How can a Non-Const Reference not Bind to a Temporary Object?

Have you ever wondered why C++ prohibits non-const references from binding to temporary objects? What is it about rvalues that makes them so tricky? And how can function overloading be used to your advantage?

If you don’t handle temporary objects properly, you can run into compilation errors and undefined behavior. This article will cover the topic of non-const references, rvalues, move semantics, and function overloading to avoid running into these situations and write safer and more efficient C++ code.

Table of Contents:

• What is a Non-const Reference?
• Why Can’t a Non-const Reference Bind to a Temporary?
  • Non-const Reference Fails to Bind to a Temporary
  • Using a Named Variable (Works Fine)
  • Using a const Reference to Bind to a Temporary
• Avoiding Temporary Object Issues in Function Overloading
• Conclusion

What is a Non-const Reference?

In C++, a non-const reference is a reference mostly used for modifications of the object it refers to. This reference is denoted by using the & symbol, and it should be bound only to a valid lvalue (a named variable with a continuing memory address). The objects that do not have stable memory locations can easily be destroyed at the end of the expression, which is the reason why non-const references cannot bind to temporary objects like rvalues. 

Why Can’t a Non-const Reference Bind to a Temporary?

The temporary object (rvalue) in C++ is an unnamed object that is assigned during the expression evaluation. These temporary objects are present only within the expression when they are created and destroyed immediately after. There is a high chance of getting undefined behavior when a non-const reference is allowed to bind with the temporary object, which would create a reference to an object that no longer exists in the program. To prevent this issue, C++ allows the non-const reference to bind only to the lvalues. However, if you want to bind a reference to a temporary object, then you must use a const reference.

1. Non-const Reference Fails to Bind to a Temporary

This may lead to compilation errors.

Example: 

Output: 

In the above code, the compilation error occurs because we assigned a temporary value(rvalue) 5. The non-const reference allows only an lvalue, the temporaries (rvalues) are mostly destroyed after the expression, so binding a non-const reference can be unsafe. 

2. Using a Named Variable (Works Fine)

In this example, we use the lvalue to avoid the errors.

Example: 

Output: 

Here, the modify(int& x) takes a non-const reference, and the variable num(lvalue) is passed to modify(), which modifies the value from 5 to 10. Finally, the std::cout prints 10. 

3. Using a const Reference to Bind to a Temporary

A temporary (rvalue) can bind to a const& reference, and the lifetime of that temporary is extended until the lifetime of the reference ends. A temporary object is typically destroyed at the end of the expression that created it. However, a const reference prevents immediate destruction. By allowing the temporary to be safely accessible without modifying it. Since const int& does not allow for any modification operations to the const int&, there is no concern about modifying a short-lived value, which permits C++ to allow binding the temporary through const.

Example: 

Output: 

The above code, const int& ref = 5; allows the const reference to the temporary value as 5, it extends as long as the reference exists. This allows the temporary without modifying it and using std::cout it prints the ref value as 5. 

Avoiding Temporary Object Issues in Function Overloading

In C++, function overloading allows multiple functions with the same name but different parameters. Special care needs to be taken when we are dealing with temporary objects to avoid issues such as binding non-const reference to temporaries. Proper use of function overloading makes sure the temporary objects for safely handled without leading to any dangling references or undefined modifications. 

To handle these issues, we can overload by: 

• Lvalues using a non-const reference 
• Rvalues using a const reference 
• Rvalues reference to optimize the move semantics 

Example: 

Output: 

The above program demonstrates the function of overloading for correctly handling the lvalues and rvalues. It modifies the lvalues while calling the process and passes the temporary value like 15, it will bind to a const int&.

Conclusion

In C++, temporary objects cannot be bound to non-const references because the lifetime of a temporary is limited, and they are destroyed after the expression. To work safely with temporaries, we can either use a const reference, which will extend the temporary’s lifetime, or we can use an rvalue reference (&&) utilizing move semantics. If we provide a proper overload of functions that handle both lvalues and rvalues, then we can work correctly and safely, avoiding dangling references and undefined behavior.

You can learn more about C++ in the C++ article, and also explore C++ Interview Questions prepared by industry experts.

FAQs on How can a Non-Const Reference not Bind to a Temporary Object?