what does the error mean when I am compiling c++ w

问题:

using the following code:

#include<iostream>
#include<vector>

using namespace std;

int main()
{
    vector<int> ivec;
    for(vector<int>::size_type ix = 0; ix != 10; ix++)
    {
        ivec.push_back(ix);
    }
    vector<int>::iterator mid = (ivec.begin() + ivec.end()) / 2;
    cout << *mid << endl;
    return 0;
}

I get an error compiling with g++:

iterator_io.cpp: In function `int main()':
iterator_io.cpp:13: error: no match for 'operator+' in '(&ivec)->std::vector<_Tp,               _Alloc>::begin [with _Tp = int, _Alloc = std::allocator<int>]() + (&ivec)->std::vector<_Tp, _Alloc>::end [with _Tp = int, _Alloc = std::allocator<int>]()'
/usr/lib/gcc/x86_64-redhat-linux/3.4.5/../../../../include/c++/3.4.5/bits/stl_iterator.h:654: note: candidates are: __gnu_cxx::__normal_iterator<_Iterator, _Container> __gnu_cxx::__normal_iterator<_Iterator, _Container>::operator+(const typename std::iterator_traits<_Iterator>::difference_type&) const [with _Iterator = int*, _Container = std::vector<int, std::allocator<int> >]
/usr/lib/gcc/x86_64-redhat-linux/3.4.5/../../../../include/c++/3.4.5/bits/stl_bvector.h:261: note:                 std::_Bit_iterator std::operator+(ptrdiff_t, const std::_Bit_iterator&)
/usr/lib/gcc/x86_64-redhat-linux/3.4.5/../../../../include/c++/3.4.5/bits/stl_bvector.h:345: note:                 std::_Bit_const_iterator std::operator+(ptrdiff_t, const std::_Bit_const_iterator&)
/usr/lib/gcc/x86_64-redhat-linux/3.4.5/../../../../include/c++/3.4.5/bits/stl_iterator.h:765: note:                 __gnu_cxx::__normal_iterator<_Iterator, _Container> __gnu_cxx::operator+(typename __gnu_cxx::__normal_iterator<_Iterator, _Container>::difference_type, const __gnu_cxx::__normal_iterator<_Iterator, _Container>&) [with _Iterator = int*, _Container = std::vector<int, std::allocator<int> >]

I know the ivec.end() can not be used as a normal vector element. But I can't understand what the error information means...something about operator+?

回答1:

You cannot add two iterators together.

operator+ is not defined for two iterators, because that operation wouldn't make sense. Iterators are a kind of generalization over pointers - they point to the specific element stored in container. At which element the sum of iterators is pointing?

However, when you use a vector, you can add integers to iterators, like that:

vec.begin() + vec.size() / 2

and that is why you have candidates are: (...) in your error message, followed by some definitions of operator+.

In your case the best, and cleanest way will be not using the iterators, but simple getting the value from specified position:

int mid = vec[vec.size() / 2];

回答2:

You cannot add iterators. What you can do:

vector<int>::iterator mid = ivec.begin() + distance(ivec.begin(), ivec.end()) / 2;

回答3:

It simply means that vector iterators have no addition operator (+). You cannot add ivec.begin() and ivec.end().

To get the middle element, you can simply use the subscript operator:

cout << ivec[ivec.size()/2] << endl;

If you insist on using an iterator, you can get an iterator that points to the middle in this fashion:

vector<int>::iterator mid = ivec.begin();
mid += ivec.size()/2;
cout << *mid << endl;

You can do this, because the vector iterator is a random access iterator (in all implementations I'm aware of it encapsulates an actual pointer to the raw container-data).

回答4:

You can't add iterators.

What you'll want to use is a combination of std::distance() and std::advance():

vector<int>::iterator mid = std::advance(ivec.begin(), std::distance(ivec.begin(), ivec.end()) / 2);

Why use std::advance() instead of the iterator's addition operator? std::advance() works optimally regardless of the iterator type (random access, forward only, bidirectional, etc), so if you switch from std::vector to std::list the above code can remain the same, and still work optimally.

回答5:

not possible to add two iterators

use at to get the middle item:

ivec.at(ivec.size()/2);

回答6:

You can not add two iterators together, because iterators are not integers. That's what the error message means. If you want to access the element in the middle of a vector, use ivec[ivec.size() / 2].