PODs, non-PODs, rvalue and lvalues

Question

Could anyone explain the details in terms of rvalues, lvalues, PODs, and non-PODs the reason why the first expression marked below is not ok while the secon

Answer 1

In my understanding both int() and A() should be rvalues, no?

Correct, the epxression T() is always an rvalue for scalar and user-defined types T. As long as no const is involved, the expression T() is a modifiable rvalue, to be more precise.

Assignment involving scalar types requires a modifiable lvalue on the left hand side of the assignment operator. Since int() isn't an lvalue, you can't assign to int().

For user-defined types, assignment is a special member function, and member functions can also be called on rvalues (see §3.10 section 10). That's why A().operator=(a) is well formed.