std::ostream {aka std::basic_ostream} Ivalue

问题:

In this code I try to move the iterator by 10 elements.

   #include <iostream>
    #include <string>
    #include <vector>
    int main()
    {
        using namespace std;
       vector<int> v(20);
       auto mid = v.begin() + 10;
        cout<<mid;


    }

On running this code, I get the error mentioned in the title. I'm a beginner. I experience this error in almost every program I write. Where am I going wrong?

回答1:

An iterator "points" to an element, what you want to be doing is:

cout << *mid;

You have to "dereference" the iterator to print what it points to. Trying to print it directly gives you the error you mentioned.

Edit: here's a little demo:

#include <iostream>
#include <vector>

int main(int argc, char* argv[])
{
    std::vector<int> numbers;
    numbers.push_back(4);
    numbers.push_back(3);
    numbers.push_back(2);

    auto beg = numbers.begin();
    auto mid = numbers.begin() + 1;
    std::cout << *beg << std::endl;
    std::cout << (beg < mid) << std::endl;      // True because beg (index 0) points to an element earlier than mid (index 1)
    std::cout << (*beg < *mid) << std::endl;    // False because the element pointed-to by beg (4) is bigger than the one pointed-to by mid (3)

    return 0;
}

Output The first line shows 4 which is the value of the first element! The second line shows 1 (all non-zero values mean true) and the last line shows 0 (zero is the only value that means false).