Iterating over the output of a member function in std::for_each

Question

I have a class with an accessor member function that I want to call and apply the result to a functor using std::for_each. I have a working version below that uses a for loop and for_each, but the for_each version is cryptic and cumbersome. Is there a way I can make the for_each version more concise, considering I have access to boost, but not C++11?

#if 0
   // for loop version:
   for(value_vector_type::iterator it = values.begin(); it!=values.end(); it++){
     avg(it->getValue());  // I want to put this in a for_each loop
   }
#else
  //  bind version:
  std::for_each(values.begin(), values.end(), // iterate over all values
    boost::bind(
      boost::mem_fn(&average_type::operator()), // attach the averaging functor to the output of the getvalue call
      &avg, 
      boost::bind(
        boost::mem_fn(&value_wrapper_type::getValue), // bind the getValue call to each element in values
        _1
      )
    )
  );
#endif    

Here is the full working implementation:

#include <vector>
#include <algorithm>
#include <iostream>
#include <boost/bind.hpp>
#include <boost/bind/mem_fn.hpp>

// A value wrapper
template<typename T>
struct Value {
  Value(){}
  Value(const T& value, bool valid = true):m_value(value),m_valid(valid){}

  T getValue(){ return m_value; }
  bool getValid(){ return m_valid; }
  void setValue(const T& value){ m_value = value; }
  void setValid(const T& valid){ m_valid = valid; }

private:
  T m_value;
  bool m_valid;   
};

// Class that calculates the average piecewise
template<typename T>
struct Average {
private:
    T m_numPoints;
    T m_ChannelSum;

public:

    Average() : m_numPoints(0), m_ChannelSum(0.0){}

    void operator()(T value){
        m_numPoints++;
        m_ChannelSum+=value;
    }

    double getAverage(){ return m_ChannelSum/m_numPoints; }
    T getCount(){ return m_numPoints; }
    T getSum(){ return m_ChannelSum; }
};

// Run the average computation on several values
int main(int argc, char** argv){
  typedef int value_type;
  typedef Value<value_type> value_wrapper_type;
  typedef std::vector<value_wrapper_type> value_vector_type;
  value_vector_type values;
  values.push_back(value_wrapper_type(5));
  values.push_back(value_wrapper_type(7));
  values.push_back(value_wrapper_type(3));
  values.push_back(value_wrapper_type(1));
  values.push_back(value_wrapper_type(2));

  typedef Average<value_type> average_type;
  average_type avg;

#if 0
   // for loop version:
   for(value_vector_type::iterator it = values.begin(); it!=values.end(); it++){
     avg(it->getValue());  // I want to put this in a for_each loop
   }
#else
  //  bind version:
  std::for_each(values.begin(), values.end(), // iterate over all values
    boost::bind(
      boost::mem_fn(&average_type::operator()), // attach the averaging functor to the output of the getvalue call
      &avg, 
      boost::bind(
        boost::mem_fn(&value_wrapper_type::getValue), // bind the getValue call to each element in values
        _1
      )
    )
  );
#endif    
  std::cout << "Average: " << avg.getAverage() << " Count: " << avg.getCount() << " Sum: " << avg.getSum() << std::endl;
}

note: my original question was how to construct a for_each at all, but I've found that solution and a whole new question did not make much sense.

Thanks, all help is really appreciated!

Answer 1

Credit goes to Mathias Gaunard on the boost.users mailing list for pointing me towards this solution:

  std::for_each(values.begin(), values.end(),
    boost::bind(boost::ref(avg), boost::bind(&value_wrapper_type::getValue, _1))
  );

Wrapping avg with boost::ref is required because otherwise a copy of avg is filled out with the results of getValue(), rather than avg itself.

Here is the full compiled and tested solution:

#include <stdexcept>
#include <vector>
#include <algorithm>
#include <iostream>
#include <boost/bind.hpp>
#include <boost/bind/mem_fn.hpp>

// A value wrapper
template<typename T>
struct Value {
  Value(){}
  Value(const T& value, bool valid = true):m_value(value),m_valid(valid){}

  T getValue(){ return m_value; }
  bool getValid(){ return m_valid; }
  void setValue(const T& value){ m_value = value; }
  void setValid(const T& valid){ m_valid = valid; }

private:
  T m_value;
  bool m_valid;   
};

// Class that calculates the average piecewise
template<typename T>
struct Average {
private:
    T m_numPoints;
    T m_ChannelSum;

public:
  typedef void result_type;

    Average() : m_numPoints(0), m_ChannelSum(0.0){}

    result_type operator()(T value){
        m_numPoints++;
        m_ChannelSum+=value;
    }

    double getAverage(){ 
    if (m_ChannelSum==0) {
      throw std::logic_error("Cannot get average of zero values");
    }

    return m_ChannelSum/m_numPoints; 
  }
    T getCount(){ return m_numPoints; }
    T getSum(){ return m_ChannelSum; }
};

// Run the average computation on several values
int main(int argc, char** argv){
  typedef int value_type;
  typedef Value<value_type> value_wrapper_type;
  typedef std::vector<value_wrapper_type> value_vector_type;
  value_vector_type values;
  values.push_back(value_wrapper_type(5));
  values.push_back(value_wrapper_type(7));
  values.push_back(value_wrapper_type(3));
  values.push_back(value_wrapper_type(1)); 
  values.push_back(value_wrapper_type(2));

  typedef Average<value_type> average_type;
  average_type avg;

#if 0
  // for loop version:
  for(value_vector_type::iterator it = values.begin(); it!=values.end(); it++){
   avg(it->getValue());  // I want to put this in a for_each loop
  }
#else
  //  bind version:
  std::for_each(values.begin(), values.end(),
    boost::bind(boost::ref(avg), boost::bind(&value_wrapper_type::getValue, _1))
  );
#endif    
  std::cout << "Average: " << avg.getAverage() << " Count: " << avg.getCount() << " Sum: " << avg.getSum() << std::endl;
}

Answer 2

If you don't have C++11 but Boost you could try a bind() expression (which would also work with C++2011 as bind() is part of C++2011):

std::for_each(a.begin(), a.end(), bind(&avg<value_type>, bind(&Value<value_type>::getValue, _1)));

Answer 3

if you are using c++11 then you can try

for(auto& a: values)
    avg(a->getValue());

or

std::for_each(a.begin(), a.end(), [](whatever_type& wt){
    avg(wt->getValue());
});

If you are not, then I think that toy have as good as your going to get although formatting wont hurt.

for(value_vector_type::iterator it = values.begin(); 
    it!=values.end(); 
    ++it)
{
    avg(it.getValue());  // I want to put this in a for_each loop
}

Trying to be too clever with function object and the like can often have the inverse effect of obscuring your code.

Answer 4

One way to make it look neater is to use Boost.Phoenix. You can shorten down to this:

std::for_each(values.begin(), values.end(), lazy(avg)(arg1.getValue()));

Heres how to do that. First thing you need to do is make the avg function object lazy. The simplest way to that is in-place with a function, defined like this:

template<class Function>
function<Function> lazy(Function x)
{
    return function<Function>(x);
}

Next thing you need to do is write a function object for getValue, that can be lazy, like this:

struct get_value_impl
{
    // result_of protocol:
    template <typename Sig>
    struct result;

    template <typename This, typename T>
    struct result<This(Value<T>&)>
    {
        // The result will be T
        typedef typename T type;
    };

    template <typename V>
    typename result<get_value_impl(V &)>::type
    operator()(V & value) const
    {
        return value.getValue();
    }
};

Thirdly, we extend the phoenix actors, using our get_value_impl class, so it will have a getValue method, like this:

template <typename Expr>
struct value_actor
    : actor<Expr>
{
    typedef actor<Expr> base_type;
    typedef value_actor<Expr> that_type;

    value_actor( base_type const& base )
        : base_type( base ) {}

    typename expression::function<get_value_impl, that_type>::type const
    getValue() const
    {
        function<get_value_impl> const f = get_value_impl();
        return f(*this);
    }
};

Finally, we put it all together by defining the argument and passing it into the for_each algorithm:

expression::terminal<phoenix::argument<1>, value_actor>  arg1;
std::for_each(values.begin(), values.end(), lazy(avg)(arg1.getValue()));

Answer 5

If you can use boost, but not C++11 features, then I would consider using the BOOST_FOREACH macro

Yes, it's a macro, but as macros go it's well behaved

It's also reads quite nicely and is hard to get wrong

BOOST_FOREACH(const Value& rValue, values)
{
    avg(rValue.getValue());
}

C++11 range based for loops will replace it