Почему «using namespace std;» считается плохой практикой?
Другие говорили мне, что писать using namespace std;
в коде неправильно и что я должен использовать std::cout
и std::cin
напрямую.
Почему using namespace std;
считается плохой практикой? Является ли это неэффективным или существует риск объявления неоднозначных переменных (переменных, которые имеют то же имя, что и функция в std
пространстве имен)? Это влияет на производительность?
Ответы
Это вообще не связано с производительностью. Но учтите это: вы используете две библиотеки под названием Foo и Bar:
using namespace foo;
using namespace bar;
Все работает нормально, и звонить Blah()
из Foo и Quux()
из Bar можно без проблем. Но однажды вы обновитесь до новой версии Foo 2.0, которая теперь предлагает функцию с именем Quux()
. Теперь у вас конфликт: и Foo 2.0, и Bar импортируются Quux()
в ваше глобальное пространство имен. Это потребует некоторых усилий, чтобы исправить, особенно если параметры функции совпадают.
Если бы вы использовали foo::Blah()
и bar::Quux()
, то введение foo::Quux()
не было бы событием.
Я согласен со всем, что написал Грег , но хочу добавить: может быть даже хуже, чем сказал Грег!
Библиотека Foo 2.0 могла бы ввести функцию, Quux()
которая однозначно лучше подходит для некоторых из ваших вызовов, Quux()
чем bar::Quux()
код, который вызывается годами. Тогда ваш код все равно компилируется , но он молча вызывает неправильную функцию и делает бог знает что. Это настолько плохо, насколько это вообще возможно.
Имейте в виду , что std
пространство имен имеет тонн идентификаторов, многие из которых являются очень распространенными из них (думаю list
, sort
, string
, iterator
и т.д.) , которые очень вероятно, появится в другом коде тоже.
Если вы считаете это маловероятным: здесь был задан вопрос о переполнении стека, где в значительной степени именно это произошло (неправильная функция вызвана из-за пропущенного std::
префикса) примерно через полгода после того, как я дал этот ответ. Вот еще один, более свежий пример такого вопроса. Так что это настоящая проблема.
Вот еще одна информация: много-много лет назад меня раздражало то, что все, что было в стандартной библиотеке, было префиксом std::
. Затем я работал в проекте, где с самого начала было решено, что и using
директивы, и объявления запрещены, за исключением областей видимости функций. Угадай, что? Большинству из нас потребовалось всего несколько недель, чтобы привыкнуть к написанию префикса, и еще через несколько недель большинство из нас даже согласились, что это действительно сделало код более читаемым . Для этого есть причина: нравится ли вам более короткая или более длинная проза - это субъективно, но префиксы объективно добавляют ясности коду. Не только компилятору, но и вам легче увидеть, на какой идентификатор имеется ссылка.
За десять лет этот проект вырос до нескольких миллионов строк кода. Поскольку эти обсуждения возникают снова и снова, мне однажды было любопытно, как часто (разрешенная) область видимости функции using
фактически использовалась в проекте. Я поискал его в источниках и нашел только один или два десятка мест, где он использовался. Для меня это указывает на то, что после std::
попытки разработчики не находят достаточно болезненным использование директив using даже один раз в 100 kLoC, даже если это разрешено.
Итог: явное добавление префиксов ко всему не приносит никакого вреда, требует очень небольшого привыкания и имеет объективные преимущества. В частности, это упрощает интерпретацию кода компилятором и читателями - и это, вероятно, должно быть основной целью при написании кода.
Проблема с помещением using namespace
файлов заголовков ваших классов заключается в том, что это заставляет любого, кто хочет использовать ваши классы (путем включения ваших файлов заголовков), также «использовать» (т.е. видеть все внутри) эти другие пространства имен.
Однако вы можете свободно помещать оператор using в свои (личные) файлы * .cpp.
Помните, что некоторые люди не согласны с моим высказыванием «не стесняйтесь» вроде этого - потому что, хотя using
выражение в файле cpp лучше, чем в заголовке (потому что оно не влияет на людей, которые включают ваш файл заголовка), они думают, что это все еще не так. хорошо (потому что в зависимости от кода это может затруднить поддержку реализации класса). В этой записи C ++ Super-FAQ говорится:
Директива using существует для устаревшего кода C ++ и упрощает переход к пространствам имен, но вам, вероятно, не следует использовать ее на регулярной основе, по крайней мере, в вашем новом коде C ++.
FAQ предлагает две альтернативы:
Объявление использования:
using std::cout; // a using-declaration lets you use cout without qualification cout << "Values:";
Просто набери std ::
std::cout << "Values:";
Недавно я столкнулся с жалобой на Visual Studio 2010 . Оказалось, что почти все исходные файлы содержат эти две строки:
using namespace std;
using namespace boost;
Многие функции Boost входят в стандарт C ++ 0x, а Visual Studio 2010 имеет множество функций C ++ 0x, поэтому внезапно эти программы перестали компилироваться.
Следовательно, избегание using namespace X;
- это форма защиты от изменений, способ убедиться, что изменение используемых библиотек и / или файлов заголовков не нарушит работу программы.
Краткая версия: не используйте глобальные using
объявления или директивы в файлах заголовков. Не стесняйтесь использовать их в файлах реализации. Вот что Херб Саттер и Андрей Александреску говорят об этой проблеме в Стандартах кодирования C ++ (выделено мной жирным шрифтом):
Резюме
Использование пространства имен предназначено для вашего удобства, а не для того, чтобы вы навязывали его другим: никогда не пишите объявление using или директиву using перед директивой #include.
Следствие: в файлах заголовков не записывайте уровень пространства имен с помощью директив или с помощью объявлений; вместо этого явно уточняйте все имена по пространству имен. (Второе правило следует из первого, потому что заголовки никогда не могут знать, какой другой заголовок #includes может появиться после них.)
Обсуждение
Вкратце: вы можете и должны свободно использовать пространство имен, используя объявления и директивы в своих файлах реализации после директив #include, и вам это нравится. Несмотря на неоднократные утверждения об обратном, декларации и директивы, использующие пространства имен, не являются злом и не противоречат цели пространств имен. Скорее они делают пространства имен пригодными для использования .
Не следует использовать using
директиву в глобальной области видимости, особенно в заголовках. Однако бывают ситуации, когда это уместно даже в заголовочном файле:
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));
}
Это лучше, чем явная квалификация ( std::sin
, std::cos
...), потому что она короче и позволяет работать с определяемыми пользователем типами с плавающей запятой (через поиск, зависящий от аргументов (ADL)).
Не используйте его глобально
Это считается «плохим» только при глобальном использовании . Потому как:
- Вы загромождаете пространство имен, в котором программируете.
- Читателям будет сложно понять, откуда берется тот или иной идентификатор, если вы их используете
using namespace xyz
. - Все, что верно для других читателей вашего исходного кода, еще более верно для наиболее частого читателя: для вас самих. Вернитесь через год или два и посмотрите ...
- Если вы говорите только о том, что
using namespace std
вы можете не знать обо всем, что берете, и когда вы добавляете еще одну#include
или переходите к новой ревизии C ++, у вас могут возникнуть конфликты имен, о которых вы не знали.
Вы можете использовать его локально
Идите вперед и используйте его локально (почти) свободно. Это, конечно, мешает вам повторяться std::
- и повторение тоже плохо.
Идиома для локального использования
В C ++ 03 была идиома - шаблонный код - для реализации swap
функции для ваших классов. Было предложено использовать локальный using namespace std
- или, по крайней мере 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(...)`
}
Это делает следующую магию:
- Компилятор выбрать
std::swap
FORvalue_
, то естьvoid std::swap(int, int)
. - Если у вас
void swap(Child&, Child&)
реализована перегрузка, компилятор выберет ее. - Если вы не имеете , что перегрузка компилятор будет использовать
void std::swap(Child&,Child&)
и попробовать все эти замены.
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.
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.
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 complete 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.
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.
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".
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.
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.
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
.
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 forusing 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 useusing 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
andstd
introduces a class namedfoo
, 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 insteadAKFoo()
. 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 prefixedWin
, 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.
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.
It depends on where it is located. If it is a common header, then you are diminishing the value of the namespace by merging it into the global namespace. Keep in mind, this could be a neat way of making module globals.
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. :)
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.