Re: [RFC] add push/pop pragma to control the scope of "using"

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On Wed, 15 Jan 2020 at 08:15, 马江 <majiang31312@gmail.com> wrote:
>
> Hello,
>   After  some google, I find there is no way to control the scope of
> "using" for the moment.  This seems strange as we definitely need this
> feature especially when writing inline member functions in c++
> headers.
>
>   Currently I am trying to build a simple class in a c++ header file
> as following:
>
> #include <string>
> using namespace std;
> class mytest
> {
>   string test_name;
>   int test_val;
> public:
>   inline string & get_name () {return test_name;}
> };
>
>   As a experienced  C coder, I know that inline functions must be put
> into headers or else users could only rely on LTO. And I know that to
> use "using" in a header file is a bad idea as it might silently change
> meanings of other codes. However, after I put all my inline functions
> into the header file, I found I must write many "std::string" instead
> of "string" which is totally a torture.

It's really not that bad.

>   Can we add something like "#pragma push_using"  (just like #pragma
> pop_macro)? I believe it's feasible and probably not hard to
> implement.

It would create a non-standard, non-portable dialect of C++. We prefer
to avoid adding non-standard extensions these days. You could propose
it to the C++ committee, but I'm pretty sure they would not want such
a thing.

You can already do it anyway, if you put your own code in a namespace
(which is a good idea anyway):

#include <string>

namespace foo
{
  using std::string;
  class bar
  {
  string name_;
  public
    const string& name() const noexcept { return name_; }
  };
}

This doesn't have all the problems of "using namespace" in a header.