Difference between const int &x = 4 and const int x = 4

Question

The sole purpose of references is aliasing. Assigning a reference (referring to a constant int) to a integer seems absurd since it is not an alias (and it doesn't give an error!). I suppose it is similar to defining a constant int itself. Is there any difference?

Answer 1

Within a function body or file scope, the only difference is decltype(x) . In one case, it is int const and the other int const& .

The const int & x=7; creates a temporary anonymous int with value 7 . It then binds a reference x to it. The lifetime of the temporary is the extended to that of the reference. This is basically indistinguishable from x being the name of a const int with value 7 .

An exception to it being nigh identical is when the binding occurs within an object's constructor as part of member initialization. In that case, the lifetime is not extended.

I suspect you can induce this with:

struct Foo{
  int const& x=7;
  Foo(){};
};

Either the above syntax is illegal or it dangles (I do not recall if there is a corner case in the standard for references), while:

struct Foo{
  int const x=7;
  Foo(){};
};

is both legal and does not dangle. So there is a difference.

There would also be a difference as a parameter to a function, wbhere =7 simply provides a default.