Using a tempororary object with operator<<

Consider a class:

class RunningSum {
public:
    void Add(int i) { m_sum += i; }
    operator int() { return m_sum; }
private:
    int m_sum;
};

Used like this:

RunningSum rs;
rs.Add(5);
rs.Add(13);
int x = rs; // x = 18

Now I'd like to be able to say:

RunningSum rs;
rs << 5 << 13;
int x = rs; // x = 18

so define:

RunningSum& operator<<(RunningSum& rs, int i) { rs.Add(i); return
rs; }

But now I try:

int x = RunningSum() << 5 << 13;

I get a warning from the compiler that I'm trying to cast a temporary
RunningSum object to RunningSum&.
The warning seems legitimate (normally I wouldn't want to bind a
reference to a temporary), but in this particular case, this IS what I
want.

I could change RunningSum::Add to return a RunningSum& and write:

int x = RunningSum().Add(5).Add(13); // x = 18

but that isn't the syntax I want.
Is there a way to get the operator syntax and allow the use of the
temporary here without violating language rules?

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