c++11新特性之左值、右值、左值引用、右值引用、引用折叠、std::move()、std::forward()

c++11新特性之左值、右值、左值引用、右值引用、引用折叠、std::move()、std::forward()

参考

1.程序喵大人
2.
https://zhuanlan.zhihu.com/p/461031135
https://zhuanlan.zhihu.com/p/461361869
https://zhuanlan.zhihu.com/p/461285071
https://zhuanlan.zhihu.com/p/461098279
3.
https://blog.csdn.net/qq_34954047/article/details/123636179?utm_medium=distribute.pc_relevant.none-task-blog-2_defaultbaidujs_title~default-5.pc_relevant_aa&spm=1001.2101.3001.4242.4&utm_relevant_index=8

实践示例：

#include 
#include 
#include 
#include 
#include 


void PrintV(int& t) 
{ 
	std::cout << "lvalue" << std::endl;
}
void PrintV(int&& t) 
{ 
	std::cout << "rvalue" << std::endl;
}

template<typename T>
void test1(T t) 
{
	PrintV(t);
}

template<typename T>
void test2(T& t)
{
	PrintV(t);
}

//这个是万能引用模板，与test1的区别是不发生拷贝（本菜鸡猜测的，并没有测试自定义类）
template<typename T>
void test3(T&& t)
{
	PrintV(t);
}

int main() {

	int a = 2;
	
	//全为左值
	std::cout << "--------------------test3-----------------------" << std::endl;
	test3(2);
	test3(a);
	test3(std::move(2));
	test3(std::move(a));
	test3(std::forward<int>(2));
	//test3(std::forward(2)); forward error
	test3(std::forward<int&&>(2));
	test3(std::forward<int>(a));
	test3(std::forward<int&>(a));
	test3(std::forward<int&&>(a));
	std::cout << "--------------------test3-----------------------" << std::endl;
	std::cout << "\n\n\n\n\n\n\n\n" << std::endl;


	std::cout << "--------------------test2-----------------------" << std::endl;
	//test2(2); error 不可以传纯右值
	test2(a); //左值
	//test2(std::move(2)); error 不可以传纯右值
	//test2(std::move(a)); error 不可以传纯右值
	//test2(std::forward(2)); error 不可以传纯右值
	//test1(std::forward(2)); forward error
	//test2(std::forward(2)); error 不可以传纯右值
	//test2(std::forward(a)); error 不可以传纯右值
	test2(std::forward<int&>(a)); //左值
	//test2(std::forward(a)); error 不可以传纯右值
	std::cout << "--------------------test2-----------------------" << std::endl;
	std::cout << "\n\n\n\n\n\n\n\n" << std::endl;

	//拷贝-》全为左值
	std::cout << "--------------------test1-----------------------" << std::endl;
	test1(2);
	test1(a);
	test1(std::move(2));
	test1(std::move(a));
	test1(std::forward<int>(2));
	//test1(std::forward(2)); forward error
	test1(std::forward<int&&>(2));
	test1(std::forward<int>(a));
	test1(std::forward<int&>(a));
	test1(std::forward<int&&>(a));
	std::cout << "--------------------test1-----------------------" << std::endl;
	std::cout << "\n\n\n\n\n\n\n\n" << std::endl;


	//左值
	std::cout << "--------------------lvalue-----------------------" << std::endl;
	PrintV(a);
	//PrintV(std::forward(2)); forward error
	PrintV(std::forward<int&>(a));
	std::cout << "--------------------lvalue-----------------------\n" << std::endl;

	
	//右值
	std::cout << "\n\n--------------------rvalue-----------------------" << std::endl;
	PrintV(2);

	PrintV(std::move(2));
	PrintV(std::move(a));

	PrintV(std::forward<int>(2));
	PrintV(std::forward<int&&>(2));
	PrintV(std::forward<int>(a));
	PrintV(std::forward<int&&>(a));
	std::cout << "--------------------rvalue-----------------------\n" << std::endl;

	return 0;
}