c – 我可以从基于循环的范围内获取项目的索引吗？

for(auto item : make_indexable(v))
{
      //std::get<0>(item) is the index
      //std::get<1>(item) is the object
}

这里是最小的实现(它只是为了演示基本的想法)：

#include <tuple>
#include <functional>

template<typename C>
struct indexed_container
{
    struct indexed_iterator
    {
        typedef typename C::value_type value_type;
        typedef std::tuple<size_t,std::reference_wrapper<value_type>> tuple_type;

        typename C::iterator _it;
        size_t _index;

        indexed_iterator(typename C::iterator it) : _it(it),_index(0) {}

        indexed_iterator& operator++()
        {
            ++_it;
            ++_index;
            return *this;
        }
        bool operator == (indexed_iterator const & other)
        {
            return _it == other._it;
        }
        bool operator != (indexed_iterator const & other)
        {
            return _it != other._it;
        }
        tuple_type operator*()
        {
            return std::make_tuple(_index,std::ref(*_it));
        }
    };

    indexed_container(C & c) : _c(c) {}

    indexed_iterator begin()
    {
        return indexed_iterator(_c.begin());
    }
    indexed_iterator end()
    {
        return indexed_iterator(_c.end());
    }
private:
    C & _c;
};

template<typename C>
auto make_indexable(C & c) -> indexed_container<C>
{
    return indexed_container<C>(c); 
}

测试代码：

#include <iostream>
#include <vector>

int main()
{
    std::vector<int> v{1,2,3};
    for(auto item : make_indexable(v))
    {
        std::cout << std::get<0>(item) << " => " << std::get<1>(item) << std::endl;

        std::get<1>(item) *= 10; //modify value!
    }
    std::cout << "nModifiedn";
    for(auto item : make_indexable(v))
    {
        std::cout << std::get<0>(item) << " => " << std::get<1>(item) << std::endl;
    }
}

输出：

0 => 1
1 => 2
2 => 3

Modified
0 => 10
1 => 20
2 => 30

Online demo

请注意,此解决方案并不完美,因为它不适用于临时和const容器(以及const对象的容器).此外,即使您编写自动项而不是auto&,也会立即返回基础对象作为参考. item(事实上,你不能写auto& item).但我认为这些问题可以通过更多努力和精心设计来解决.毕竟,这只是一个基本想法的示范.