range-based for loop with a const container issue

Question

I'm implementing some example from the book "A Tour of C++" by Bjarne Stroustrup. I have a template function to compute sum of elements (copied from the book):

template<typename Container, typename Value>
Value sum(const Container& c, Value v)
{   
  for (auto x : c)
    v+=x;
  return v;
}

I want to apply it to a template class Vector (defined in the book) which has a "begin" and "end" functions to support range-for loop (copied from the book):

   template<typename T>
   T∗ begin(Vector<T>& x)
   {
    return x.size() ? &x[0] : nullptr;
   }
   template<typename T>
   T∗ end(Vector<T>& x)
   {
    return begin(x)+x.size();
   }

when applied to Vector, for example :

Vector<int> v={1,2,3,4};
int s=sum(v, 0);

this lead to a compilation error. However, when I remove const from the sum template function

 template<typename Container, typename Value>
 Value sum( Container& c, Value v)

it compiles.

1- why it didn't work with the "const" ? If it is a problem with the "begin", "end" implementation, how to implement them?

Casey · Accepted Answer · 2014-10-15 18:11:39Z

2

Are these the only versions of begin and end? Notice they take the vector by reference to non-const. To make sum compile, you should also add const overloads:

template<typename T>
const T∗ begin(const Vector<T>& x)
{
  return x.size() ? &x[0] : nullptr;
}

template<typename T>
const T∗ end(const Vector<T>& x)
{
  return begin(x)+x.size();
}

Note that this requires Vector to have a const overload of operator[]. That's a very reasonable requirement; if the original Vector doesn't have it (doubtful), you should add it.

edited Oct 15, 2014 at 18:11

Casey

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answered Oct 15, 2014 at 14:39

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1 Comment

adel Over a year ago

That works! Thank you! This is missing in the book and should be added to the errata.

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