是否有可能使用boost :: filter_iterator输出? [英] Is it possible to use boost::filter_iterator for output?
问题描述
我使用的std ::变换
与的std :: back_inserter
追加元素添加到的std :: deque的
。现在的转型可能会失败,会返回一个无效的对象(比如未初始化的的boost ::可选的
或一个空指针)在某些情况下。我想从得到附加过滤掉无效的对象。
我想过使用的boost :: filter_iterator
,但不知道如何present的端()
过滤的范围的参数。
的boost :: filter_iterator
的文件表明,输出滤波是可能的。如果我只是专注运算符==
为的std :: back_insert_iterator
在这种情况下总是返回false?
除此之外,如果我要追加初始化的boost ::可选的值
或指针,我可以链接的boost :: filter_iterator
和的boost :: indirect_iterator
?
我试图避免推出我自己的 transform_valid
函数,它接受一个可选的提取
功能。
它甚至有可能使用 filter_iterator
作为一个输出迭代器?
我建议使用升压范围(算法和放大器;适配器)为方便使用,你会写:
的boost ::复制(
数据|转化(MAKET)|过滤(验证)/ * | *间接寻址/,
的std :: back_inserter(队列));
下面就是一个完整的工作示例:
的#include<升压/ range.hpp>
#包括LT&;升压/范围/ adaptors.hpp>
#包括LT&;升压/范围/ algorithm.hpp>
#包括LT&;升压/ optional.hpp>#包括LT&;矢量>
#包括LT&;&双端GT;TYPEDEF提振::可选< INT>笔;
的typedef的std :: deque的< T> Q;静态ŧMAKET(int i)以
{
如果(I%2)返回T();
否则返回我;
}静态布尔验证(常量T&安培;可选)
{
回报(布尔)可选; //选择有一个值设置可选
}诠释的main()
{
静态const int的数据[] = {1,2,3,4,5,6,7,8,9}; Q Q; 使用boost ::适配器::过滤;
使用boost ::适配器::转化; //注意如何提升范围典雅支持一个int []作为输入范围
提高::复制(数据|转换(MAKET)|过滤(验证),性病:: back_inserter(Q)); //演示输出:2,4,6,8的印刷
对于(Q ::常量性吧= q.begin(!);它= q.end(); ++吧)
{
性病::法院LT&;< (*为一套:未设置)<< \\ t的<< IT-> get_value_or(0)&所述;&下;的std :: ENDL;
} 返回0;
}
更新
从这个答案有点帮助:<一href=\"http://stackoverflow.com/questions/6123327/use-boostoptional-together-with-boostadaptorsindirected\">Use推动::可选一起的boost ::适配器:: 间接寻址
我现在包括使用优雅的演示进行间接寻址
系列适配器以及队列立即输出(取消引用自选):
<子>请注意,(智能)指针类型会有的显然的不需要提供
指针对象&LT;&GT;
专业化。我认为这是由设计: <$c$c>optional<>不,一点也不模型,指针 的
块引用>的#include&LT;升压/ range.hpp&GT;
#包括LT&;升压/范围/ adaptors.hpp&GT;
#包括LT&;升压/范围/ algorithm.hpp&GT;#包括LT&;升压/ optional.hpp&GT;名字空间boost {
模板&LT; typename的P&GT;结构指针对象&LT;可选&LT; P&GT; &GT; {
的typedef typename的可选&LT; P&GT; :: value_type的类型;
};
}TYPEDEF提振::可选&LT; INT&GT;笔;静态ŧMAKET(int i)以{回报我%2? T():我; }
静态布尔验证(常量T&安培;可选){返回(布尔)可选; }诠释主(){
使用空间boost ::适配器; 静态int数据[] = {1,2,3,4,5,6,7,8,9};
提高::复制(数据|转换(MAKET)
|过滤(验证)
|进行间接寻址,
的std :: ostream_iterator&LT; INT&GT;(STD ::法院));
}I am using
std::transform
with anstd::back_inserter
to append elements to anstd::deque
. Now the transformation may fail and will return a invalid object (say an uninitializedboost::optional
or a null pointer) in some cases. I would like to filter out the invalid objects from getting appended.I thought about using
boost::filter_iterator
, but not sure how to present theend()
parameter of the filtered range.The documentation of
boost::filter_iterator
suggests that output filtering is possible. Should I just specializeoperator ==
forstd::back_insert_iterator
in this case to always return false?In addition to this, if I want to append values of initialized
boost::optional
or pointers, can I chainboost::filter_iterator
andboost::indirect_iterator
?I am trying to avoid rolling out my own
transform_valid
function that takes an optionalextractor
function.Is it even possible to use
filter_iterator
as an output iterator?解决方案I suggest using boost range (algorithms & adaptors) for ease of use, you'd write:
boost::copy( data | transformed(makeT) | filtered(validate) /* | indirected */, std::back_inserter(queue));
Here is a complete working example of that:
#include <boost/range.hpp> #include <boost/range/adaptors.hpp> #include <boost/range/algorithm.hpp> #include <boost/optional.hpp> #include <vector> #include <deque> typedef boost::optional<int> T; typedef std::deque<T> Q; static T makeT(int i) { if (i%2) return T(); else return i; } static bool validate(const T& optional) { return (bool) optional; // select the optional that had a value set } int main() { static const int data[] = { 1,2,3,4,5,6,7,8,9 }; Q q; using boost::adaptors::filtered; using boost::adaptors::transformed; // note how Boost Range elegantly supports an int[] as an input range boost::copy(data | transformed(makeT) | filtered(validate), std::back_inserter(q)); // demo output: 2, 4, 6, 8 printed for (Q::const_iterator it=q.begin(); it!=q.end(); ++it) { std::cout << (*it? "set" : "unset") << "\t" << it->get_value_or(0) << std::endl; } return 0; }
Update
With a little help from this answer: Use boost::optional together with boost::adaptors::indirected
I now include an elegant demonstration of using the
indirected
range adaptor as well for immediate output of the queue (dereferencing the optionals):Note that for (smart) pointer types there would obviously be no need to provide the
pointee<>
specialisation. I reckon this is by design:optional<> is not, and does not model, a pointer
#include <boost/range.hpp> #include <boost/range/adaptors.hpp> #include <boost/range/algorithm.hpp> #include <boost/optional.hpp> namespace boost { template<typename P> struct pointee<optional<P> > { typedef typename optional<P>::value_type type; }; } typedef boost::optional<int> T; static T makeT(int i) { return i%2? T() : i; } static bool validate(const T& optional) { return (bool) optional; } int main() { using namespace boost::adaptors; static int data[] = { 1,2,3,4,5,6,7,8,9 }; boost::copy(data | transformed(makeT) | filtered(validate) | indirected, std::ostream_iterator<int>(std::cout, ", ")); }
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