What is "using namespace::std" in C-CodePudding

I am reading some code snippets from others and I find a line:

using namespace::std;

I suspect its purpose is using namespace std;, with some typos. But to my surprise the compiler accepts this code without any complaint. I build with:

$ g   --version
g   (Ubuntu 9.4.0-1ubuntu1~20.04) 9.4.0

$ /usr/bin/g   ${SRC} -std=c  11 -pthread -Wall -Wno-deprecated -o ${OUT}

I wonder why is this code valid, and what effects will it make? I suspect it is a bad practice.

CodePudding user response：

It's just same as using namespace ::std;, then has the same effect with using namespace std; in fact. The :: refers to the global namespace, and std is put in the global namespace indeed.

As the syntax of using-directives:

(emphasis mine)

attr(optional) using namespace nested-name-specifier(optional) namespace-name ;   
... ...
nested-name-specifier - a sequence of names and scope resolution operators ::, ending with a scope resolution operator. A single :: refers to the global namespace.
... ...

CodePudding user response：

using namespace::std is the same as using namespace std;

The :: symbol is the scope resolution operator. When used without a scope name before it , it refers to the global namespace. This means that std is a top level namespace, and is not enclosed in another.

The spaces before and after the :: are optional in this case because the lexer can deduce the tokens from the context.

For example, all of the following are valid:

namespace A { namespace notstd{} } // define my own namespaces A and A::notstd
using namespace::std; // the standard library std
using namespace A;
using namespace ::A;
using namespace::A;
using namespace A::notstd;