iterators can't access problems properly

问题:

I am trying to access the element of a vector using the iterator. But I get strange outputs.

std::vector<int> ivec{ 7, 6 , 8, 9} ; 

std::vector<int>::iterator beg = ivec.begin(); 
std::vector<int>::iterator last = ivec.end(); 

std::cout << *beg << *last << std::endl; 

But, In the above case the program shows me error: debug assertion failed. Vector iterator not dereferencable. and this error is particularly for *last. If I just print *beg that seems to wrong fine. But can't dereference the last one.

Other problem I get with iterator is during increment.

std::vector<int>::iterator beg = ivec.begin(); 

cout << *(beg++) ;  // in this case it prints me value of 7
cout << *(++beg) ;  // in this case it print me the right value of second place i.e. is 6
cout << *(beg+=1) ; // in this case we also print the second value i.e. 6 

回答1:

For your first example, std::vector<T>::end points to the theoretical element after the actual last element, so dereferencing it does not make sense. It is primarily used for checking when you have got past the end of the vector in a loop.

For your second example, the results are as you would expect:

cout << *(beg++) ; //increments beg after dereferencing
cout << *(++beg) ;  //increments beg before dereferencing
cout << *(beg+=1) ; //dereferences the result of adding one to beg

回答2:

end iterators are not iterators that you can de-reference. They point past the last element into the container. There's a good reason why this needs to be true; but long and short, the end iterator does not actually point to any element. If you want the last element, you need to decrement the end iterator.

回答3:

As it is stated in here,

Return iterator to end, Returns an iterator referring to the past-the-end element in the vector container.

The past-the-end element is the theoretical element that would follow the last element in the vector. It does not point to any element, and thus shall not be dereferenced.

Because the ranges used by functions of the standard library do not include the element pointed by their closing iterator, this function is often used in combination with vector::begin to specify a range including all the elements in the container.

If the container is empty, this function returns the same as vector::begin.