What would 'std:;' do in c++?

Question

I was recently modifying some code, and found a pre-existing bug on one line within a function:

std:;string x = y;

This code still compiles and has been working as expected.

The string definition works because this file is using namespace std;, so the std:: was unnecessary in the first place.

The question is, why is std:; compiling and what, if anything, is it doing?

@Xeo: He said in the question he has `using namespace std;`. — Fred Larson, Oct 09 '12 at 19:36
@Fred: Derp! That part wasn't marked as code, so I just overlooked it. :) I'd still like to point to [this question](http://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-a-bad-practice-in-c), however. — Xeo, Oct 09 '12 at 19:36
@KerrekSB That happened to a friend of mine while browsing reddit! The link turned out to be [this](http://i.imgur.com/q1nKF.jpg) :D — mmirzadeh, Oct 09 '12 at 19:44

K-ballo · Accepted Answer · 2012-10-09T19:41:30.057

91

std: its a label, usable as a target for goto.

As pointed by @Adam Rosenfield in a comment, it is a legal label name.

C++03 §6.1/1:

Labels have their own name space and do not interfere with other identifiers.

edited Oct 09 '12 at 19:41

answered Oct 09 '12 at 19:34

K-ballo

4

AFAIK labels has their own name space, so they never conflict with anything else, other than keywords or macros, of course. – rodrigo Oct 09 '12 at 19:38
11

C++03 §6.1/1 says "Labels have their own name space and do not interfere with other identifiers." – Adam Rosenfield Oct 09 '12 at 19:40
6.1/1: "Labels have their own name space and do not interfere with other identifiers" – John Dibling Oct 09 '12 at 19:43

score 32 · Answer 2 · answered Oct 09 '12 at 19:34

32

It's a label, followed by an empty statement, followed by the declaration of a string x.

answered Oct 09 '12 at 19:34

Fred Larson

score 12 · Answer 3 · answered Oct 09 '12 at 19:38

12

Its a label which is followed by the string

answered Oct 09 '12 at 19:38

Rahul Tripathi

score 8 · Answer 4 · edited Oct 09 '12 at 19:52

8

(expression)std: (end of expression); (another expression)string x = y;

edited Oct 09 '12 at 19:52

John Conde

answered Oct 09 '12 at 19:47

Polymorphism

4

Grammar-wise, it's actually a `(label-)statement`, not an `expression`. – GManNickG Oct 09 '12 at 20:08

score 2 · Answer 5 · answered Feb 15 '14 at 10:57

The compiler tells you what is going on:

#include <iostream>
using namespace std;
int main() {
  std:;cout << "Hello!" << std::endl;
}

Both gcc and clang give a pretty clear warning:

std.cpp:4:3: warning: unused label 'std' [-Wunused-label]
  std:;cout << "Hello!" << std::endl;
  ^~~~
1 warning generated.

The take away from this story: always compile your code with warnings enabled (e.g. -Wall).

5 Answers5