为 map<T1,T2> 迭代器创建一个模板函数

Question

所以我试图制作 4 个模板函数来执行以下操作：对通用容器求和、对映射求和、对通用容器迭代器求和以及对通用映射迭代器求和。我设法做到了前三个，但我无法弄清楚 sum map 迭代器。这是我的代码：

Sum.hpp

#ifndef SUM_HPP
#define SUM_HPP

#include <iostream>
#include <map>
#include <string>
using namespace std;

//Sum Generic Container
template <typename T>
double Sum(const T& cont) {
    double sum = 0; 

    typename T::const_iterator iter = cont.begin();
    for (iter; iter != cont.end(); iter++) {
        sum += *iter;
    }

return sum;
}

//Sum Generic Map
template <typename T1, typename T2>
double Sum(const map<T1, T2>& mp) {
    double sum = 0;

    typename map<T1,T2>::const_iterator iter = mp.begin();
    for (iter; iter != mp.end(); iter++) {
        sum += iter->second;
    }

    return sum;
}

//Sum Generic Container Iterators
template <typename T>
double Sum(T& begin, const T& end) {
    double sum = 0;

    for (begin; begin != end; begin++) {
        sum += *begin;
    }

    return sum;
}

//Sum Generic Map Iterators
template <typename T1, typename T2>
double Sum(map<T1, T2>::iterator& begin, map<T1, T2>::iterator& end) {
    double sum = 0;

    for (begin; begin != end; begin++) {
        sum += begin->second;
    }

    return sum;
}


#endif

Test.cpp:

#include "Sum.hpp"
#include <iostream>
#include <map>
using namespace std;

int main() {
    //Map
    cout << "map" << endl;
    map<string, double> mp;

    mp["weight"] = 5.5;
    mp["height"] = 6.7;
    mp["length"] = 8.4;

    map<string, double>::iterator mp_iter_begin = mp.begin();
    map<string, double>::iterator mp_iter_end = mp.end();
    cout << Sum(mp_iter_begin, mp_iter_end) << endl;

    return 0;
}

当我运行 Sum() 函数尝试调用 Sum Generic Container Iterators 函数时，如果我注释掉 Sum Generic Container Iterators 函数，我会收到 "no instance of overloaded function" 错误。谁能发现我做错了什么？

Answer 1

这属于non-deduced contexts:

1) The nested-name-specifier (everything to the left of the scope resolution operator ::) of a type that was specified using a qualified-id:

也就是说对于map<T1, T2>::iterator，模板参数T1和T2无法推导。

您可以通过SFINAE解决问题，例如

//Sum Generic Container Iterators
template <typename T>
auto Sum(T begin, T end) -> remove_reference_t<decltype(*begin += *begin)> {

    remove_reference_t<decltype(*begin)> sum = 0;

    for (; begin != end; begin++) {
        sum += *begin;
    }

    return sum;
}

//Sum Generic Map Iterators
template <typename T>
auto Sum(T begin, T end) -> remove_reference_t<decltype(begin->second += begin->second)> {

    remove_reference_t<decltype(begin->second)> sum = 0;

    for (; begin != end; begin++) {
        sum += begin->second;
    }

    return sum;
}

LIVE

Answer 2

我想出了如何按照我想要的方式进行专业化：

//Sum Generic Container Iterators
template <typename T>
double Sum(T& begin, const T& end) {
    double sum = 0;

    for (begin; begin != end; begin++) {
        sum += *begin;
    }

    return sum;
}

//Sum Map<string, double> Iterators
template <>
double Sum(map<string, double>::iterator& begin, const map<string, double>::iterator& end) {
    double sum = 0;

    for (begin; begin != end; begin++) {
        sum += begin->second;
    }

    return sum;
}

请注意确保通用容器迭代器和映射迭代器函数具有匹配参数（两个参数都是引用，第二个参数是两个模板函数的常量）或 map<string,double> 的特化的重要性工作。

为 map<T1,T2> 迭代器创建一个模板函数

Create a template function for map<T1,T2> iterators

c++

templates

generics

template-function