What would 'std:;' do in c++?
C++StdColonC++ Problem Overview
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?
C++ Solutions
Solution 1 - C++
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.
Solution 2 - C++
It's a label, followed by an empty statement, followed by the declaration of a string x.
Solution 3 - C++
Its a label which is followed by the string
Solution 4 - C++
(expression)std: (end of expression); (another expression)string x = y;
Solution 5 - C++
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).