错误C2783无法推断模板参数

Question

I am stuck with this error . I have found a workaround too but it kind of kills the whole purpose of exercise.

I am trying to create a function which will take two iterators pointing to same container . I will find the sum of elements between them. I created general function for the sequential containers like vector which works fine. I overloaded the same function for associative containers. This is the one giving error.

map<string,double> myMap;
myMap["B"]=1.0;
myMap["C"]=2.0;
myMap["S"]=3.0;
myMap["G"]=4.0;
myMap["P"]=5.0;

map<string,double>::const_iterator iter1=myMap.begin();
map<string,double>::const_iterator iter2=myMap.end();

cout<<"\nSum of map using the iterator specified range is: "<<Sum(iter1,iter2)<<"\n"; 
//Above line giving error. Intellisense is saying: Sum, Error: no instance of overloaded function "Sum" matches the argument list.

//function to calculate the sum is listed below (It appears in a header file with <map> header included):
template <typename T1,typename T2>
double Sum(const typename std::map<T1,T2>::const_iterator& input_begin,const typename std::map<T1,T2>::const_iterator& input_end)
{
double finalSum=0;
typename std::map<T1,T2>::const_iterator iter=input_begin;

for(iter; iter!=input_end; ++iter)
{
    finalSum=finalSum+ (iter)->second;
}

return finalSum;
}

Compilation error is: 1>c:\\documents and settings\\ABC\\my documents\\visual studio 2010\\projects\\demo.cpp(41): error C2783: 'double Sum(const std::map::const_iterator &,const std::map::const_iterator &)' : could not deduce template argument for 'T1'

Workaround:

If call Sum(iter1,iter2) is replaced with Sum < string,double > (iter1,iter2), it compiles fine.

Was I trying to do something impossible as per C++ standards in the first place?

Answer 1

The error is actually quite clear, in the following template:

template <typename T1,typename T2>
double Sum(const typename std::map<T1,T2>::const_iterator& input_begin,
           const typename std::map<T1,T2>::const_iterator& input_end)

The types T1 and T2 cannot be deduced from the argument at the place of call. This is defined as such in the standard and if you think about it (in the general case) it makes sense.

Consider that instead of std::map<>::const_iterator you had sometemplate<T>::nested_type and that the argument at the place of call was an int . If the compiler had to deduce the type, it would have to instantiate sometemplate for all possible types T in the universe (infinite set) and find for which of them the nested type nested_type is a typedef to int .

As someone points out in the comments you can change the template so that instead of being templated on the key and value types of the map it just takes the iterators.

---

Delegating extraction of the value

This is a workaround to providing a single implementation of Sum that can deal with both sequential and associative containers.

namespace detail {
template <typename T> T valueOf( T const & t ) { return t; }
template <typename K, typename V>
V valueOf( std::pair<const K, V> const & p ) {
   return p.second;
}
}
template <typename Iterator>
double Sum( Iterator begin, Iterator end ) {
  double result = 0;
  for (; begin != end; ++begin) {
     result += detail::valueOf(*begin);
  }
  return result;
}

I have not tested the code, but that should do it. And this is probably much simpler than using SFINAE on the Sum template.