According to section 20.4.5, the function auto_ptr::operator=() returns a reference to an auto_ptr. The reason that operator=() usually returns a reference is to facilitate code like

    int x,y,z;
    x = y = z = 1;

However, given analogous code for auto_ptrs,

    auto_ptr<int> x, y, z;
    z.reset(new int(1));
    x = y = z;

the result would be that z and y would both be set to NULL, instead of all the auto_ptrs being set to the same value. This makes such cascading assignments useless and counterintuitive for auto_ptrs.