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Re: Putting two colons before std -- is it ever needed?

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On 9/25/19 4:02 AM, Frederick Gotham wrote:
> 
> Occasionally I see people write code like this:
> 
> #include <string>
> 
> int main()
> {
>     ::std::string str;
> }
> 
> I think you would only ever need to put those two colons before 'std' if you created a class or a namespace called 'std', and of course if you did, then it would have to be inside another namespace (because otherwise you'd have a name clash).
> 
> I tried compiling the following code, and each time it created a 'string' from the C++ standard library.
> 
> So the question I ask is if there is **ever** any situation when it's necessary to put two colons before 'std'?
> 
> #include <string>
> #include <iostream>
> 
> namespace Monkey {
> 
> 	class nonstd {
> 		int i;
> 	};
> 	
> 	class mutant : public nonstd {
> 		int j;
> 	};
> 	
> 	class mongrel : public nonstd {
> 		int k;
> 	};
> 	
> 	class std {
> 	public:
> 		typedef char *string;
> 	};
> }
> 
> int SomeFunc()
> {
> 	using namespace Monkey;
> 	
> 	std::string a;
> 	
> 	::std::string b;
> 	
> 	::std::cout << sizeof(a) << " " << sizeof(b) << ::std::endl;
> }
> 
> int main()
> {
> 	std::string a;
> 	
> 	::std::string b;
> 	
> 	::std::cout << sizeof(a) << " " << sizeof(b) << ::std::endl;
> 	
> 	SomeFunc();
> }

#include <iostream>
#include <cstdio>
namespace kuyper {
  namespace std {
      void printf(const char *txt) {
	  ::std::cout << "Kuyper::std::" << txt;
      }

      void func(void)
      {
	std::printf("printf\n");
	::std::printf("::std::printf\n");
      }
  }
}

int main(void)
{
    kuyper::std::func();
    return 0;
}

Compiled using g++ (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
 with the following command line:
g++ -std=c++1y -pedantic -Wall -Wpointer-arith -Wcast-align
-fno-enforce-eh-specs -ffor-scope -fno-gnu-keywords
-fno-nonansi-builtins -Wctor-dtor-privacy -Wnon-virtual-dtor
-Wold-style-cast -Woverloaded-virtual -Wsign-promo std.cpp -o std

Execution produces the following results:
Kuyper::std::printf
::std::printf

Code where std: is simply ambiguous, and must therefore be replaced by
::std if that's what you want to use is slightly simpler to create, but
is ill-formed. You need well-formed code like that above to make an
alternative namespace std be a preferred match to an unqualified std::.

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Thread

Putting two colons before std -- is it ever needed? Frederick Gotham <cauldwell.thomas@gmail.com> - 2019-09-25 01:02 -0700
  Re: Putting two colons before std -- is it ever needed? Ian Collins <ian-news@hotmail.com> - 2019-09-25 20:15 +1200
  Re: Putting two colons before std -- is it ever needed? David Brown <david.brown@hesbynett.no> - 2019-09-25 12:22 +0200
    Re: Putting two colons before std -- is it ever needed? Frederick Gotham <cauldwell.thomas@gmail.com> - 2019-09-25 03:48 -0700
  Re: Putting two colons before std -- is it ever needed? Bo Persson <bo@bo-persson.se> - 2019-09-25 15:48 +0200
    Re: Putting two colons before std -- is it ever needed? Vir Campestris <vir.campestris@invalid.invalid> - 2019-09-26 21:51 +0100
  Re: Putting two colons before std -- is it ever needed? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-09-25 10:02 -0400
    Re: Putting two colons before std -- is it ever needed? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-09-25 10:11 -0400
    Re: Putting two colons before std -- is it ever needed? Frederick Gotham <cauldwell.thomas@gmail.com> - 2019-09-25 07:17 -0700
  Re: Putting two colons before std -- is it ever needed? Stuart Redmann <DerTopper@web.de> - 2019-09-25 19:51 +0200
    Re: Putting two colons before std -- is it ever needed? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-09-25 15:37 -0400
    Re: Putting two colons before std -- is it ever needed? Frederick Gotham <cauldwell.thomas@gmail.com> - 2019-09-26 00:01 -0700
  Re: Putting two colons before std -- is it ever needed? woodbrian77@gmail.com - 2019-09-27 08:50 -0700
    Re: Putting two colons before std -- is it ever needed? scott@slp53.sl.home (Scott Lurndal) - 2019-09-27 16:25 +0000
      Re: Putting two colons before std -- is it ever needed? woodbrian77@gmail.com - 2019-09-27 11:40 -0700
        Re: Putting two colons before std -- is it ever needed? scott@slp53.sl.home (Scott Lurndal) - 2019-09-27 20:19 +0000

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