相关疑难解决方法(0)

对于此代码:

#include<unordered_map>
#include<iostream>
using namespace std;
struct myhash {
    unsigned operator()(const unsigned&v)const {
        cout<<"Hash function is called:"<<v<<endl;
        return v;
    }
};
unordered_map<unsigned,unsigned,myhash>mp;
int main() {
    for (unsigned i=0;i<3;++i) {
        cout<<"Visiting hash table:"<<i<<endl;
        ++mp[i];
    }
}

使用g ++时,输出并不奇怪:

Visiting hash table:0
Hash function is called:0
Visiting hash table:1
Hash function is called:1
Visiting hash table:2
Hash function is called:2

但MSVC++(2015)的输出震惊了我:

Visiting hash table:0
Hash function is called:0
Hash function is called:0
Visiting hash table:1
Hash function is called:1
Hash function is …