C++中函数的这个用法有些不解，望解惑，具体请见题干

Answer 1

The f function returns a reference to a, and a is a global variable with a fixed address, so this reference can be used as an lvalue, that is, it can be assigned a value.
This is a very tricky method, but it is not recommended. , because sometimes the variables you reference are defined very early, which is difficult for people who read this code to understand.

Answer 2

1. When an object is used as an rvalue, its content (value) is used. When it is used as an lvalue, its address is used.

2. Here the variable a is a global variable, and the function f returns a reference to a.

Answer 3

Function f returns a reference to a, which is equivalent to directly assigning a value to a