STL复制，对，矢量和插入器

Question

I have an input of:

1 a
2 b
..

I would like to insert them to a vector of pairs, with copy function, like this:

#include <vector>
#include <iterator>
#include <algorithm>
#include <iostream>

int main(void) {
    std::vector<std::pair<int, char>> v;
    std::copy(std::istream_iterator<std::pair<int, char>>(std::cin), std::istream_iterator<std::pair<int, char>>(), std::back_inserter(v));
    for(auto pair: v)
        std::cout << pair.first << std::endl;
    return 0;
}

However, this will not compile: error: no match for 'operator>>' , since it probably needs an operator overloading.

Does that mean that I will have to create my own class, which inhertis from std::vector , and then overload the operator?

I would like to avoid using my own class, instead of the standard vector class.

Answer 1

The problem is not with std::vector , it's std::istream_iterator . The reason being that std::pair doesn't have a deserialization operator defined.

You can still use std::vector and std::back_insert_iterator , but you'll need to define your own input iterator. One that reads pairs of values.

Some people may suggest that you define operator>> for your pairs, but that is an unreliable technique. It will depend on the operator being defined before you include <algorithm> and <iterator> .

Answer 2

You could copy it through a proxy object:

#include <vector>
#include <iterator>
#include <algorithm>
#include <iostream>

struct proxy
{
    friend auto operator>>(std::istream& is, proxy& prox) -> std::istream&
    {
        is >> std::get<0>(prox.target);
        is >> std::get<1>(prox.target);
        return is;
    }

    operator std::pair<int, char>() const {
        return target;
    }

    std::pair<int, char> target;
};

int main(void) {
    std::vector<std::pair<int, char>> v;
    std::copy(std::istream_iterator<proxy>(std::cin), 
              std::istream_iterator<proxy>(), 
              std::back_inserter(v));
    for(auto pair: v)
        std::cout << pair.first << std::endl;
    return 0;
}