结合boost range算法，例如。boost::copy和boost::remove_if

Question

我正在尝试编写一个复杂的算法列表，这些算法必须应用于范围序列。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能编译。有什么建议吗？

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

为了让这个语法起作用，我不想为内部算法指定模板参数。这可以通过使用auto关键字来简化，但我需要保持代码向后兼容。

以下是错误消息的片段：

1>c:\dev\thirdparty\boost\boost-1.48.0-windows-vc90-x32-p64925\installed\include\boost-1_48\boost/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:\dev\thirdparty\boost\boost-1.48.0-windows-vc90-x32-p64925\installed\include\boost-1_48\boost/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:\dev\thirdparty\boost\boost-1.48.0-windows-vc90-x32-p64925\installed\include\boost-1_48\boost/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:\dev\thirdparty\boost\boost-1.48.0-windows-vc90-x32-p64925\installed\include\boost-1_48\boost/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:\dev\thirdparty\boost\boost-1.48.0-windows-vc90-x32-p64925\installed\include\boost-1_48\boost/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>        ..\..\..\..\..\source\yotta\libraries\snl\unittests\StackOverflow.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   
....

Answer 1

问题是，如果您没有指定应该返回的范围类型(您似乎并不需要这样做)，那么remove_if (以及所有的算法)会返回一个迭代器，而不是一个范围。

一种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[]={1,2,3,4};
  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