C++——STL内存清除

1.vector元素的清除

看代码。在vector中添加若干元素，然后clear()

 #include<iostream>
 #include<list>
 #include<vector>
 #include<iterator>
 #include<string>
 using std::vector;
 using std::list;
 using std::iterator;
 using std::string;
 using std::cout;
 using std::endl;

 int main()
 {
     vector<string> vecStr;
     string pStr1 = "Robb";
     vecStr.push_back(pStr1);
     string pStr2 = "Bran";
     vecStr.push_back(pStr2);
     string pStr3 = "Snow";
     vecStr.push_back(pStr3);
     string pStr4 = "Sansa";
     vecStr.push_back(pStr4);
     string pStr5 = "Arya";
     vecStr.push_back(pStr5);

     /*打印*/
     for(auto unit:vecStr)
     {
         cout<<"-----"<<unit<<"-----"<<endl;
     }
     /*释放前vector的容量*/
     cout<<"释放前vector的容量"<<endl;

     cout<<"vecStr.size()    :"<<vecStr.size()<<endl;
     cout<<"vecStr.capacity():"<<vecStr.capacity()<<endl;

     /*释放*/
     vecStr.clear();
     cout<<endl<<"clear后vector的容量"<<endl;

     cout<<"vecStr.size()    :"<<vecStr.size()<<endl;
     cout<<"vecStr.capacity():"<<vecStr.capacity()<<endl;
     system("pause");
     return ;
 }

输出结果如图：

size是变小了，但是capacity并没有变小。

我们加一下代码

     /*swap*/
     vector<string>().swap(vecStr);
     cout<<endl<<"swap后vector的容量"<<endl;

     cout<<"vecStr.size()    :"<<vecStr.size()<<endl;
     cout<<"vecStr.capacity():"<<vecStr.capacity()<<end

使用swap之后，就清空了capacity。

这是为什么呢？

vector由于是一个不定长存储的数据结构，每一次分配的大小都是比面前输入的数据个数略大一点（实际上也并不准确，参看2)code中注释，是介于2^n与2^（n+1)之间），所以每一次push_back()且发现当被分配的存储空间已装满数据时，都是将包含现有数据的vector进行拷贝，进入一个更大一点的vector，而原有的vector就会被自然销毁，我们用.swap()释放内存的原理其实是相似的，即手动进行了一次人工拷贝的操作。（https://blog.csdn.net/a272846945/article/details/51182144 ）

由于vector的空间是阶梯递增式管理的，而且基本只增不减，也就是说，虽然调用remove、erase或者clear等方法（他们会调用所存元素对象的析构函数），确实会释放掉一些内存，但是，容器之前分配的空间仍不会被回收，大小不变，仍旧不能被其他程序使用，这是由STL的内存管理机制决定的，目的是为了提高效率。 （https://www.cnblogs.com/EE-NovRain/archive/2012/06/12/2546500.html）

下面有一个调用函数的例子，感觉还是有很多东西

①内存增长方式，指数增长。2^n。数量增大到capacity的时候，整体拷贝，然后析构之前的内存。

②push_back,调用复制构造函数。

 class CUnit
 {
 private:
     /* data */
     string m_name;
 public:

     CUnit(string name)
     {
         m_name = name;
         //cout<<this<<",create"<<endl;
     }
     ~CUnit()
     {
         //delete member
         cout<<this<<",destroy"<<endl;
     }
     CUnit(const CUnit & c)
     {
         //cout<<&c<<",param"<<endl;
         //cout<<this<<",copy"<<endl;
     }
     string& getName(){return m_name;}
 };

 int main()
 {
     vector<CUnit> vecStr;
     CUnit cUnit1 = CUnit("Robb");
     vecStr.push_back(cUnit1);    //此处调用复制构造函数

     cout<<"push one"<<endl;

     cout<<"vecStr.size()    :"<<vecStr.size()<<endl;
     cout<<"vecStr.capacity():"<<vecStr.capacity()<<endl;

     CUnit cUnit2 = CUnit("Bran");//size不够，多次复制构造
     vecStr.push_back(cUnit2);

     cout<<"push two"<<endl;

     cout<<"vecStr.size()    :"<<vecStr.size()<<endl;
     cout<<"vecStr.capacity():"<<vecStr.capacity()<<endl;
     CUnit cUnit3 = CUnit("Snow");//size不够，多次复制构造
     vecStr.push_back(cUnit3);
     CUnit cUnit4 = CUnit("Arya");//size不够，多次复制构造
     vecStr.push_back(cUnit4);
     CUnit cUnit5 = CUnit("Sansa");//size不够，多次复制构造
     vecStr.push_back(cUnit5);
     cout<<"push five"<<endl;
     cout<<"vecStr.size()    :"<<vecStr.size()<<endl;
     cout<<"vecStr.capacity():"<<vecStr.capacity()<<endl;
     cout<<endl;

     /*打印*/
     for(auto unit:vecStr)
     {
         cout<<"-----"<<unit.getName()<<"-----"<<endl;
     }
     /*释放前vector的容量*/
     cout<<"释放前vector的容量"<<endl;

     cout<<"vecStr.size()    :"<<vecStr.size()<<endl;
     cout<<"vecStr.capacity():"<<vecStr.capacity()<<endl;

     /*释放*/
     vecStr.clear();
     cout<<endl<<"clear后vector的容量"<<endl;

     cout<<"vecStr.size()    :"<<vecStr.size()<<endl;
     cout<<"vecStr.capacity():"<<vecStr.capacity()<<endl;

     /*swap*/
     vector<CUnit>().swap(vecStr);
     cout<<endl<<"swap后vector的容量"<<endl;

     cout<<"vecStr.size()    :"<<vecStr.size()<<endl;
     cout<<"vecStr.capacity():"<<vecStr.capacity()<<endl;

     system("pause");
     return ;
 }

2.list的清内存

 class CUnit
 {
 private:
     /* data */
     string m_name;
 public:

     CUnit(string name)
     {
         m_name = name;
         //cout<<this<<",create"<<endl;
     }
     ~CUnit()
     {
         //delete member
         cout<<this<<",destroy"<<endl;
     }
     CUnit(const CUnit & c)
     {
         //cout<<&c<<",param"<<endl;
         //cout<<this<<",copy"<<endl;
     }
     string getName(){cout<<this<<",copy"<<endl;return m_name;}
 };

 int main()
 {
     list<CUnit*> listStr;
     CUnit *cUnit1 = new CUnit("Robb");
     listStr.push_back(cUnit1);    //此处调用复制构造函数

     cout<<"push one"<<endl;
     cout<<"listStr.size()    :"<<listStr.size()<<endl;

     CUnit* cUnit2 = new CUnit("Bran");//size不够，多次复制构造
     listStr.push_back(cUnit2);

     cout<<"push two"<<endl;
     cout<<"listStr.size()    :"<<listStr.size()<<endl;
     CUnit *cUnit3 = new CUnit("Snow");//size不够，多次复制构造
     listStr.push_back(cUnit3);
     CUnit *cUnit4 = new CUnit("Arya");//size不够，多次复制构造
     listStr.push_back(cUnit4);
     CUnit *cUnit5 = new CUnit("Sansa");//size不够，多次复制构造
     listStr.push_back(cUnit5);

     cout<<"push five"<<endl;
     cout<<"listStr.size()    :"<<listStr.size()<<endl;
     cout<<endl;

     /*打印*/
     for(list<CUnit*>::iterator it = listStr.begin(); it!=listStr.end();it++)
     {
         cout<<"-----"<<(*it)->getName()<<"-----"<<endl;
     }
     /*释放前list的容量*/
     cout<<"释放前list的容量"<<endl;
     cout<<"listStr.size()    :"<<listStr.size()<<endl;
     cout<<"~~~~~~~~~~~~~~~~~~~~~~~~"<<endl;
 #if 1              //调用析构函数，清掉了list的内存
     for(list<CUnit*>::iterator it = listStr.begin(); it!=listStr.end();)
     {
          delete *it;
         listStr.erase(it++);
         //cout<<"-----"<<(*it)->getName()<<"-----"<<endl;
     }

     cout<<"释放后list的容量"<<endl;
     cout<<"listStr.size()    :"<<listStr.size()<<endl;
     cout<<"~~~~~~~~~~~~~~~~~~~~~~~~"<<endl;
 #endif

 #if 0              //并不会调用析构函数，只是清掉了list的内存
     /*释放*/
     listStr.clear();
     cout<<endl<<"clear后list的容量"<<endl;
     cout<<"listStr.size()    :"<<listStr.size()<<endl;
     cout<<"~~~~~~~~~~~~~~~~~~~~~~~~"<<endl;

     /*swap*/
     cout<<endl<<"swap后list的容量"<<endl;
     cout<<"listStr.size()    :"<<listStr.size()<<endl;
     cout<<"~~~~~~~~~~~~~~~~~~~~~~~~"<<endl;
 #endif
     system("pause");
     return ;
 }

①自己new的空间，自己delete，然后再释放容器。

②不是new出来的，直接erase、remove和clear即可。这类链式存储，一个元素一个元素递增空间的结构，这些函数可以真正地改变list占用的内存大小。

感觉好多东西啊！今天的结束了！

参考文献：

https://www.cnblogs.com/EE-NovRain/archive/2012/06/12/2546500.html

https://philoscience.iteye.com/blog/1456509

https://blog.csdn.net/a272846945/article/details/51182144

https://blog.csdn.net/HW140701/article/details/76704583