使用STL容器的c ++函数模板特化

Question

I'm learning STL and templates. Here's my problem. I wrote this function calculating sum of elements "between" two iterators:

template <typename Iter> double PartialSum(Iter itBegin, Iter itEnd){
    if (itBegin == itEnd) return 0.;
    double dSum = 0;
    while(itBegin != itEnd){
        dSum += (*itBegin);
        ++itBegin;
    }
    return dSum;
}

And this works fine (I know I can use std::accumulate but this is for learning purposes). Now, I would like to have the same functionality for std:map but iterators there work differently than in the case of std::vector and std::list . Therefore, I'd like to write overloaded/specialized PartialSum . What I tried and failed is this (minimal example):

template <typename T1, typename T2> double PartialSum(std::map<T1,T2>::iterator itBegin{
    return 0.;
}

This is the error log:

Main.cpp(42): error: nontype "std::map<_Key, _Tp, _Compare, _Alloc>::iterator [with _Key=T1, _Tp=T2, _Compare=std::less<T1>, _Alloc=std::allocator<std::pair<const T1, T2>>]" is not a type name template <typename T1, typename T2> double PartialSum(std::map<T1,T2>::iterator itBegin){ Main.cpp(83): error: no instance of overloaded function "PartialSum" matches the argument list argument types are: (std::_Rb_tree_iterator<std::pair<const std::string, int>>) std::cout<<"Map partial sum: "<<PartialSum(myMap.begin())<<std::endl;

Since it's so simple I probably don't undersatnd something very fundamental. Would be happy to hear your opinion :-)

Answer 1

Trying to formulate in another way.

Consider you have function

template<typename T>
T f(){
    return T();
}

It's impossible here to automatically get T, so you need call it as f<T>() . Same goes with

template <typename T>
int f(typename type<T>::inner){
     //
}

For example, if you have

struct type{
    typedef int inner;
}

It's easily to see here, that if you call f(0) it's impossible to get T.
You may say that's possible to get it in that particular case with map , but how will you define it?

You should read c++ standard to read which type should be deducable.

---

In your case you may call in following way

PartialSum<std::string, int>(m.begin());

---

BTW, It seems, that map is just uncommon case, you may try to do something more general, that will work with any iterator type. You may see std::accumulate sources to get some ideas.

template<typename _InputIterator, typename _Tp, typename _BinaryOperation>
inline _Tp
accumulate(_InputIterator __first, _InputIterator __last, _Tp __init,
       _BinaryOperation __binary_op)
{
  // concept requirements
  __glibcxx_function_requires(_InputIteratorConcept<_InputIterator>)
  __glibcxx_requires_valid_range(__first, __last);

  for (; __first != __last; ++__first)
__init = __binary_op(__init, *__first);
  return __init;
}

Answer 2

Apart from the problem that T1 and T2 are not deducible, there is another problem that you're missing the typename keyword on a dependent name

template<typename T1, typename T2>
void MyFunction(typename std::map<T1, T2>::iterator it /*, ...*/)
//              ^^^^^^^^^          

You see, a dependent name is a name dependent on the template arguments. Indeed, there could theoretically be such types T1 and T2 for which the name map::iterator is not a type but is, say, a static data member. The compiler will always assume a data member unless you explicitly specify that it is a type.

You should simply do something like this

template<class ValueType, class IteratorType, class Func>
ValueType partialSum(IteratorType first, IteratorType last, ValueType startingValue = ValueType(), Func func = std::plus<ValueType>())

And this will cover all cases. To sum a map you will need to provide func that adds two pairs.

Answer 3

When you dereference a std::map<T1, T2>::iterator , you get a std::pair<const T1, T2> , where the .first element is the key, and the .second element is the value.

General structure like this: (code untested, even uncompiled)

template <typename T1, typename T2> double PartialSum(std::map<T1,T2>::iterator itBegin, std::map<T1,T2>::iterator itEnd)
{
double dSum = 0;
while(itBegin != itEnd){
    dSum += (itBegin->second);
    ++itBegin;
}
return dSum;
}