在boost :: spirit :: x3中用作標籤的奇怪結構

Question

在以下上下文中，什麼是奇怪語言結構X< class Y > Y;？

#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 the C++ ABI mangling rules"; 
    case -3: return "Invalid argument to 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; 
} 

它的目的和語義是什麼？

Answer 1

讓我們來分析一下：

X <class Y> Y 
| ^^^^^^^^^ | 
|  |  | 
|  |  | 
|  |  ---> name of the variable 
|  | 
|  ---> the struct is a template which is instantiated with a type 
|   (in this case, the type is an incomplete class named Y) 
| 
---> struct declared globally 

如前所述Agent_L，有類名和因爲它可以從上下文推斷出你的真正用意是什麼變量之間沒有衝突：類型的變量或。