第三章主要讲vector和string

1. string到底是不是以\0结尾的？c++11中规定是。但是不要依赖这个特性，乖乖的用size()；但是getline的时候换行符确实丢掉了

2. auto for循环，注意效率，有些时候auto for效率更高（迭代器最慢～～）

   #include <iostream>
   #include <chrono>
   #include <string>
   int main(){
       int n;
       char ch;
       std::cout<<"input n: ";
       std::cin>>n;
       std::cout<<std::endl;
       std::string str(n,'c');
       std::chrono::high_resolution_clock::time_point t1,t2;
       t1=std::chrono::high_resolution_clock::now();
       for(int i=0;i<n;++i){
           ch=str[i];
       }
       t2=std::chrono::high_resolution_clock::now();
       std::cout<<"for: "<<std::chrono::duration_cast<std::chrono::microseconds>(t2-t1).count()<<" microseconds"<<std::endl;
       t1=std::chrono::high_resolution_clock::now();
       for(auto t:str){
           ch=t;
       }
       t2=std::chrono::high_resolution_clock::now();
       std::cout<<"auto for: "<<std::chrono::duration_cast<std::chrono::microseconds>(t2-t1).count()<<" microseconds"<<std::endl;
       t1=std::chrono::high_resolution_clock::now();
       for(auto &t:str){
           ch=t;
       }
       t2=std::chrono::high_resolution_clock::now();
       std::cout<<"auto& for: "<<std::chrono::duration_cast<std::chrono::microseconds>(t2-t1).count()<<" microseconds"<<std::endl;
       t1=std::chrono::high_resolution_clock::now();
       for(const auto &t:str){
           ch=t;
       }
       t2=std::chrono::high_resolution_clock::now();
       std::cout<<"const auto& for: "<<std::chrono::duration_cast<std::chrono::microseconds>(t2-t1).count()<<" microseconds"<<std::endl;
       return 0;
   }
   

   $ ./fortest.out
   input n: 1000000000
   
   for: 9104902 microseconds
   auto for: 5651329 microseconds
   auto& for: 5445633 microseconds
   const auto& for: 5508764 microseconds
   

   但是切记范围for不能改变循环序列的大小！！！

3. string的c_str()返回的char*字符串有可能不保证一直有效！！！