Why int& a = is not allowed in C++?

Question

I am reading about references in C++. It says that int& a = 5 gives compile time error.

In Thinking in C++ - Bruce Eckel, author says that

Answer 1

"The storage must be const because changing it would make no sense."

If you want a be a reference to a const value, you must declare it as const, because a is referencing to a temporary constant value, and changing it is not possible.

const int &a = 123;
a = 1000; // `a` is referencing to temporary 123, it is not possible to change it
          // We can not change 123 to 1000
          // Infact, we can change a variable which its value is 123 to 1000
          // Here `a` is not a normal variable, it's a reference to a const
          // Generally, `int &a` can not bind to a temporary object

For non-const bindings:

int x = 1;
int &a = x;

a is a reference to a lvalue. Simple speaking, it's an alias name for another variable, so on the right hand you should give a variable. The reference a can not change and bind to another variable after it's first binding;

In C++11, you can reference to temporary objects/values by rvalue references:

int &&a = 123;