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 0;
}

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 0;
}

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 0;
}

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