引用方式访问STL容器潜在隐患

C++ STL map与vector操作详解

最新推荐文章于 2025-11-26 09:00:00 发布

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map<int , vector<int> >& iMap；

如果用引用访问vector里的元素，必须分配内存先，而且分配不够会数据丢失；push_back则不会存在这个问题。

示例代码：

#include <iostream>
#include <vector>
#include <map>
using namespace std;
void fun_print(map<int , vector<int> >& iMap,int flag , const char* str = "null"){
        cout<<str<<endl;
        cout<<"*******"<<endl;
        for(int i = flag; i <iMap.size(); i++){
                vector<int>& ap = iMap[i];
                cout<<"\tlist["<<i<<"],";
                cout<<"list_size="<<ap.size()<<"_\t";
                for(int j = 0; j<ap.size(); j++)
                        cout<<ap[j]<<"\t";
                cout<<endl;
        }
}
int main()
{
        int flag = 0;
        cout<<"___________________________"<<endl;
        map<int,vector<int> > iMap;
        iMap[0].push_back(1);                                                                                                                        
        fun_print(iMap,flag,"push_back");

        flag++;
        cout<<"___________________________"<<endl;
        iMap[1].reserve(2);
        fun_print(iMap,flag,"reserve_2");	//size = 0
        iMap[1].push_back(10);	//size = 1
        fun_print(iMap,flag,"push_back_1");
        iMap[1].push_back(12);	//size = 2
        iMap[1].push_back(15);`//size = 3
        fun_print(iMap,flag,"cout");

        flag++;
        cout<<"___________________________"<<endl;
        iMap[2].resize(2);
        fun_print(iMap,flag,"resize_2");
        iMap[2][0]= 20;	//size = 2
        fun_print(iMap,flag,"[x][0]");
        iMap[2][1]= 22;	//size = 2
        iMap[2][2]= 24;	//size = 2，这个数据丢失
        fun_print(iMap,flag,"resize");
        
        flag++;
        cout<<"___________________________"<<endl;                                                                                                   
        vector<int>& tpp = iMap[3];
        tpp.push_back(1);
        tpp[0] = 30;	//size = 1，如果上行不push_back，会core！ => Segmentation fault (core dumped)
        tpp[1] = 31;	//size = 1，这个数据丢失！
        fun_print(iMap,flag,"push_back&[x]");

        flag++;
        cout<<"___________________________"<<endl;
        iMap[4].resize(2);
        int& app = iMap[4][0];
        app = 42;	//size = 2，第二个数：默认构造0
        fun_print(iMap,flag,"resize&[x][x]");
        cout<<"___________________________"<<endl;
        return 0;
}

运行结果：