Odtnhj

I am trying to write a generic function to compute an average over a certain range.

template <typename Range, typename Ret, typename Func>
Ret average(Range range, Ret zero, Func extract) 
 Ret sum = zero;
 int numElements = 0;
 for (const auto& elem : range) 
 sum += extract(elem);
 ++numElements;
 
 if (numElements > 0)
 sum /= numElements;
 return sum;

The problam I am having is with the usage of the /= operator, but to better clarify the arguments of this function, let me clarify them:

• 
  Range range is any object that defines a range through begin() and end() member funcitons. I may need to add const& to avoid unnecessary copying.


• 
  Ret zero defines the neutral element of the addition used when computing the average. It could be just a scalar, but will work with vectors or matrices too for example.


• 
  

  Func extract is a function (usually given as a lambda function) that converts the elements of the range into Ret values that I average over. In practice I use it as a getter of a specific field in big objects that I iterate over.

  
  
  

  I could probably define it as std::function<Ret(decltype(*range.begin()))> or something similar, if C++ didn't have problems deducting types this way.

  
  

I assume that Ret provides some /= operator that the above function can work with, but I do not want to require it to take an int specifically.

In my use case, for example, Ret works with float-s and this gives me an annoying warning:

warning: 'argument': conversion from 'int' to 'float', possible loss of data

So, what are my options to make the above function clean and work with any suitable operator/=?

I tried, for example, to deduct the type of the right argument of the operator and explicitly cast to it:

template <typename Range, typename Ret, typename Func>
Ret average(Range range, Ret zero, Func extract) 
 Ret sum = zero;
 int numElements = 0;
 for (const auto& elem : range) 
 sum += extract(elem);
 ++numElements;
 
 using F = std::remove_pointer<decltype(&Ret::operator/=)>;

 if (numElements > 0)
 sum /= static_cast<typename boost::function_traits<F>::arg1_type>(numElements);
 return sum;

But I get a lot of compile errors, suggesting that I don't know what I am doing. Starts with:

error: 'boost::detail::function_traits_helper<std::remove_pointer<SpecificTypeUsedAsRet &(__cdecl SpecificTypeUsedAsRet::* )(float)> *>': base class undefined

That's probably because boost::function_traits does not work with member functions, just regular ones?

I am also concerned that this solution may not work when:

• The operator/= is not given as a member function, but as a regular function with two arguments.


• The operator/= is overloaded with respect to its right operand. An int may match only one of the overloads - so there is no ambiguity, but decltype won't know which overload to take.


• I would prefer not to use boost but stick to the powers provided by newest C++ standards

You could simply declare Ret numElements = 0; instead of making it an int. If it has /= operator, it probably has an ++ operator; or you could use num_elements += 1 instead.