c++输入与运算符<<和[]混淆

我试图将数组元素的值打印为 cout << array[0]

example.cpp:44:20: error: no match for ‘operator<<’ in ‘std::cout << a_0.fixedarr<T, N>::operator[] [with T = int, long unsigned int N = 5ul, size_t = long unsigned int](0ul)’

(整个源代码来自更复杂的东西,但我想我已经把它缩减到一个最小的例子)。

#include <assert.h>
#include <cassert>
#include <climits>
#include <cstdio>
#include <iostream>

using namespace std;

template<typename T>
class fixedarrRef{
    T* ref;
    int sz;
    public:
        fixedarrRef(T* t, int psz){ ref = t; sz = psz;}

        T val(){ return ref[0]; }
};

template<typename T, size_t N>
class fixedarr{
    public:
        T arr[N];
        fixedarr(){
            for(int i=0; i<N; ++i){
                arr[i] = 0;
            }
        }
        inline fixedarrRef<T> operator[] (const size_t i) const{
            assert ( i < N);
            return fixedarrRef<T>((T*)&arr[i], N-i);
        }
};

template <typename T>
ostream & operator << (ostream &out, fixedarrRef<T> &v)
{
    return (out << v.val());
}   

int main() {
    fixedarr<int, 5> a_0;
    fixedarrRef<int> r = a_0[0];
    cout << (a_0[0]) << endl;
    // cout << r << endl;
    return 0;
}

请注意,最后的注释代码是有效的。提前谢谢。