C++ unordered_set 使用struct或者class

雨点打透心脏的1/2处 2022-06-13 03:45 172阅读 0赞

本文介绍如何将unordered\_set应用于strcut或者class

先看看其声明

template < class Key,                        // unordered_set::key_type/value_type
               class Hash = hash<Key>,           // unordered_set::hasher
               class Pred = equal_to<Key>,       // unordered_set::key_equal
               class Alloc = allocator<Key>      // unordered_set::allocator_type
               > class unordered_set;

对于没有特殊需求的non-POD的int、string等类型来说，实用默认的模板参数即可，当我们要使用struct或者class等数据结构作为输入时，则需要一些特定的步骤。

假设我们需要为下面的结构体设置unordered\_set容器：

struct record{
        string num;
        string file;
        mutable int count;
        record(string n,string f):num(n),file(f),count(1){}
    };

### **1.指定hasher** ###

将作为模板第二个参数

struct record_hash{
        size_t operator()(const struct record& _r) const {
        string tmp=_r.file+_r.num;
        return std::hash<string>()(tmp);
        }
    };

### **2.指定符合hash函数的`operator==`重载** ###

bool operator==(const struct  record & X,const struct record & Y)
    {
        return hash<string>()(X.num+X.file)==hash<string>()(Y.num+Y.file);
        //or
        //return (Y.num==X.num)&&(Y.file==X.file);
    }

完成这两步之后，`struct record`以`unordered_set`为容器了：

struct record{
        string num;
        string file;
        mutable int count;
        record(string n,string f):num(n),file(f),count(1){}
    };
    bool operator==(const struct  record & X,const struct record & Y)
    {
        //return hash<string>()(X.num+X.file)==hash<string>()(Y.num+Y.file);
        return (Y.num==X.num)&&(Y.file==X.file);
    }
    
    
    struct record_hash{
        size_t operator()(const struct record& _r) const {
        string tmp=_r.file+_r.num;
        return std::hash<string>()(tmp);
        }
    };
    int main()
    {
        unordered_set<record,record_hash> records;
        records.insert(record("zhang","xiao"));
        record r("zhang","xiao");
        auto it = records.find(r);
        cout<<it->num<<" "<<it->file<<endl;
        return 0;
    }

## **参考** ##

[https://stackoverflow.com/questions/15869066/inserting-into-unordered-set-with-custom-hash-function][https_stackoverflow.com_questions_15869066_inserting-into-unordered-set-with-custom-hash-function]

[https_stackoverflow.com_questions_15869066_inserting-into-unordered-set-with-custom-hash-function]: https://stackoverflow.com/questions/15869066/inserting-into-unordered-set-with-custom-hash-function