Overloading reference operator

[Samkit Jain <samkit.feeds@gmail.com>]

Hi,
I overloaded reference operator in a templatized class like below:
template<class T>
MyClass
{
 public:
    MyClass() : m_data(10) { }
    operator T&() { return m_data; }
 private:
    T m_data;
};
Now I wrote a code to utilize reference operator as below:
int main()
{
    MyClass instance;
    instance++; // this works fine
    ++instance; // this too works fine
    instance = 105;    // this gives compile error. Why ?
    return 0;
}

The pre and post increment operators work fine, but a simple
assignment operator is giving compilation error.
Is there a valid reason for this or is this undefined behavior ?
Thank you.
Regards,
Samkit