C++11:来自第二个元素的范围循环向量? [英] C++11: Range-looping vector from the second element?

问题描述

我有一个 std::vectorv;(已初始化).如何使用 range-for 循环访问除第一个元素(索引为零)以外的所有元素.对于所有元素:

I have a std::vector<std::string> v; (initialized). How can I use the range-for loop for accessing all elements except the first one (on index zero). For all elements:

for (const string & s: v)
    process(s);

可以使用范围表达式代替v.如何编写范围表达式以跳过第一个元素(或跳过前n个元素)?

Instead of the v a range expression can be used. How can I write the range expression to skip the first element (or skip the first n elements)?

我知道如何使用 v.begin() + 1 和经典循环来获得效果.我正在寻找新的、更易读的、推荐的替代方法来做到这一点.可能类似于 Python 切片?...喜欢:

I know how to get the effect using v.begin() + 1 and using the classic loop. I am searching for the new, more readable, recommended alternative to do that. Possibly something similar to Python slicing? ...like:

for s in v[1:]:
    process(s)

推荐答案

创建一个包装器，使其 begin() 和 end() 返回正确的迭代器，然后您可以将其用作第二个参数.

Create a wrapper for which begin() and end() return the correct iterators and then you can use that as the second argument.

#include <iostream>
#include <vector>

template< typename Collection >
class FromNth
{
    Collection& coll_;
    size_t offset_;

public:
    FromNth( Collection& coll, size_t offset )
        : coll_( coll ), offset_( offset )
    {
    }

    // will nicely resolve to const_iterator if necessary
    auto begin() const -> decltype( coll_.begin() ) 
       { return coll_.begin() + offset_; }

    auto end() const -> decltype( coll_.end() )
       { return coll_.end(); }
};

template< typename Collection >
FromNth<Collection> makeFromNth( Collection& collection, size_t offset )
{
     return FromNth<Collection>( collection, offset );
}

template< typename Collection >
auto begin( const FromNth<Collection> & wrapper ) -> decltype( wrapper.begin() )
{   
   return wrapper.begin();
}

template< typename Collection >
auto end( const FromNth<Collection> & wrapper ) -> decltype( wrapper.end() )
{  
   return wrapper.end();
}

int main()
{
   std::vector< int > coll { 2, 3, 5, 7, 11, 13, 17, 19, 23 };

   for( auto x : makeFromNth( coll, 1 ) )
   {
       std::cout << x << '
';
   }
   return 0;
}

请注意，如果输入的大小小于偏移量，则我的 fromNth "begin" 是未定义的行为.(如果它相等，则它定义良好并且开始 == 结束).因此，请先检查尺寸.

Note that my fromNth "begin" is undefined behaviour if the size of the input is less than the offset. (If it's equal then it's well defined and begin == end). Therefore do a size check first.

注意:如果您使用的是最新版本的 boost，那么 iterator_range 可能已经为您提供了类似于我的FromNth"的集合".

Note: if you are using a recent enough version of boost then iterator_range may already provide you such a "collection" that is similar to my "FromNth".

for( auto const& s : boost::make_iterator_range( v.begin() + 1, v.end() ) )
{
    process( s );
}

注意:上述代码适用于使用 C++11 GNU 4.8.3 的 CodingGround.(虽然那个网站很慢).从 C++14 开始，您将不需要 ->decltype 语句(在 C++11 模板中需要这些语句).

Note: the code above worked on CodingGround using C++11 GNU 4.8.3. (That site is very slow though). From C++14 you will not need the ->decltype statements (which are needed in C++11 for templates).

输出:

sh-4.3$ g++ -std=c++11 -o main *.cpp
sh-4.3$ main
3
5
7
11
13
17
19
23 

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