Question

Why does this code fail to compile?

 double d ;
 int & i1 = d // Compilation FAILS  

While this one does?

 double d ;
 const int & i = d // Compilation Succeeds

Please, I am interested in knowing what was in mind of C++ designers that they allowed one behavior while disallowed another.

I know in either case it’s immoral, independent of technically possible or not.
Also FYI I am using GCC on mac with “-O0 -g3 -Wall -c -fmessage-length=0”

Answer 1

Because it creates a temporary int where the double is converted, and mutable references cannot bind to temporaries, whereas const ones can.

The problem with allowing mutable references to bind to temporaries is relatively clear.

Derived* ptr;
Base*& baseptr = ptr;
baseptr = new Base;
ptr->SomeDerivedFunction();