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#include <iostream>
#include <algorithm> 
#include <utility>
using namespace std;
 
 
struct A
{
	bool bValid;
	mutable bool singleTimeInvert;
	int  value;
	bool isValid() { return bValid; }
 
	A(bool b = false) : bValid(b), singleTimeInvert(false) {}
	A(int v) : bValid(true), singleTimeInvert(false), value(v)  {} 
	A(const A & a) : bValid(a.bValid), singleTimeInvert(a.singleTimeInvert), value(a.value) {}
	A(const A & a, bool i) : bValid(a.bValid), singleTimeInvert(i), value(a.value) {}
 
	explicit operator bool() const
	{
		bool tmp = singleTimeInvert;
		singleTimeInvert = false;
		return tmp ? !bValid : bValid;
	}
 
	A operator!() const
	{
		return A(*this, true);
	}
 
	A& operator!=(const A & a)
	{
		*this = a;
		singleTimeInvert = true;
		return *this;
	}
 
};
 
A foo()
{
    return 5;	
}
 
A bar()
{
	return false;
}
 
int main() {
 
	if (auto a != foo())
	{
		cout<<"a is not valid\n";
	}
	else
	{
		cout<<"a = "<<a.value<<"\n";
	}
 
	if (auto b != bar())
	{
		cout<<"b is not valid\n";
	}
	else
	{
		cout<<"b = "<<b.value<<"\n";
	}
 
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:52:6: error: expected primary-expression before ‘auto’
  if (auto a != foo())
      ^~~~
prog.cpp:52:6: error: expected ‘)’ before ‘auto’
prog.cpp:58:17: error: ‘a’ was not declared in this scope
   cout<<"a = "<<a.value<<"\n";
                 ^
prog.cpp:61:6: error: expected primary-expression before ‘auto’
  if (auto b != bar())
      ^~~~
prog.cpp:61:6: error: expected ‘)’ before ‘auto’
prog.cpp:67:17: error: ‘b’ was not declared in this scope
   cout<<"b = "<<b.value<<"\n";
                 ^

Standard output is empty