c++ - 如何根据条件对优先级队列使用不同的比较器

Question

我正在处理一个任务，我有一个优先级队列，我希望它像这样工作:

if(field == '0')
    priority_queue<record_t*,vector<record_t*>, CompareRecordID > pq;
else if(field == '1')
    priority_queue<record_t*,vector<record_t*>, CompareRecordNum > pq;
else if(field == '2')
    priority_queue<record_t*,vector<record_t*>, CompareRecordStr > pq;
else if(field == '3')
    priority_queue<record_t*,vector<record_t*>, CompareRecordNumStr > pq;

record_t 在哪里:

typedef struct {
   unsigned int recid;
   unsigned int num;
   char str[STR_LENGTH];
   bool valid;  // if set, then this record is valid
   int blockID; //The block the record belongs too -> Used only for minheap
} record_t;

这意味着，根据函数参数字段，队列将“排序”record_t 的不同字段。但是，我不能在 if 语句中声明队列，因为它显然会给我一个错误“pq 未在此范围内声明”。我能做什么？

Answer 1

您可以使用将比较器对象作为参数的std::priority_queue 构造函数。然后你可以像这样给它一个可配置的比较器:

#include <vector>
#include <queue>
#include <cstring>
#include <iostream>

const int STR_LENGTH = 20;

struct record_t
{
   unsigned int recid;
   unsigned int num;
   char str[STR_LENGTH];
   bool valid;  // if set, then this record is valid
   int blockID; //The block the record belongs too -> Used only for minheap
};

// switchable priority comparator
struct CompareRecord
{
    int field;

    CompareRecord(int field = 0): field(field) {}

    bool operator() (const record_t* lhs, const record_t* rhs) const
    {
        switch(field)
        {
            case 0: return lhs->recid < rhs->recid;
            case 1: return lhs->num < rhs->num;
            case 2: return std::strcmp(lhs->str, rhs->str) < 0;
        }
        return true;
    }
};

int main()
{
    // physical records
    std::vector<record_t> records;

    record_t r;

    r.recid = 1;
    r.num = 1;
    std::strcpy(r.str, "banana");

    records.push_back(r);

    r.recid = 2;
    r.num = 4;
    std::strcpy(r.str, "orange");

    records.push_back(r);

    r.recid = 3;
    r.num = 2;
    std::strcpy(r.str, "apple");

    records.push_back(r);

    // input priority type: 0, 1 or 2

    int field;

    std::cout << "Sort type [0, 1, 2]: " << std::flush;
    std::cin >> field;
    std::cout << '\n';

    // build priority view

    CompareRecord cmp(field);
    std::priority_queue<record_t*, std::vector<record_t*>, CompareRecord> pq(cmp);

    for(auto& r: records)
        pq.push(&r);

    while(!pq.empty())
    {
        std::cout << "rec: " << pq.top()->recid << '\n';
        std::cout << "num: " << pq.top()->num << '\n';
        std::cout << "str: " << pq.top()->str << '\n';
        std::cout << '\n';
        pq.pop();
    }
}

输出:

Sort type [0, 1, 2]: 0

rec: 3
num: 2
str: apple

rec: 2
num: 4
str: orange

rec: 1
num: 1
str: banana