为什么我不能做std :: map.begin（）+ 1？ [英] Why can't I do std::map.begin() + 1?

问题描述

我有一个 std :: map ，我想从第二个条目开始迭代。

I have a std::map, which I want to iterate over starting at the second entry.

我可以解决这个罚款，但我很困惑为什么明显的语法不编译。错误消息没有帮助，因为它指的是 std :: string ，这里不使用。

I can workaround this fine, but I'm confused about why the "obvious" syntax doesn't compile. The error message doesn't help because it refers to std::string, which I'm not using here.

这里有一些代码

// suppose I have some map ...
std::map<int, int> pSomeMap;

// this is fine ...
std::map<int, int>::const_iterator pIterOne = pSomeMap.begin();
++pIterOne;

// this doesn't compile ...
std::map<int, int>::const_iterator pIterTwo = pSomeMap.begin() + 1;

VS2012在上述行中出现以下错误

VS2012 gives the following error on the above line

错误C2784：'std :: _ St​​ring_iterator< _Mystr> std :: operator +（_ String_iterator <_Mystr> :: difference_type，std :: _ St​​ring_iterator< _Mystr>）：无法推导出模板参数for'std :: _ St​​ring_iterator< _Mystr>'from'int'

error C2784: 'std::_String_iterator<_Mystr> std::operator +(_String_iterator<_Mystr>::difference_type,std::_String_iterator<_Mystr>)' : could not deduce template argument for 'std::_String_iterator<_Mystr>' from 'int'

任何人都可以解释这里发生了什么？

Can anyone explain what's happening here?

推荐答案

std :: map< T> :: iterator > bidirectional iterator 。那些只有 ++ 和 - 运算符。 + N 和 [] 仅适用于随机存取迭代器 in std :: vector< T> ）。

std::map<T>::iterator is of the iterator-class bidirectional iterator. Those only have ++ and -- operators. +N and [] is only available for random access iterators (which can be found in e.g. std::vector<T>).

c> N 到 是恒定时间（例如添加 N * sizeof（T） code> T * ），而对于双向迭代器做同样的事情将需要应用 ++ N 次。

The reason behind this is that adding N to a random access iterator is constant time (e.g. add N*sizeof(T) to a T*), whereas doing the same thing for a bidirectional iterator would require applying ++ N times.

你可以做什么（如果你有C ++ 11） >

What you can do though (if you have C++11) is:

std::map<int, int>::const_iterator pIterTwo = std::next(pSomeMap.begin(),1);

这对于所有的迭代器类型都是正确的。

which does the right thing for all iterator types.

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