C++11 STL容器新用法

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/HERO_CJN/article/details/82254206

一、常用

1、using用法

using Vector = Eigen::Matrix<FloatType, 3, 1>;
// 类型别名，等同于
typedef Eigen::Matrix<FloatType, 3, 1> Vector;

2、std::move() 使用

通过std::move，可以避免不必要的拷贝操作。

std::move是为性能而生。

std::move是将对象的状态或者所有权从一个对象转移到另一个对象，只是转移，没有内存的搬迁或者内存拷贝。

#include <iostream>
#include <utility>
#include <vector>
#include <string>
 
int main()
{
    std::string str = "Hello";
    std::vector<std::string> v;
 
    // uses the push_back(const T&) overload, which means 
    // we'll incur the cost of copying str
    v.push_back(str);
    std::cout << "After copy, str is \"" << str << "\"\n";
 
    // uses the rvalue reference push_back(T&&) overload, 
    // which means no strings will be copied; instead, the contents
    // of str will be moved into the vector.  This is less
    // expensive, but also means str might now be empty.
    v.push_back(std::move(str));
    std::cout << "After move, str is \"" << str << "\"\n";
 
    std::cout << "The contents of the vector are \"" << v[0]
                                         << "\", \"" << v[1] << "\"\n";
}

输出：

After copy, str is "Hello"
After move, str is ""
The contents of the vector are "Hello", "Hello"

二、map

1、std::map::at

//Map中元素取值主要有at和[]两种操作，at会作下标检查，而[]不会。
	std::cout<< _map.at(100).c_str()<< std::endl;//使用at会进行关键字检查，因为没有100因此该语句会报错
	std::cout << _map.at(4).c_str() << std::endl;//因为已经有4了，不会报错
	
	std::cout << _map[300].c_str() << std::endl;//ID_Name中没有关键字200，使用[]取值会导致插入，因此不会报错，但打印结果为空

// map::at
#include <iostream>
#include <string>
#include <map>

int main ()
{
  std::map<std::string,int> mymap = {
                { "alpha", 0 },
                { "beta", 0 },
                { "gamma", 0 } };

  mymap.at("alpha") = 10;
  mymap.at("beta") = 20;
  mymap.at("gamma") = 30;

  for (auto& x: mymap) {
    std::cout << x.first << ": " << x.second << '\n';
  }

  return 0;
}

输出：  

alpha: 10
beta: 20
gamma: 30

三、queue

1、std::deque::pop_front

删除deque容器中的第一个元素

// deque::pop_front
#include <iostream>
#include <deque>

int main ()
{
  std::deque<int> mydeque;

  mydeque.push_back (100);
  mydeque.push_back (200);
  mydeque.push_back (300);

  std::cout << "Popping out the elements in mydeque:";
  while (!mydeque.empty())
  {
    std::cout << ' ' << mydeque.front();
    mydeque.pop_front();
  }

  std::cout << "\nThe final size of mydeque is " << int(mydeque.size()) << '\n';

  return 0;
}

输出：  

Popping out the elements in mydeque: 100 200 300
The final size of mydeque is 0

2、std::queue::front

返回位于队列开头的第一个元素的引用。

// queue::front
#include <iostream>       // std::cout
#include <queue>          // std::queue

int main ()
{
  std::queue<int> myqueue;

  myqueue.push(77);
  myqueue.push(16);

  myqueue.front() -= myqueue.back();    // 77-16=61

  std::cout << "myqueue.front() is now " << myqueue.front() << '\n';

  return 0;
}

输出： 

myqueue.front() is now 61

3、std::queue::back

返回队列最后一个元素的引用。

// queue::back
#include <iostream>       // std::cout
#include <queue>          // std::queue

int main ()
{
  std::queue<int> myqueue;

  myqueue.push(12);
  myqueue.push(75);   // this is now the back

  myqueue.back() -= myqueue.front();

  std::cout << "myqueue.back() is now " << myqueue.back() << '\n';

  return 0;
}

输出：  

myqueue.back() is now 63