c++ - Strange construct that used as tag in boost::spirit::x3 -

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what strange language construct x< class y > y; in following context?

#include <iostream> #include <sstream> #include <typeinfo> #include <type_traits> #include <cstdlib> #include <cxxabi.h>  template< typename t > std::string const type_info_str() {     int status = 0;     auto realname_(abi::__cxa_demangle(typeid(t).name(), nullptr, nullptr, &status));     switch (status) {     case -1: return "could not allocate memory";     case -2: return "invalid name under c++ abi mangling rules";     case -3: return "invalid argument demangle";     }     std::ostringstream oss;     if (std::is_volatile< t >::value) {         oss << "volatile ";     }     oss << realname_;     std::free(realname_);     if (std::is_const< t >::value) {         oss << " const";     }     if (std::is_rvalue_reference< t >::value) {         oss << " &&";     } else if (std::is_lvalue_reference< t >::value) {         oss << " &";     }     return oss.str(); }  template< typename t > struct x { };  int main() {     x< class y > y;     std::cout << type_info_str< decltype(y) >() << std::endl; // x<main::y>     return exit_success; }

what purpose , semantics?

let's break down:

x  <class y> y  |  ^^^^^^^^^ | |      |     | |      |     | |      |     ---> name of variable  |      | |      ---> struct template instantiated type  |          (in case, type incomplete class named y) | ---> struct declared globally

as agent_l mentioned, there no conflict between class name , variable because it can inferred context meant: type or variable.


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