About iterator of containers

Question

I do not know, why does it output 1024?

vector<int> default_container = { 1,2,3,4,5,6,7,78,8,1024 };
cout << *default_container.end() << endl; // 0
default_container.pop_back();

for (auto it : default_container) 
{
    cout << it << ",";
}
cout << endl;

cout << *default_container.end() << endl;   // 1024 why？why？why？why？
cout << *--default_container.end() << endl; // 8

Answer 1

Your program has Undefined behavior!

You are de-referencing the end iterator, at the lines

cout << *default_container.end() << endl;
...
cout << *default_container.end() << endl;

which gives you undefined behavior. Form cppreference.com the std::vector::end, std::vector::cend

Returns an iterator to the element following the last element of the vector. This element acts as a placeholder; attempting to access it results in undefined behavior.

Means, anything can be happened; therefore you shouldn't be relaying on its result and should not be doing it!

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That being said, it looks like that you want to access the last element in the vector. If that the case, for a non-empty vector you have multiple other (safe) choices:

1. Using std::vector::back

   // prints first 8 from last; due to "default_container.pop_back()"  
   std::cout << default_container.back(); 
   

2. Using std::prev

   #include <iterator>
   
   // prints first 8 from last; due to "default_container.pop_back()"  
   std::cout << *std::prev(default_container.end()); 
   

3. Using reverse iterator std::rbegin

   // prints first 8 from last; due to "default_container.pop_back()"  
   std::cout << *std::rbegin(default_container);
   

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As a side note, see: Why is "using namespace std;" considered bad practice?