template

#include <iostream>
#include <functional>
 
using namespace std;
 
template <typename T>
constexpr bool is_lvalue(T&) {
	return true;
}
 
template <typename T>
constexpr bool is_lvalue(T&&) {
	return false;
}
 
class MyStr
{
public:
	template<class T>
	MyStr(T&& t)
	{
		set(forward<T>(t));
	}
	template<class T>
	void set(T&& t)
	{
		if constexpr (conjunction_v<is_convertible<T&, MyStr&>>)
		{
			if (!is_lvalue(forward<T>(t)))
			{
				str_ = forward<string>(t.str_);
				cout << "is rvalue, type MyStr." << endl;
			}
			else
			{
				str_ = t.str_;
				cout << "is lvalue, type MyStr." << endl;
			}
		}
		else
		{
			str_ = forward<T>(t);
			cout << "is string." << endl;
		}
	}
	template<class T=char>
	void test()
	{
		cout << "test T size is " << sizeof(T) << endl;
	}
	string str_;
};
 
int main()
{
	cout << "call constructor:" << endl;
 
	MyStr str1("str1");
	MyStr str2(str1);
	MyStr str3(move(str1));
 
	cout << "call member function:" << endl;
 
	str1.set("set1");
	str2.set(str1);
	str3.set(move(str1));
 
	str3.test();
 
	return 1;
}

g++ -o rvalue -g -O0 -std=c++17 rvalue.cpp

call constructor:
is string.
is lvalue, type MyStr.
call member function:
is string.
is lvalue, type MyStr.
is rvalue, type MyStr.
test T size is 1