{"mangled(std::mutex)"}    expands to    "St5mutex"

-> this changes, how curly braces or how strings are interpreted! And that you can resolve expressions (e.g. decltype and its parameter) within strings.

-> In which translation phase should the expansion be done?

 

The _ZTV prefix is ABI specific (so are virtual tables). Why standardize the {mangled}, if the further usage of the mangled name is implementation-specific anyway?


 

-----Ursprüngliche Nachricht-----
Von: Frederick Virchanza Gotham via Std-Proposals <std-proposals@lists.isocpp.org>
Gesendet: Fr 23.02.2024 14:11
Betreff: [std-proposals] operator mangled
An: std-proposals <std-proposals@lists.isocpp.org>;
CC: Frederick Virchanza Gotham <cauldwell.thomas@gmail.com>;
Imagine if C++ had a new operator called '{mangled}', which sits just
above 'sizeof' on the operator precedence table, which must be
followed by a type in parentheses, for example:

   {mangled(std::mutex)}      expands to    St5mutex
   {"mangled(std::mutex)"}    expands to    "St5mutex"

The latter behaves exactly like a string literal, such that you can do:

   std::stringstream ss;
   char const *str = "_ZTV" {"mangled(decltype(ss))"};

and it's as if you wrote:

   std::stringstream ss;
   char const *str = "_ZTV"
"NSt7__cxx1118basic_stringstreamIcSt11char_traitsIcESaIcEEE";

which of course is the same as:

   std::stringstream ss;
   char const *str =
"_ZTVNSt7__cxx1118basic_stringstreamIcSt11char_traitsIcESaIcEEE";

What would make it more interesting though is if we could use it with
'extern' as follows:

   #include <sstream>
   int main(void)
   {
       std::stringstream ss;

       // On the next line, we declare the mangled name of the vtable
       extern void (*_ZTV{mangled(decltype(ss))})(void);

       // On the next line, we get the address of the vtable
       void (**vtable)(void*) = &_ZTV{mangled(decltype(ss))};

       vtable[0](&ss);    // Invoke the destructor
       ::new(&ss) std::stringstream();   // Invoke the constructor
   }
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