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.
It's just same as using namespace::std;
, then has the same effect with using namespace std;
.
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 .
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.
For example, all of the following are valid:
namespace A { namespace std{} }
using namespace::std;
using namespace A;
using namespace ::A;
using namespace::A;
using namespace A::std;
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