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c++11中auto并不代表一种实际的数据类型，它只是一个类型声明的占位符，auto也并不是再所有场景下都能推导出变量的实际类型，使用auto不需要进行初始化，让编译器推导出它的实际类型，再编译阶段将auto占位符替换为真正的类型。
auto temp = 2;
auto还可以和指针，引用以及const，在不同的场景下有对应的推导规则.
当变量不是指针或者引用时，推导的结果中不会保留const关键字
当变量是指针或者引用时，推导结果中会保留const关键字
int temp = 110; auto *a = &temp; //int auto b = &temp; //int * auto &c = temp;
//int auto d = temp; //int int tmp = 250; const auto a1 = tmp; //int auto a2 =
a1; //int 因为没有声明是指针或者引用所以是int const auto &a3 = tmp; //int auto &a4 = a3;
//const int
auto的限制
auto 不能作为函数参数，因为函数参数只有在函数调用的时候才会将实参传入，auto要求必须给修饰的变量赋值
不能定义数组
不能用于类的非常量静态成员变量的初始化
class A { public: auto c = 2;//报错 static auto a;//报错，因为静态成员变量需要在类外赋值 const
static auto b = 2; };
auto经常应用在STL容器的遍历，其他地方尽量少用，不利于代码阅读
增强for循环
在增强for循环中不需要传递容器需要遍历的范围，循环会自动以容器为范围展开，并且循环屏蔽掉了迭代器的遍历细节，直接抽取容器中的元素进行运算。
如果想要修改遍历的容器，则需要使用引用的方式遍历
如果只读数据，不修改元素的值for(const auto &it:vec)比非引用的效率高一些
注意遍历哈希表时获取的是对象不是迭代器所以要用it.first或者it.second
