c – 组合增压范围算法,例如. boost :: copy和boost :: remove_i

发布时间：2020-12-16 09:40:31 所属栏目：百科来源：网络整理

导读：我正在尝试编写一个必须应用于范围序列的复杂算法列表.我想使用类似于以下代码的语法嵌套许多算法.我唯一的问题是它不会编译.有什么建议？ bool pred(double x);double d[]={1,2,3,4};std::vectordouble x(d,d+4);std::vectordouble y;boost::copy( x,std::b

我正在尝试编写一个必须应用于范围序列的复杂算法列表.我想使用类似于以下代码的语法嵌套许多算法.我唯一的问题是它不会编译.有什么建议？

bool pred(double x);

double d[]={1,2,3,4};
std::vector<double> x(d,d+4);
std::vector<double> y;
boost::copy( x,std::back_inserter(y) );   // OK
boost::copy( boost::remove_if(x,&pred),std::back_inserter(y) ); // ERROR

要使此语法起作用,我不希望必须为内部算法指定模板参数.这可以通过使用auto关键字简化,但我需要保持代码向后兼容.

这是错误消息的一个片段：

1>c:devthirdpartyboostboost-1.48.0-windows-vc90-x32-p64925installedincludeboost-1_48boost/range/iterator.hpp(63) : error C2039: 'type' : is not a member of 'boost::mpl::eval_if_c<C,F1,F2>'
1>        with
1>        [
1>            C=true,1>            F1=boost::range_const_iterator<std::_Vector_iterator<double,std::allocator<double>>>,1>            F2=boost::range_mutable_iterator<const std::_Vector_iterator<double,std::allocator<double>>>
1>        ]

1>        c:devthirdpartyboostboost-1.48.0-windows-vc90-x32-p64925installedincludeboost-1_48boost/range/concepts.hpp(256) : see reference to class template instantiation 'boost::range_iterator<C>' being compiled
1>        with
1>        [
1>            C=const std::_Vector_iterator<double,std::allocator<double>>
1>        ]
1>        c:devthirdpartyboostboost-1.48.0-windows-vc90-x32-p64925installedincludeboost-1_48boost/concept/detail/has_constraints.hpp(42) : see reference to class template instantiation 'boost::SinglePassRangeConcept<T>' being compiled
1>        with
1>        [
1>            T=const std::_Vector_iterator<double,std::allocator<double>>
1>        ]
1>        c:devthirdpartyboostboost-1.48.0-windows-vc90-x32-p64925installedincludeboost-1_48boost/concept/detail/msvc.hpp(53) : see reference to class template instantiation 'boost::concepts::not_satisfied<Model>' being compiled
1>        with
1>        [
1>            Model=boost::SinglePassRangeConcept<const std::_Vector_iterator<double,std::allocator<double>>>
1>        ]
1>        c:devthirdpartyboostboost-1.48.0-windows-vc90-x32-p64925installedincludeboost-1_48boost/range/algorithm/copy.hpp(33) : see reference to class template instantiation 'boost::concepts::require<Model>' being compiled
1>        with
1>        [
1>            Model=boost::SinglePassRangeConcept<const std::_Vector_iterator<double,std::allocator<double>>>
1>        ]
1>        ..........sourceyottalibrariessnlunittestsStackOverflow.cpp(7) : see reference to function template instantiation 'OutputIterator boost::range::copy<std::_Vector_iterator<_Ty,_Alloc>,std::back_insert_iterator<_Container>>(const SinglePassRange &,OutputIterator)' being compiled
1>        with   
....

解决方法

问题是,如果您没有指定应返回的范围类型(??您似乎不需要),则 remove_if(以及所有算法)返回单个迭代器而不是范围.

一个简单的解决方法是使用the adaptor filtered：

#include <boost/range/adaptor/filtered.hpp>
#include <boost/range/algorithm/copy.hpp>
#include <iterator>
#include <vector>
#include <iostream>

bool pred(double x){
  return x > 2.0;
}

int main(){

  double d[]={1,4};
  std::vector<double> x(d,d+4);
  std::vector<double> y;
  boost::copy( x | boost::adaptors::filtered(pred),std::back_inserter(y) );
  boost::copy( y,std::ostream_iterator<double>(std::cout," ") );
}

输出：3 4

（编辑：李大同）

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