Modern C++ - Value Categories

Value Categories

• glvalue (generalized lvalue)
  • identity : polymorphic, name
  • lvalue
    • identity
    • copy
    • lvalue 리턴
  • xvalue (eXpiring value)
    • identity
    • move
    • rvalue 리턴
• rvalue
  • move
  • xvalue (eXpiring value)
    • identity
    • move
    • rvalue 리턴
  • prvalue (pure rvalue)
    • no identity : non-polymorphic, no name
    • move
    • 값 리턴
• lvalue
  • ++a, a += b, *p, a[n]
  • “Hello, World!”
• xvalue
  • std::move(x), a[n], Test().m
• prvalue
  • a++, a + b, a < b
  • static_cast(a)
  • this
  • lambda expression
  • requires expression

int x = 0;
int   f1() { return x; }
int&  f2() { return x; }
int&& f3() { return move(x); }

Derived d;
Base   f4() { return d; }
Base&  f5() { return d; }
Base&& f6() { return move(d); }

int main()
{
	f1() = 1; // error
	f3() = 2; // ok
	f3() = 3; // error

	Base b1 = f4(); // move, 임시객체
	Base b2 = f5(); // copy
	Base b3 = f6(); // move

	f4().foo(); // Base::foo, 임시객체
	f5().foo(); // Derived::foo
	f6().foo(); // Derived::foo
}