C ++模板函数专业化错误

Question

I am new in using C++ templates. I need to write a template function specialization for my project. It is a simple Sum function for different type inputs and it calculates the sum between two iterators. The original function is generic and so accepts a template argument. The template specialization is written for Maps.

#include <map>
#include <string>

template <typename T>
double Sum(T &it_beg, T &it_end) {
    double sum_all = 0;

    for(it_beg++; it_beg != it_end; it_beg++)
        sum_all += *it_beg;

    return sum_all;
};

template <>
double Sum(std::map<std::string, double> &it_beg, std::map<std::string, double> &it_end) {
    double sum_all = 0;

    for(it_beg++; it_beg != it_end; it_beg++)
        sum_all += it_beg->second;

    return sum_all;
};

when I try to run the code, I get the following errors

...\sum.h(21): error C2676: binary '++' : 'std::map<_Kty,_Ty>' does not define     this operator or a conversion to a type acceptable to the predefined operator
 1>          with
 1>          [
 1>              _Kty=std::string,
 1>              _Ty=double
 1>          ]

I appreciate if anyone could give me a hint ! thanks

Answer 1

Your function signature should look like this (possibly without references) so you can pass in rvalues (iterators are cheap to copy anyway):

template <>
double Sum(std::map<std::string, double>::iterator it_beg,
           std::map<std::string, double>::iterator it_end)

std::map does not define operator++ , clearly your arguments are meant to be std::map::iterator s.

Don't forget to remove the references from the main template function parameters too.

There's also this:

for(it_beg++; it_beg != it_end; it_beg++)

Why are you incrementing it_beg as you enter the loop? You can leave initialization statement empty.