c++ : comma separated list in using declaration

Question

Why is it a bad practice to use a comma separated list in a using declaration statement in c++?

For example

using std::cout;
using std::cin;
using std::endl;

is considered better code than

using std::cout,std::cin,std::endl;

Some compiliers(gcc, for instance) even issue warnings if the code contains a comma separated list of using declarations.

Ofcourse, the best practice would be to use fully qualified names for identifiers.

Is `using std::cout,std::cin,std::endl;` legal syntax? I've never seen this before, and VS doesn't accept it. — François Andrieux, Dec 13 '17 at 15:44
You could post one of these warnings... It might already indicate the answer. Anyway, [cppreference](http://en.cppreference.com/w/cpp/language/using_declaration) states that `using declarator-list ;` is C++17 only. So that's probably why. — underscore_d, Dec 13 '17 at 15:44
@FrançoisAndrieux it works with gcc (I've tried it), but it issues warnings — iambrj, Dec 13 '17 at 15:46

score 6 · Accepted Answer · answered Dec 13 '17 at 15:49

6

As documented at cppreference, the form

using declarator-list;

is available from C++17 onwards only.

Your compiler is probably telling you this, if you'd read the warning!

answered Dec 13 '17 at 15:49

underscore_d

Well done. Indeed, the warning might suggest OP to turn on `-std=c++17` ;) – YSC Dec 13 '17 at 16:14

user4581301 · Answer 2 · 2017-12-14T16:47:01.147

Approaching from the other side, Reasons for disapproving of horizontal lists:

using std::cout; is about an inch wide on my screen. This allows the eye to read it with very little horizontal movement and the vertical distance traveled to the next line is much shorter than having to read one item and then the next and the next.... Most people can read the vertical scroll faster.
Next, lined up vertically you tend to get more easily recognized patterns. A rogue stb:: will stand out better in a line-up of std::s to the majority of readers. Not much, but it may save you a compile and that compile might take a long, long time under some circumstances.
Inserting, removing, or altering a using declaration will be easier to pick out in file difference output if it's on its own line. If the changes are buried all on line you have to scan both lines and run a Mark 1 Eyeball diff to find out what changed, and the Mark 1 Eyeball and the human CPU can really suck at doing this.

I was thinking that we could just point readers at the objections given to multi-line variable declarations and cover most of the common ground (though `using` is not anywhere near so vulnerable to mistakes as that). — underscore_d, Dec 14 '17 at 09:52

2 Answers2