c++ - 为长值数组重载 operator<

Question

我对 operator< 重载的实现对于我围绕 long 值数组的包装类无法正常工作。

当在 std::map 中使用此包装器类作为键时，如果我混合不同长度的数组，调用 at/find 函数将不起作用。

const oid oid1[] = { 1,3,6,1,4,1,42570,1,3,1 };
oid_wrapper wrapper1(oid1, 10);
const oid oid2[] = { 1,3,6,1,4,1,42570,1,3,1,9,1 };
oid_wrapper wrapper1(oid2, 12);

当先添加wrapper1再添加wrapper2到map时，总是会用at/find找到wrapper1，但是对于wrapper2它会抛出一个out_of_range exception with at和iterator.end with find。
当先添加 wrapper2 再添加 wrapper1 到 map 时，它总是会用 at/find 找到 wrapper2，但是对于 wrapper1 它会抛出一个 out_of_range exception with at and iterator.end with find。

oid类型来自Net-SNMP，实际上是unsigned long int。
那么我的实现缺失或完全错误的是什么？

#include <ostream>
#include <sstream>
#include <net-snmp/net-snmp-config.h>
#include <net-snmp/net-snmp-includes.h>

class oid_wrapper {
public:
    oid_wrapper(const oid* _oid_value, int _oid_length) : oid_value(0), oid_length  (_oid_length) {
        oid_value = snmp_duplicate_objid(_oid_value, oid_length);
    }

    const oid* get_oid() const { return oid_value; }
    const int& get_length() const { return oid_length; }

    /**
     * copy operator overloader
     */
    oid_wrapper& operator=(const oid_wrapper& wrapper) {
        oid_value = snmp_duplicate_objid(wrapper.get_oid(), wrapper.get_length());
        oid_length = wrapper.get_length();

        return *this;
    }

    /**
     * equal operator overloader
     */
    bool operator==(const oid_wrapper& rhs) const {
        const oid* rhs_oid = rhs.get_oid();
        const int rhs_length = rhs.get_length();

        if (netsnmp_oid_equals(oid_value, oid_length, rhs_oid, rhs_length) == 0) {
            return true;
        } else {
            return false;
        }
    }

    /**
     * less-than operator overloader
     */
    bool operator<(const oid_wrapper& rhs) const {
        const oid* rhs_oid = rhs.get_oid();
        const int rhs_length = rhs.get_length();

        if (oid_length < rhs_length) { return true; }

        for (int i = 0; i < oid_length; i++) {
            if (oid_value[i] < rhs_oid[i]) { return true; }
        }

        return false;
    }

    const long getLastNumberInOid() const { return oid_value[oid_length - 1]; }

    bool operator!=(const oid_wrapper& rhs) { return !operator==(rhs); }
    bool operator>(const oid_wrapper& rhs) { return !operator<(rhs); }
    bool operator<=(const oid_wrapper& rhs) { return !operator>(rhs); }
    bool operator>=(const oid_wrapper& rhs) { return !operator<(rhs); }

private:
    const oid* oid_value;
    int oid_length;
};

这是我设计的测试类，用于检查它是否有效:

#include <iostream>
#include <ostream>
#include <string>
#include <map>
#include <stdexcept>

#include <net-snmp/net-snmp-config.h>
#include <net-snmp/net-snmp-includes.h>

#include "oid_wrapper.h"

using namespace std;

int main(int argc, char **argv) {

    cout << "Testing first set of OID" << endl;
    const oid oid1[] = { 1,3,6,1,4,1,40850,1,3 };
    const oid_wrapper parent1 (oid1, 9);
    const oid_wrapper parent1_child1 (parent1, 40);
    const oid_wrapper parent1_child2 (parent1, 41);
    const oid_wrapper parent1_child3 (parent1, 42);
    const oid_wrapper parent1_child4 (parent1, 43);

    map<oid_wrapper, string> test;
    test.insert(make_pair(parent1_child1, "parent1_child1"));
    test.insert(make_pair(parent1_child2, "parent1_child2"));
    test.insert(make_pair(parent1_child3, "parent1_child3"));
    test.insert(make_pair(parent1_child4, "parent1_child4"));
    if (test.size() != 4) { cout << "FAIL" << endl; }
    cout << "Number of OID in list: " << test.size() << endl;

    const oid_wrapper parent11 (oid1, 9);
    const oid_wrapper parent1_child11 (parent11, 40);
    try {
        string teststring = test.at(parent1_child11);
        cout << "Success" << endl;
    } catch (out_of_range& e) {
        cout << "Failure" << endl;
    }

    const oid_wrapper parent1_child12 (parent11, 41);
    try {
        string teststring = test.at(parent1_child12);
        cout << "Success" << endl;
    } catch (out_of_range& e) {
        cout << "Failure" << endl;
    }

    const oid_wrapper parent1_child13 (parent11, 42);
    try {
        string teststring = test.at(parent1_child13);
        cout << "Success" << endl;
    } catch (out_of_range& e) {
        cout << "Failure" << endl;
    }

    const oid_wrapper parent1_child14 (parent11, 43);
    try {
        string teststring = test.at(parent1_child14);
        cout << "Success" << endl;
    } catch (out_of_range& e) {
        cout << "Failure" << endl;
    }

    cout << "Testing second set of OID" << endl;
    const oid oid2[] = { 1,3,6,1,4,1,40850,2,1,9,1 };
    const oid_wrapper parent2 (oid2, 11);
    const oid_wrapper parent2_child1 (parent2, 1);
    const oid_wrapper parent2_child2 (parent2, 2);
    const oid_wrapper parent2_child3 (parent2, 3);
    const oid_wrapper parent2_child4 (parent2, 4);

    test.insert(make_pair(parent2_child1, "parent2_child1"));
    test.insert(make_pair(parent2_child2, "parent2_child2"));
    test.insert(make_pair(parent2_child3, "parent2_child3"));
    test.insert(make_pair(parent2_child4, "parent2_child4"));
    if (test.size() != 8) { cout << "FAIL" << endl; }
    cout << "Number of OID in list: " << test.size() << endl;

    const oid_wrapper parent21 (oid2, 11);
    const oid_wrapper parent1_child21 (parent21, 1);
    try {
        string teststring = test.at(parent1_child21);
        cout << "Success" << endl;
    } catch (out_of_range& e) {
        cout << "Failure" << endl;
    }

    const oid_wrapper parent1_child22 (parent21, 2);
    try {
        string teststring = test.at(parent1_child22);
        cout << "Success" << endl;
    } catch (out_of_range& e) {
        cout << "Failure" << endl;
    }

    const oid_wrapper parent1_child23 (parent21, 3);
    try {
        string teststring = test.at(parent1_child23);
        cout << "Success" << endl;
    } catch (out_of_range& e) {
        cout << "Failure" << endl;
    }

    const oid_wrapper parent1_child24 (parent21, 4);
    try {
        string teststring = test.at(parent1_child24);
        cout << "Success" << endl;
    } catch (out_of_range& e) {
        cout << "Failure" << endl;
    }

    //#################################################

    //Adding 100 OIDs
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2,1,9,1 };
        const oid_wrapper wrapper(newoid, 11, i);
        test.insert(make_pair(wrapper, "wrapper"));
    }

    //Query for the 100 added OIDs
    int run1 = 0;
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2,1,9,1 };
        const oid_wrapper wrapper(newoid, 11, i);

        try {
            string teststring = test.at(wrapper);
            run1++;
        } catch (out_of_range& e) {
            //FAIL
        }
    }
    if (run1 == 100) { cout << "Run 1 Success" << endl; }
    else { cout << "Run 1: " << run1 << endl; }

    //#################################################

    //Adding 100 OIDs
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,1,3 };
        const oid_wrapper wrapper(newoid, 9, i);
        test.insert(make_pair(wrapper, "wrapper"));
    }

    //Query for the 100 added OIDs
    int run2 = 0;
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,1,3 };
        const oid_wrapper wrapper(newoid, 9, i);

        try {
            string teststring = test.at(wrapper);
            run2++;
        } catch (out_of_range& e) {
            //FAIL
        }
    }
    if (run2 == 100) { cout << "Run 2 Success" << endl; }
    else { cout << "Run 2: " << run2 << endl; }

    //#################################################

    //Adding 100 OIDs
    for (int i = 0; i < 10; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2 };
        const oid_wrapper wrapper(newoid, 8, i);
        test.insert(make_pair(wrapper, "wrapper"));
    }

    //Query for the 100 added OIDs
    int run3 = 0;
    for (int i = 0; i < 10; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2 };
        const oid_wrapper wrapper(newoid, 8, i);

        try {
            string teststring = test.at(wrapper);
            run3++;
        } catch (out_of_range& e) {
            //FAIL
        }
    }
    if (run3 == 10) { cout << "Run 3 Success" << endl; }
    else { cout << "Run 3: " << run3 << endl; }

    //#################################################

    //Adding 100 OIDs
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2,2,5,1 };
        const oid_wrapper wrapper(newoid, 11, i);
        test.insert(make_pair(wrapper, "wrapper"));
    }

    //Query for the 100 added OIDs
    int run4 = 0;
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2,2,5,1 };
        const oid_wrapper wrapper(newoid, 11, i);

        try {
            string teststring = test.at(wrapper);
            run4++;
        } catch (out_of_range& e) {
            //FAIL
        }
    }
    if (run4 == 100) { cout << "Run 4 Success" << endl; }
    else { cout << "Run 4: " << run4 << endl; }

    //#################################################

    //Adding 100 OIDs
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2,3,5,2 };
        const oid_wrapper wrapper(newoid, 11, i);
        test.insert(make_pair(wrapper, "wrapper"));
    }

    //Query for the 100 added OIDs
    int run5 = 0;
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2,3,5,2 };
        const oid_wrapper wrapper(newoid, 11, i);

        try {
            string teststring = test.at(wrapper);
            run5++;
        } catch (out_of_range& e) {
            //FAIL
        }
    }
    if (run5 == 100) { cout << "Run 5 Success" << endl; }
    else { cout << "Run 5: " << run5 << endl; }

    //#################################################

    //Adding 100 OIDs
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2,2,5,2 };
        const oid_wrapper wrapper(newoid, 10, i);
        test.insert(make_pair(wrapper, "wrapper"));
    }

    //Query for the 100 added OIDs
    int run6 = 0;
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2,2,5,2 };
        const oid_wrapper wrapper(newoid, 10, i);

        try {
            string teststring = test.at(wrapper);
            run6++;
        } catch (out_of_range& e) {
            //FAIL
        }
    }
    if (run6 == 100) { cout << "Run 6 Success" << endl; }
    else { cout << "Run 6: " << run6 << endl; }

    //#################################################

    //Adding 100 OIDs
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2,5,2,3,5 };
        const oid_wrapper wrapper(newoid, 12, i);
        test.insert(make_pair(wrapper, "wrapper"));
    }

    //Query for the 100 added OIDs
    int run7 = 0;
    for (int i = 0; i < 100; i++) {
        const oid newoid[] = { 1,3,6,1,4,1,40850,2,5,2,3,5 };
        const oid_wrapper wrapper(newoid, 12, i);

        try {
            string teststring = test.at(wrapper);
            run7++;
        } catch (out_of_range& e) {
            //FAIL
        }
    }
    if (run7 == 100) { cout << "Run 7 Success" << endl; }
    else { cout << "Run 7: " << run7 << endl; }
}

Answer 1

问题可能是您的 operator<不是反对称的。想象一下这段代码:

const oid oid1[] = {5};
oid_wrapper wrapper1(oid1, 1);

const oid oid2[] = {1, 4};
oid_wrapper wrapper2(oid2, 2);

bool b1 = wrapper1 < wrapper2;
bool b2 = wrapper2 < wrapper1;

b1是true , 因为 wrapper1 的长度小于 wrapper2 的长度.

b2也是true , 因为 wrapper2 中的第一个 oid小于wrapper1中的.

你必须确保operator<是总排序。它应该返回 false如果oid_length > rhs_length .

此外，您不能实现 operator>作为! < ，因为这不适用于相等的值。运营商最好的选择是实现 operator==和 operator<并将其余部分表示为(伪代码):

operator!=(a, b)  {! (a == b)}
operator<=(a, b)  {a < b || a == b}
operator>(a, b)   {! (a < b)}
operator>=(a, b)  {! (a < b)}