C++ 0X学习：auto关键词
auto并不是一个新关键词，是一个把旧关键词（貌似都没人用它了
）赋予新的作用，新的作用修饰变量声明，指示编译器根据变量的初始化表达式推导变量应有的类型。auto 声明的变量必须“在声明处完成初始化”，编译器才可根据初始化表达式推导变量的类型。
二话不说，先看看例子呗。
对于现在，我们一般通过迭代器 for 容器的时候，都会写出这样的代码
#include <map>
#include <string>
struct people {
int age;
char sex;
};
typedef std::map<std::string, people> people_map;
int main() {
people_map my_map;
for (people_map::iterator it = my_map.begin(); it != my_map.end(); ++it) {
/**
* do some thing
*/
}
return 0;
}
获取容器的迭代器之前，我们要写出容器的类型，在这里用了 typedef，把类型缩短了=，=，要是懒点的话，很可能写成 std::map<std::string, people>::iterator。我的天啊，我仅仅想获取它的迭代器，却要写那么多代码。很多时候，我们并不需要知道容器的类型，或者，我们已经非常清楚类型了，那么，除了用 typedef 简化，还有啥方法呢？
这就是 auto 关键词的新作用了，用来推倒表达式的应有类型，代码改进后是这样子了。
#include <map>
#include <string>
struct people {
int age;
char sex;
};
int main() {
std::map<std::string, people> my_map;
for (auto it = my_map.begin(); it != my_map.end(); ++it) {
/**
* do some thing
*/
}
return 0;
}
在这里，把 typedef 咔嚓掉了（但是声明变量的时候麻烦了点）=，=，然后获取迭代器，直接用 auto 了，不用管它是什么类型了。
在这里，it 自动推导成 std::map<std::string, people>::iterator 了。
auto 是不是很强大的一个关键词呢，在新的作用下，可以写出更简洁的代码。
但是 auto 也有局限性，比如上面的例子，他是推导成 std::map<std::string, people>::iterator，不会推导成 std::map<std::string, people>::const_iterator。
关于 auto 的使用，还有一些细节问题，才开始学习用，还没弄得明白
使用 auto关键词也有注意的地方（摘自网络）：
注意一：auto 不能做为模板参数。因为这违背了 auto 需要由初始化表达式来推导类型的原则。
注意二：auto 不能做为函数的参数类型和返回类型。同样是因为违背了 auto 推导类型的原则。函数在编译时要实例化，此时便需要确定参数的类型，以方便安排内存。声明为 auto 的话如何确定其类型呢？没法确定，所以这样用是不允许的。
2018年5月22日 14:34
auto 不能做为模板参数。因为这违背了 auto 需要由初始化表达式来推导类型的原则。!!
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