使用模板类时 C++ 解析错误

Question

I have a school assignment where I have to code a template class that would store the minimum of a list of ints. When running it, I get parse errors but I don't understand why. Could someone explain why I'm getting these errors?

ListMin.hpp

template <int ... list>
class ListMin;

template <int first, int ... others>
class ListMin <first, others ...> {
public:
    enum : long { value = std::min(first, ListMin<others>::value) };
};

template <int first>
class ListMin <first> {
public:
    enum : long { value = first };
};

Main.cpp

std::cout << "min( [ 1, -5, 3 ] ) = " << ListMin< 1, -5, 3 >::value << std::endl;

The errors

ListMin.hpp:4: parse error before `...'
ListMin.hpp:7: parse error before `...'
ListMin.hpp:14: parse error before `<'

Thanks in advance

Answer 1

First problem: you need to expand the others pack.

Clang tells it up front:

<source>:9:27: error: enumerator value contains unexpanded parameter pack 'others'
    enum : long { value = std::min(first, ListMin<others>::value) };
                          ^                       ~~~~~~

Solution:

enum : long { value = std::min(first, ListMin<others...>::value) };
//                                                  ^~~

Second problem: std::min requires the arguments to have the same type. (Currently have int vs an unnamed enum ).

enum : long { value = std::min(first, int(ListMin<others...>::value)) };
//                                    ^~~~                         ^

---

A much better solution would be to use std::min({1, 2, 3}) , which is constexpr since C++14 (just like std::min(1, 2) , which you already use).

Or at least, drop the archaic C-style unnamed enum and replace it with static constexpr int value = ...; .