C++函数模板的问题

Question

问题遇到的现象和发生背景

刚学习C++，对它的理解还停留在数据成员和成员函数，这里遇到的函数模板问题，解决不了

问题相关代码，请勿粘贴截图

The function template min_elem() finds the smallest element in the range [first, last) and returns the iterator to the smallest element. The way how smallest is understood can be customized.

 

#include <functional>
#include <iostream>
#include <iterator>
#include <string>
#include <list>
#include <vector>

template<class ForwardIt, class Compare>
ForwardIt min_elem(ForwardIt first, ForwardIt last, 
 comp)
{
  if (first == last)
    return 
;

  ForwardIt smallest = first++;
  for (; 
; ++first) {
    if (comp(*first, *smallest)) {
      smallest = first;
    }
  }
  return 
;
}

template<class ForwardIt>
ForwardIt min_elem(ForwardIt first, ForwardIt last)
{
  return min_elem(first, last, std::less<
 std::iterator_traits<ForwardIt>::value_type>());
}

struct Student {
  int id;
  std::string name;
  bool 
(const Student& s2) const {
    return id < s2.id;
  }
};

std::ostream& operator<<(std::ostream& out, const Student& s)
{
  out << s.name << s.id;
  return out;
}

int main()
{
  std::vector<int> v = { 3, 1, 4, 2, 5, 9 };
  std::cout << "min element is: " << *min_elem(std::begin(v), std::end(v)) << std::endl;
  
  std::list<double> l = { 3.1, 1.2, 4.3, 2.4, 5.5, 9.7 };
  std::cout << "min element is: " << *min_elem(std::begin(l), std::end(l), std::greater<double>()) << std::endl;
  
  Student s[] = { {30, "Curry"}, {23, "Lebron"}, {35, "Durant"} };
  std::cout << "min element is: " << *min_elem(std::begin(s), std::end(s)) << std::endl;
}

运行结果及报错内容

我的解答思路和尝试过的方法

还不太明白函数模板的原理

我想要达到的结果

希望给出题目解答的同时，给出一些思路上的指点

Answer 1

// The function template min_elem() finds the smallest element in the range [first, last) and returns the iterator to the smallest element. The way how smallest is understood can be customized.
 
 
 
#include <functional>
#include <iostream>
#include <iterator>
#include <string>
#include <list>
#include <vector>
 
template<class ForwardIt, class Compare>
ForwardIt min_elem(ForwardIt first, ForwardIt last, 
Compare // 比较类实例
comp)
{
  if (first == last)
    return first; // 返回第一个.
 
  ForwardIt smallest = first++;
  for (; first != last // 当前不是迭代器末尾.
    ; ++first) {
    if (comp(*first, *smallest)) {
      smallest = first;
    }
  }
  return smallest // smallest保存着最小值。
;
}
 
template<class ForwardIt>
ForwardIt min_elem(ForwardIt first, ForwardIt last)
{
  return min_elem(first, last, std::less< typename // 去除语义二义性。
 std::iterator_traits<ForwardIt>::value_type>());
}
 
struct Student {
  int id;
  std::string name;
  bool operator< //运算符重载
(const Student& s2) const {
    return id < s2.id;
  }
};
 
std::ostream& operator<<(std::ostream& out, const Student& s)
{
  out << s.name << s.id;
  return out;
}
 
int main()
{
  std::vector<int> v = { 3, 1, 4, 2, 5, 9 };
  std::cout << "min element is: " << *min_elem(std::begin(v), std::end(v)) << std::endl;
  
  std::list<double> l = { 3.1, 1.2, 4.3, 2.4, 5.5, 9.7 };
  std::cout << "min element is: " << *min_elem(std::begin(l), std::end(l), std::greater<double>()) << std::endl;
  
  Student s[] = { {30, "Curry"}, {23, "Lebron"}, {35, "Durant"} };
  std::cout << "min element is: " << *min_elem(std::begin(s), std::end(s)) << std::endl;
}