地图C ++输出错误

Question

I wrote the following code for constructing std::pair as key to unordered_map. However, I dont know why I am getting all 0's as output of vector. Can someone please suggest as to where am I going wrong?

struct key_hash
{
    size_t operator()(const std::pair<unsigned,unsigned>& key) const
    {
        return uint64_t((key.first << 32) | key.second);
    }
};

typedef std::unordered_map<std::pair<unsigned,unsigned>, std::vector<unsigned>, key_hash> MyMap;


int main()
{    
    MyMap m;
    vector<unsigned> t;
    t.push_back(4);
    t.push_back(5);
    m[make_pair(4294967292,4294967291)]=t;

    for(vector<unsigned>::iterator i=m[make_pair(4294967292,4294967291)].begin(),j=m[make_pair(2147483645,2147483643)].end();i!=j;++i)
        cout<<"vec="<<(*i)<<"\n";

    cout<<"vector empty. \n";
}

Answer 1

i and j are iterators to 2 different vector's and they cannot be compared. Using debug iterators might catch this under visual studio.

This code: j=m[make_pair(2147483645,2147483643)].end(); will create a new empty vector since the key is different from the previously used one.

Whem initializing j like this: j=m[make_pair(4294967292,4294967291)].end(); the results are fine:

vec=4 vec=5 vector empty.

Answer 2

您将获得不确定的行为，因为m[make_pair(4294967292,4294967291)]和m[make_pair(2147483645,2147483643)]可能是不同的对象（除非发生与溢出包装非常奇怪的事情）。

Answer 3

1. You may have a typo with the literals - just do key = make_pair(...u,...u) .
2. Your hash functor probably has an overflow - unsigned int is probably 32-bit on your system).
3. Your literals probably exceed the maximum value of signed integers, and are not specified to be unsigned.

Answer 4

try changing your hash function to something along those lines:

struct key_hash
 {
     size_t operator()(const std::pair<unsigned,unsigned>& key) const
     {
         uint64_t tmp = key.first;
         tmp = tmp << 32;
         return uint64_t(tmp | key.second);                                                                                                                                                              
      }
 };

I have also added the one instance of the pair so changed the main to :

MyMap m;
vector<unsigned> t;
t.push_back(4);
t.push_back(5);
auto a =  make_pair(4294967292,4294967291);
m[a]=t;

for(vector<unsigned>::iterator i=m[a].begin(),j=m[a].end();i!=j;++i)
    cout<<"vec="<<(*i)<<"\n";

cout<<"vector empty. \n";

This gave me the correct output:

vec=4
vec=5
vector empty.

Answer 5

Tidying up the loop to make it clear (and ignoring the warning about a 32-bit left-shift on a 32-bit value...)

What you are doing is this:

const auto& first_vector = m[make_pair(4294967292,4294967291)];
const auto& second_vector = m[make_pair(2147483645,2147483643)];

for(auto iter = begin(first_vector) ; 
    iter != end(second_vector) ; // <<=== SEE THE PROBLEM?
    ++iter)
{
  // ...
}

Incrementing the iterator of one vector will never yield the end() of a different one so your loop is infinite, until you get a segfault because you've accessed memory that does not belong to you.