std::vector删除元素迭代器以及内存的变化

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/sinat_39088557/article/details/81532895

 std::vector是顺序容器，当用erase成员函数删除一个迭代器指向的元素时，会自动移动（不是std::move）后面的元素到前面来，而迭代器的指向不变（如果是std::map、std::list这类关联型容器，迭代器会失效），并且不会回收内存（也就是capacity不会变）。当用swap成员函数跟自身的拷贝交换时，才能释放多余内存。

代码如下，自定义一个foo类以监视构造、赋值、复制、移动、析构等情况。

#include <iostream>
#include <vector>
class foo{
    int a;
public:
    foo(int x=0):a(x){std::cout<<a<<" is constructed\n";}
    foo(foo &&from){a=std::move(from.a);std::cout<<a<<" is moved\n";}
    foo(const foo& from){a=from.a;std::cout<<a<<" is copied\n";}
    foo& operator=(const foo&from){
        std::cout<<a<<" is assigned by "<<from<<"\n";
        if(this == &from)return *this;
        a=from.a;
        return *this;
    }
    foo& operator=(foo &&from){
        std::cout<<a<<" is move-assigned by "<<from<<"\n";
        a=std::move(from.a);
        return *this;
    }
    friend std::ostream &operator<<(std::ostream&o,const foo&f){o<<f.a;return o;}
    operator int(){return a;}
    ~foo(){std::cout<<a<<" is destructed\n";}
};
int main(){
/*1*/std::vector<foo> vec = {0,0,1,2};
    std::cout<<"Go through:\n";
    std::cout<<"Capacity:"<<vec.capacity()<<std::endl;
    for(auto& v:vec) /*printf("%d\n",v);*/std::cout<<v<<std::endl;
    std::cout<<"Erase:\n";
    for(std::vector<foo>::iterator iter=vec.begin();
 /*2*/   iter!=vec.end();/*iter++*/){
        if(*iter == 0){
            std::cout<<"Erasing:\n";
/*3*/         vec.erase(iter);
            std::cout<<"Erased.\n";
       }
        else/* printf("%d\n",*iter);*/std::cout<<*iter++<<",";
    }
    std::cout<<"\nGo through again:\n";
    std::cout<<"Capacity:"<<vec.capacity()<<std::endl;
    for(auto& v:vec) /*printf("%d\n",v);*/std::cout<<v<<std::endl;
    std::cout<<"Swapping:\n"<<std::endl;
/*4*/    std::vector<foo>(vec).swap(vec);
    std::cout<<"Capacity after swapping:"<<vec.capacity()<<std::endl;
   return 0;
}

运行情况如下（在VS 2010 和 VS 2017上跑会出错，在Linux下没问题。不知道为什么这篇文章不能继续加字了。。。