Wrong output with map C++ - c++

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

You are getting undefined behaviour, since m[make_pair(4294967292,4294967291)] and m[make_pair(2147483645,2147483643)] are probably different objects (unless something very strange with overflow wrapping is happening).

Answer 3

You may have a typo with the literals - just do key = make_pair(...u,...u).
Your hash functor probably has an overflow - unsigned int is probably 32-bit on your system).
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.