Home > Articles > Programming > Java

This chapter is from the book

2.4. Operator overloading in C++

2.4.1. Member function versus independent function

C++ provides two ways of defining the meaning of an operator applied to one or more objects. We can define an operator function either as a member function or as an independent function. Consider the following code:

Angle theta, phi;
      .
      .
phi = theta * 2.0;

Unless we’ve defined a meaning for the * operator, the compiler will complain that the operator is not defined for an Angle left operand. To legitimize the above code, we might define an independent function:

Angleoperator* (const Angle Is, const double rs)
{Angleresult = Is;
 result, value *= rs;// (needs friend access)
 result.normalize();
 return result; }

We conventionally use the names Is and rs for the left and right operands of binary operator functions.

Alternatively, we could define * as a member function:

Angle Angle::operator*(const double rs) const
{Angleresult = *this;
 result.value *= rs;
 result.normalize();
 return result; }

Here, the left-side operand is implied: It’s the object for which the function was invoked.

In either case, the compiler simply transforms the normal expression syntax into a function call, so that . . theta * 2.0 . . becomes either

. . operator* (theta, 2.0). . // Operator* defined
                                 // as independent

or

. . theta.operator* (2.0). . // Operator* defined
                                // as member

As a general rule, I recommend

  • the member function, whenever

    • the left operand must be an object of the class, or
    • the function needs access to private members
  • the independent function, whenever

    • the left-side parameter is not a member of the class, or
    • we want to allow either operand to be converted implicitly, by invoking a single-parameter constructor

C++ requires a member function for the assignment operator.

Neither version above takes care of all legitimate multiplications of an Angle by a pure number. The client program might have this coded:

.. 2.0 * theta . .

That won’t match the parameter signature of either the member or the nonmember version. We need two multiplication functions: one of the above and

Angle operator* (const double Is, const Angle rs);

Because multiplication is commutative, implementing the second function is trivial, regardless of whether the other one is a member or an independent function. We simply define it in terms of the other function:

Angle operator* (const double Is, const Angle rs)
    {return rs * Is;}

Problems and exercises

2.4-1 The last multiplication operator above would be valid for any class as long as multiplication is commutative. Some designers might suggest, therefore, a global function template:

template<class T>
 T  operator* (const T Is, const T rs)
    {return rs * Is;}

What’s wrong with that suggestion?

2.4.2. Sequence and localization

The last example illustrates defining some overloaded operator functions in terms of others. In order to simplify future maintenance, we should do this whenever it doesn’t compromise efficiency.

Another obvious candidate is the combination of a binary arithmetic operator such as + and the corresponding compound assignment operator +=. Some programmers are irritated when they learn that they have to define both. If we’ve defined +, they argue, shouldn’t the compiler know what += means?

Well, it doesn’t, and tedious as it is, you still have to define both operators. An obvious but somewhat inefficient approach is to define the compound assignment operator as a member function in terms of the simple arithmetic operator:

Money  operator+ (const Money rs) const
  {Money result;
  result.value = value + rs.value; // (or whatever)
  return result;
  }
Money&operator+= (const Money rs)
  {Money result = *this + rs; return *this;}

That works, but as Scott Meyers and others point out, it’s unnecessarily expensive.4 The efficient approach is to define the compound assignment operator first as primitive, and then define the simple binary operator in terms of it:

Money& operator+= (const Money rs)
      {value += rs.value;
      return *this;}//Note: no new object
Money  operator+(const Money rs) const
      (Money result = *this;
       return result += rs;
      }

The latter approach avoids creating a new object in the compound assignment operator function. With that in mind, we advise client programs to prefer compound assignments, especially where the expression contains only one binary operator.

Note that the second version of the simple binary operator function above knows nothing about the object’s internal representation. It could therefore be implemented as a nonmember, non-friend inline function. Some smart compilers may be able to optimize away the resulting object if we rewrite the simple + operator to use an unnamed temporary object by explicitly calling the copy constructor:

Money  operator+(const Money rs) const
      {return Money(*this) += rs;
      }

Most examples in this book follow Meyers’ recommendation.

Problems and exercises

2.4-2 Both versions of the compound assignment operator += return a reference to the object, while the simple + operator returns an actual object. Are both of those conventions necessary? Why?

2.4-3 Suppose we learn that the project for which we’re developing a class needs only the simple operators and not the compound assignment ones. How would that knowledge alter our strategy in defining binary arithmetic operators for the class?

2.4.3. Increment and decrement operators

In later chapters, we’ll examine when it’s appropriate to overload the increment (++) and decrement (—) operators. Here, we’ll just look at some of the mechanics.

First, we have to distinguish between the prefix version (++k) and the postfix version (k++). C++ recognizes the following artifice:

const ClassName& operator++();  //  Prefix version
      ClassName operator++(int);//  Postfix version

The dummy int parameter to the postfix version is never used.

Second, note that the prefix version doesn’t create a new object; it just changes the state of the object for which it’s invoked. The result is a reference, so as to avoid creating a temporary object. We make it const for consistency with C, where the result is not an Lvalue into which the program can store a new value.

Finally, we can always define the postfix version in terms of the prefix version:

ClassNameoperator++(int)
 {ClassName result = *this;
 ++(*this);
 return result;
 }

2.4.4. In-line versus separately compiled function

In object-oriented programming, many of the methods are much smaller than typical functions in purely procedural programs. An accessor function, for example, often consists only of a return statement. Since conventional subroutine linkage would then account for an unacceptably large percentage of the function’s execution time, C++ needed a construct that provided the modularity of functions without the overhead of subroutine linkage. That construct is the inline function.

You tell the compiler that a function should be generated in-line in either of two ways:

  • For any member or independent function, code the inline specifier.
  • For a member or friend function, define the function body inside the class definition.

In Java, of course, we fully define all methods within the class definition. We trust the compiler to decide which functions should be generated in-line.

2.4.5. What about exponentiation?

Programmers often complain about C’s lack of the exponentiation operator supported by almost every other procedural programming language, even COBOL. The ability to define operators in C++ may tempt us to try to fill that need, but we’ll be unsuccessful.

Syntactic ambiguities, precedence confusion, or semantic conflicts would result if we were allowed to define, say, x**n or x^n to mean exponentiation.

When would x**p mean x* (*p) ? Should a/b**c mean a/(b**c) or should it mean (a/b) **c? When would a^b have its original Boolean exclusive or meaning? (If you’re skeptical, you can read Stroustrup’s discussion and explanation of this issue.5) So, we’re stuck with using a named function for exponentiation. The C library’s function

double pow(const double x,const double y);

takes care of the most general case of xy, but if you want to go to extra trouble for the common situation in which the exponent is an integer, you can provide an efficient specialized version, such as this recursive function template:6

template<class T>  T   power(const T x,const int n)
 {T  t;
  return  n == 0  ? 1                   //  Base cases
       :  n == 1  ? x                   //    (optional)
       :  n <  0  ? 1 / power(x, -n)    //  Negative power
       :  n%2 == 1? x * power(x,n-1)    //  Odd power
       :  t = power(x,n/2), t * t;      //  Even power
}

Note that the nested selection (?:) operators don’t require parentheses since they associate left to right. That lets us list the conditions in a column, with the corresponding actions to the right, a rather readable construct once you’re familiar with it. If you’re acquainted with LISP, you’ll recognize this construct as equivalent to the COND function.7

Problems and exercises

2.4-4 The last line of the power function above is illegal in Java. Why? How can the Java programmer change it so that it’s legal and yields the correct result?

2.4-5 Why is the second base case (n==1) optional? What would happen if we removed that line? Why should we leave it in?

2.4-6 How many multiplications will be performed for n=35? Is that optimal?

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020