必须在构造函数初始化列表中初始化引用类型吗?

As self-exercise, I have written this simple code:

#include <iostream>

int gIndex = 3;

template <class T> class Array
{
public:
    explicit Array(int size);
    T& operator[](int i) { return m_data[i]; }
    T operator[](int i) const { return m_data[i]; }
    T getAnchorPoint() const { return m_data[m_anchor]; }
private:
    T* m_data;
    int m_size;
    int& m_anchor;
};

template <class T> Array<T>::Array(int size) : m_size(size), m_data(new T[size])
{
    memset(m_data, 0, size*sizeof(T));
    m_anchor = gIndex;
}

int main()
{
    Array<double> a(10);
    return 0;
}

我收到一个编译错误，内容为: