.
The choice between the three is somewhat subjective. Yes, using namespace <ns>; is sometimes frowned upon for polluting the global namespace, but I think it's safe to use for small files.
I tend to use the second one for testing purposes where I'm expecting conflicts, but I'd just remove it afterwards. This can get messier because you can end up with combinations of both qualified and un-qualified names:
vector<std::string> x;
because you have a using std::vector; at the top, but not a using std::string;.
I preffer the third.
There is zero impact on performance of your code, this is purely a compile-time thing. It might (in theory) have some impact on compilation times, but I doubt that would ever reach measurable proportions.
using namespace std (or any other namespace for that matter) is definitely to be avoided in header files, which can be included anywhere and introducing the symbols in them could result in ambiguities.
Generally, namespaces exist to avoid name clashes and using namespace destroys this purpose. So does using the_namespace::some_id, to a lesser degree. There is no definite answer to your question, but I generally follow these rules:
Never put using namespace in a header file.
Avoid using namespace and using, unless it can save tremendous amounts of typing. Use namespace aliases if necessary (that is, namespace abbrv = some_really::long_and::nested_namespace;).
Try to limit the scope of using: you can put this into functions and blocks as well as namespace scope. That is, if you have a logging function in your .cpp file, put using std::cout; and using std::endl; (or whatever you're using) into the function body, not into the file scope.
Main reason is that it could lead to ambiguities (both for compiler and for human reader) and it can also slowdown the compilation itself (but that's not that big problem as the first thing)
I'm beginner at C++ so if the answer is obvious it possibly is the one I'm looking for. I was reading the second response in this thread and got confused.
#include <algorithm>
#include <cassert>
int
main()
{
using std::swap;
int a(3), b(5);
swap(a, b);
assert(a == 5 && b == 3);
}
What I don't get is "This is just a defined function. What I meant was, why it is not directly built-in" but there was no need to include a new library so isn't it built-in? Does the std library automatically get imported (if yes, why doesn't the namespace automatically get set to std)?
"This is just a defined function. What I meant was, why it is not directly built-in" but there was no need to import a new library so isn't it built-in? Does the std library automatically get imported (if yes, why doesn't the namespace automatically get set to std)?
Well, by defined function it means, most likely that the function is already pre-written, and defined in the library, it isn't directly built-in probably because it was designed that way; only core essentials were included in the language, and everything else is in a library so the programmer can import what they want.
By built-in, usually it's a keyword, like for or while.
And no, std isn't automatically imported since it's designed so the programmer can choose what namespaces they want, like custom namespaces or std. An example of this being bad to automatically have std is this:
Say you automatically defined std, then you wanted to do using namespace foo;, now if foo also had function cout, you would run into a huge problem, like say you wanted to do this;
// code above
cout << "Hello, World" << endl;
// code below
how would the compiler which namespace function to use use? the default or your foo namespace cout? In order to prevent this, there is no default namespace set, leaving it up to the programmer.
Hope that helps!
This is just a defined function. What I meant was, why it is not directly built-in" but there was no need to include a new library so isn't it built-in?
The C++ library is a part of C++, by definition. However, it is not a part of the core language. C++ is a huge, huge language. Making it so the compiler knew every nook and cranny of the language right off the bat would make the compiler huge and slow to load. The philosophy instead is to keep the core somewhat small and give programmers the ability to extend the functionality by #including header files that. specify what they need.
Why doesn't the namespace automatically get set to std?
That would essentially make all kinds of very common words into keywords. The list of words you shouldn't use (keywords, global function in C, words reserved by POSIX or Microsoft, ...) is already huge. Putting the C++ standard library in namespace std is a feature. Putting all of those names in the global namespace would be a huge misfeature.
In your code, you have the line:
using std::swap;
So the call to swap doesn't need std::. For assert, it was defined as a macro, so it also would not need std::. If you did not use using, then other than macros, you would have to use std:: to refer to functions and objects provided by the standard C++ library.
The standard C++ library normally get linked into your program when you compile your program to create an executable. From this point of view, you might consider it "built-in". However, the term "built-in" would usually mean the compiler treats the word swap like a keyword, which is not the case here. swap is a template function defined in the algorithm header file, and assert is a macro defined in cassert.
A namespace is a convenience to allow parts of software to be easily partitioned by name. So if you wanted to define your own swap function, you could put it into your own namespace.
namespace mine {
template <typename T> void swap (T &a, T &b) { /*...*/ }
}
And it would not collide with the standard, or with some library that defined swap without a namespace.
When using C++ namespaces, do you prefer to explicitly name them, like this:
std::cout << "Hello, world!\n";
Or do you prefer using namespace:
using namespace std;
cout << "Hello, world!\n";
And if if you prefer the latter, do you declare your usings at file or function scope?
Personally I prefer to explicitly name them - it's more typing but when using a mixture of namespaces (e.g. std and boost) I find it more readable.
I use always explicit ones. Writing std does not hurt me and I clearly see where is it from. It's useful when you have some legacy project to take care of with it's own "strings", "vectors" etc to maintain. The more information the code carries with it, the better.
Extra typing is not the issue here. The problem with explicitly qualified names is the visual clutter. Let's face it, C++ syntax is untidy. No need to make this worse by needlessly making names longer and sprinkling the code generously with ::s.
I'm with Jeff Atwood: The Best Code is No Code At All. This is so true.
Namespace imports are a great way of reducing clutter with no drawback: As long as the scope of opened namespaces is reduced to a single compilation unit1, name conflicts, should they appear, can be resolved easily.
Why explicit names should (in general) be more readable has always been a mystery to me. The readers should generally know the code good enough to be able to deduce semantics. If they aren't, the code needs fixing anyway.
1) Corollary: no using in headers!
I always use using namespace for std & boost. Everything else I tend to use an explicit namespace unless it is used so much that it would clutter up the code.
In headers, I never use using namespace to avoid polluting the global namespace of the #including source.
My general rule is always explicitly use the namespace in headers, and usually use using in the code. The reason for the former is to make it explicitly clear in every part of the definition what is being used, and the reason for the latter is that it makes it easy to use replacements from another namespace if that becomes necessary. i.e. if we want to start using foo::string instead of std::string we just need to update the header and the using statement rather than replacing every instance of std::string with foo::string in the code.
Of course, that is less useful for classes that reside in the std:: namespace, since even if you replace one class you're still likely to use others in std, and may run into ambiguity issues, but that was just an example.
using and using namespace are Very Very useful to render the code more readable - remove clutter.
But in any case where it makes it harder to find out where a symbol comes from, I refuse importing it's whole namespace.
I try to limiit the scope of the imported namespaces:
void bar() {
// do stuff without vector
{ using std::vector;
// do stuff with vector
}
// do stuff without vector
}
For "generally known" libraries, like std, I would dare using using namespace std. There is reason to believe everyone reading this code knows these symbols.
As a sidenote, the using keyword is also used to indicate that a derived class also exports the overloaded members of its superclass.
class A {
void f( A );
void f( bool );
};
class B : public A {
using A::f; // without this, we get a compilation error in foo()
void f(bool);
};
void foo() {
B b;
b.f( A() ); // here's a compilation error when no `using` is used in B
}
I only use explicit namespaces when there's some ambiguity. It is more readable, but the extra typing is too tedious, and you have to assume other developers have a baseline level of familiarity with standard libraries.
The only other times I spell out a namespace are when I'm only using it once or twice, like adding in a quick debug statement, or if I'm using some non-standard library.
I typically declare the namespace at file scope, but if you're mixing namespaces it might make sense to put the declaration closer to the point where it's used, in function scope.
using at function scope, or if the function is very small (often is), just explicit namespace
I tend to explicitly import the names that I need at the top of the .cpp file, so...
using std::cout;
using std::endl;
etc...
That way I have control over the names that I use and it's easy to see where they come from and the code isn't cluttered at the point of use.
In the rare cases where I am using two names from different namespaces I fully qualify them at the point of use.
I always use fully qualified names in headers and hardly ever use 'using namespace x' anywhere...