(译)对于模版类，尽可能的使用Hidden Friend函数定义operator，而不是放在外侧当成模版方法

template<class V>
struct Cat {
    V value_;
};

template<class V>
bool operator<(const Cat<V>& a, const Cat<V>& b) {
    return a.value_ < b.value_;
}

template<class V>
struct Dog {
    V value_;

    friend bool operator<(const Dog& a, const Dog& b) {
        return a.value_ < b.value_;
    }
};

template<class T>
void sort_in_place(const std::vector<T>& vt) {
    std::vector<std::reference_wrapper<const T>> vr(vt.begin(), vt.end());
    std::sort(vr.begin(), vr.end());
    std::transform(vr.begin(), vr.end(),
        std::ostream_iterator<int>(std::cout), std::mem_fn(&T::value_));
}

opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.0/../../../../include/c++/11.0.0/bits/predefined_ops.h:43:23: error: invalid operands to binary expression ('std::reference_wrapper<const Cat<int>>' and 'std::reference_wrapper<const Cat<int>>')
      { return *__it1 < *__it2; }
               ~~~~~~ ^ ~~~~~~