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#include <iostream>
using namespace std;
 
/**
* @brief Defines a passkey for type T.
*/
template <typename ...Ts>
class PassKey
{
private:
    // befriend T so that only T can create a PassKey object
    friend Ts...;
 
    // make default constructor private so that only T can access it
    PassKey() {}
 
    // noncopyable, non-assignable so that key can not be re-used
    PassKey(const PassKey&) = delete;
    PassKey& operator=(const PassKey&) = delete;
};
 
struct B;
 
struct A
{
	void doSomething(PassKey<B>) const
	{}
};
 
struct B
{
	void callDoSomething(const A& a) const
	{
		a.doSomething({});
	}
};
 
 
 
int main() {
	A a;
	B b;
	b.callDoSomething(a);
	int FraudKey=0;
	return 0;
}

prog.cpp:12:14: error: expected unqualified-id before ‘...’ token
     friend Ts...;
              ^~~
prog.cpp: In member function ‘void B::callDoSomething(const A&) const’:
prog.cpp:34:19: error: ‘PassKey<Ts>::PassKey() [with Ts = {B}]’ is private within this context
   a.doSomething({});
                   ^
prog.cpp:15:5: note: declared private here
     PassKey() {}
     ^~~~~~~

Standard output is empty