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

[Marc Glisse]

On Wed, 15 Jan 2020, 马江 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;}
> };

Why is mytest in the global namespace?

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

We try to avoid extensions in gcc, you may want to propose this to the C++ 
standard committee first. However, you should first check if modules 
(C++20) affect the issue.

-- 
Marc Glisse