CODEWITHSUNDEEP
c++boostc++11boost-range
sjdowling
sjdowling

Reputation: 3022

value_type error when using STL containers with boost range adaptors

I've been trying to understand the use of boost range adaptors but all the working examples I have found only use STL containers with primitive types such as std::list<int> and trying to use my own classes makes everything fall apart. 

#define BOOST_RESULT_OF_USE_DECLTYPE
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <boost/range/adaptors.hpp>
#include <boost/range/adaptor/transformed.hpp>
#include <boost/range/adaptor/filtered.hpp>
#include <boost/range/algorithm.hpp>

struct Thing
{
  Thing() : _id(0), _name(""){}
  std::size_t _id;
  std::string _name;
};

int main()
{
  std::vector<Thing> input;
  std::vector<Thing> output;
  std::function<Thing (Thing&)> transform( [](Thing& t)->Thing{
    t._name = "changed";
    return t;});

  struct Filter
  {
    typedef bool result_type;
    typedef const Thing& argument_type;
    result_type operator()(const Thing& t)
    {
      return t._id > 1;
    }
  };
  Filter filter;

  boost::copy(input
      | boost::adaptors::filtered(filter)
      | boost::adaptors::transformed(transform)
      | boost::adaptors::reversed,
      output
      );
}

Using gcc 4.6/4.8 and boost 1.48/1.54/trunk I get the following compile error:

/usr/include/c++/4.8/bits/stl_algobase.h:382:57: error: no type named ‘value_type’ in ‘struct std::iterator_traits<std::vector<Thing> >’
       typedef typename iterator_traits<_OI>::value_type _ValueTypeO;
                                                         ^
/usr/include/c++/4.8/bits/stl_algobase.h:387:9: error: no type named ‘value_type’ in ‘struct std::iterator_traits<std::vector<Thing> >’
         && __are_same<_ValueTypeI, _ValueTypeO>::__value);

I understand the problems with decltype and result_of which might cause a problem with transformed despite my BOOST_RESULT_OF_USE_DECLTYPE define as advised by the answer to this. However I do not understand why I cannot pass in a functor struct to filtered or if there are additional requirements on my Thing class.

Upvotes: 4

Views: 1410

Answers (1)

llonesmiz
llonesmiz

Reputation: 155

According to the documentation the first argument to copy is a range and the second is an iterator, so changing your invocation to:

boost::copy(input
  | boost::adaptors::filtered(filter)
  | boost::adaptors::transformed(transform)
  | boost::adaptors::reversed,
  std::back_inserter(output)
  );

makes it compile fine with g++ 4.8.1 and boost 1.53.0.

Upvotes: 3

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