该文档举例说明了multimap的查找和删除元素的使用

该文档举例说明了multimap的查找和删除元素的使用。

其中，在使用迭代器遍历元素的时候，如果使用了删除迭代器的操作，那么需要小心迭代器失效的情况。

/*
功能说明：
    multimap的查找和删除元素的使用举例
代码说明：
    使用multimap的equal_range()方法来查找符合指定key的元素。使用erase来删除指定key的元素。
实现方式：
限制条件或者存在的问题：
    无
*/

#include <iostream>
#include <string>
#include <map>

using namespace std;

class CData
{
public:
    CData()
    {
        sequence = 0;
        this->remark = "default string";

        cout << "CData()	" << toString() << "	" << endl;
    }

    CData(int i, string &s)
    {
        this->sequence = i;
        this->remark = s;

        cout << "CData(int i,string &s)	" << toString() << "	" << endl;
    }

    CData(const CData &data)
    {
        this->sequence = data.sequence;
        this->remark = data.remark;

        cout << "CData(const CData &data)	" << toString() << "	" << endl;
    }

    CData operator = (const CData &data)
    {
        this->sequence = data.sequence;
        this->remark = data.remark;

        cout << "CData operator = (const CData &data)	" << toString() << "	" << endl;

        return *this;
    }

    void setSequence(const int i)
    {
        this->sequence = i;
    }

    void setRemark(const string &s)
    {
        this->remark = s;
    }

    string toString() const
    {
        char tmp[2048] = { 0 };
        sprintf(tmp, "[sequence:%d | remark:%s @ %p]", this->sequence, this->remark.c_str(),this);

        //此处应该有内存复制操作，所以不用担心返回后tmp数组所指向的内存被修改或者不存在的情况。
        return tmp;
    }

    ~CData()
    {
        cout << "~CData()	" << this << endl;
    }
protected:
private:
    int sequence;
    string remark;
};

int main(int argc, char ** argv)
{
    cout << "process begin @[" << (void*)&main << "]" << endl;

    multimap<string, CData *> multData;
    string strClassName = "CData11";
    CData *p11 = new CData(11, strClassName);
    strClassName = "CData12";
    CData *p12 = new CData(12, strClassName);
    strClassName = "CData13";
    CData *p13 = new CData(13, strClassName);
    strClassName = "CData23";
    CData *p23 = new CData(23, strClassName);
    strClassName = "CData24";
    CData *p24 = new CData(24, strClassName);
    strClassName = "CData26";
    CData *p26 = new CData(26, strClassName);

    multData.insert(make_pair("1", p11));
    multData.insert(make_pair("2", p23));
    multData.insert(make_pair("1", p13));
    multData.insert(make_pair("2", p24));
    multData.insert(make_pair("1", p12));
    multData.insert(make_pair("2", p26));

    cout << "multData.size()	" << multData.size() << endl;

    cout << "show all msg" << endl;
    // multimap在创建的时候，已经将元素根据key的顺序排好了（前提是key支持<比较）。
    for (multimap<string, CData*>::iterator itr = multData.begin(); itr != multData.end(); itr++)
    {
        cout << itr->first << "	" << (itr->second)->toString() << endl;
    }

    cout << "find 2" << endl;
    pair<multimap<string, CData*>::iterator, multimap<string, CData*>::iterator> pitr2 = multData.equal_range("2");
    for (multimap<string, CData*>::iterator itr = pitr2.first; itr != pitr2.second; itr++)
    {
        cout << itr->first << "	" << (itr->second)->toString() << endl;
    }

    multData.erase("2");

    pitr2 = multData.equal_range("2");
    if (pitr2.first == pitr2.second)
    {
        cout << "can not find key 2 after delete key 2" << endl;
    }

    cout << "show all msg after delete 2" << endl;
    for (multimap<string, CData*>::iterator itr = multData.begin(); itr != multData.end(); itr++)
    {
        cout << itr->first << "	" << (itr->second)->toString() << endl;
    }

    multData.insert(make_pair("1", p11));
    multData.insert(make_pair("2", p23));
    multData.insert(make_pair("1", p13));
    multData.insert(make_pair("2", p24));
    multData.insert(make_pair("1", p12));
    multData.insert(make_pair("2", p26));

    cout << "find element by key using another way" << endl;
    for (multimap<string, CData*>::iterator itr_find = multData.lower_bound("1"); itr_find != multData.upper_bound("1"); itr_find++)
    {
        cout << itr_find->first << "	" << (itr_find->second)->toString() << endl;
    }

    cout << "delete  key 1 by iterator" << endl;
    // 这是一种错误的迭代器删除元素的方式。
    /*
    for (multimap<string, CData*>::iterator itr = multData.begin(); itr != multData.end(); itr++)
    {
        cout << itr->first << "	" << (itr->second)->toString() << endl;
        if (itr->first == "1")
        {
            multData.erase(itr);
        }
    }
    */

    // 正确的使用迭代器删除元素的方式。
    for (multimap<string, CData*>::iterator itr = multData.begin(); itr != multData.end(); )
    {
        if (itr->first == "1")
        {
            cout <<"find key 1. "<< itr->first << "	" << (itr->second)->toString() << endl;
            itr = multData.erase(itr);
        }
        else
        {
            cout << "can not find key 1. " << itr->first << "	" << (itr->second)->toString() << endl;
            itr++;
        }
    }

    cout << "show all msg after erase key 1" << endl;
    for (multimap<string, CData*>::iterator itr = multData.begin(); itr != multData.end(); itr++)
    {
        cout << itr->first << "	" << (itr->second)->toString() << endl;
    }

    delete p11;
    delete p12;
    delete p13;
    delete p23;
    delete p24;
    delete p26;

    return 0;
}

程序的输出结果：

process begin @[00B2188E]
CData(int i,string &s)  [sequence:11 | remark:CData11 @ 011FCFD0]
CData(int i,string &s)  [sequence:12 | remark:CData12 @ 011FD020]
CData(int i,string &s)  [sequence:13 | remark:CData13 @ 011FD8C0]
CData(int i,string &s)  [sequence:23 | remark:CData23 @ 011FE448]
CData(int i,string &s)  [sequence:24 | remark:CData24 @ 011FE6C8]
CData(int i,string &s)  [sequence:26 | remark:CData26 @ 011FE808]
multData.size() 6
show all msg
1       [sequence:11 | remark:CData11 @ 011FCFD0]
1       [sequence:13 | remark:CData13 @ 011FD8C0]
1       [sequence:12 | remark:CData12 @ 011FD020]
2       [sequence:23 | remark:CData23 @ 011FE448]
2       [sequence:24 | remark:CData24 @ 011FE6C8]
2       [sequence:26 | remark:CData26 @ 011FE808]
find 2
2       [sequence:23 | remark:CData23 @ 011FE448]
2       [sequence:24 | remark:CData24 @ 011FE6C8]
2       [sequence:26 | remark:CData26 @ 011FE808]
can not find key 2 after delete key 2
show all msg after delete 2
1       [sequence:11 | remark:CData11 @ 011FCFD0]
1       [sequence:13 | remark:CData13 @ 011FD8C0]
1       [sequence:12 | remark:CData12 @ 011FD020]
find element by key using another way
1       [sequence:11 | remark:CData11 @ 011FCFD0]
1       [sequence:13 | remark:CData13 @ 011FD8C0]
1       [sequence:12 | remark:CData12 @ 011FD020]
1       [sequence:11 | remark:CData11 @ 011FCFD0]
1       [sequence:13 | remark:CData13 @ 011FD8C0]
1       [sequence:12 | remark:CData12 @ 011FD020]
delete  key 1 by iterator
find key 1. 1   [sequence:11 | remark:CData11 @ 011FCFD0]
find key 1. 1   [sequence:13 | remark:CData13 @ 011FD8C0]
find key 1. 1   [sequence:12 | remark:CData12 @ 011FD020]
find key 1. 1   [sequence:11 | remark:CData11 @ 011FCFD0]
find key 1. 1   [sequence:13 | remark:CData13 @ 011FD8C0]
find key 1. 1   [sequence:12 | remark:CData12 @ 011FD020]
can not find key 1. 2   [sequence:23 | remark:CData23 @ 011FE448]
can not find key 1. 2   [sequence:24 | remark:CData24 @ 011FE6C8]
can not find key 1. 2   [sequence:26 | remark:CData26 @ 011FE808]
show all msg after erase key 1
2       [sequence:23 | remark:CData23 @ 011FE448]
2       [sequence:24 | remark:CData24 @ 011FE6C8]
2       [sequence:26 | remark:CData26 @ 011FE808]
~CData()        011FCFD0
~CData()        011FD020
~CData()        011FD8C0
~CData()        011FE448
~CData()        011FE6C8
~CData()        011FE808

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原文地址：https://www.cnblogs.com/babyha/p/6445172.html