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

int& a = 5;

In order for the above code to work, int& needs to bind to a temporary object of type int created out of the expression 5. But binding int& to a temporay didn't appeal to Bjarne Stroustrup — and he gave an example, similar to the following, to illustrate his point:

void f(int &i) { ++i; }

float x = 10.0;
f(x); 
std::cout << x <<< std::endl;

What will the std::cout print¹? Looks like it will print 11.

It feels, ++i is changing the argument x, but it doesn't. This is one reason why the creator of C++ didn't permit temporaries to bind to non-const reference.

However, you can do this:

int const & i = 10;
int const & j = x; //x is float

And since C++11, you can do this:

int && i = 10;
int && i = x; //x is float

Hope that helps.

---

^{1. assuming int& can bind to the temporary created out of x.}

Answer 2

"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;

Answer 3

What you can do is

int b=5;
int &a=b;

or

const int& a = 5;