Home > Articles > Programming > C/C++

This chapter is from the book

Getting Started with C++

A C++ program consists of one or more compilation units. Each compilation unit is a separate source code file, typically with a .cpp extension (other common extensions are .cc and .cxx) that the compiler processes in one run. For each compilation unit, the compiler generates an object file, with the extension .obj (on Windows) or .o (on Unix and Mac OS X). The object file is a binary file that contains machine code for the architecture on which the program will run.

Once all the .cpp files have been compiled, we can combine the object files together to create an executable using a special program called the linker. The linker concatenates the object files and resolves the memory addresses of functions and other symbols referenced in the compilation units.

When building a program, exactly one compilation unit must contain a main() function that serves as the program's entry point. This function doesn't belong to any class; it is a global function. The process is shown schematically in Figure D.1.

app_d_fig01.gif

Figure D.1 The C++ compilation process (on Windows)

Unlike Java, where each source file must contain exactly one class, C++ lets us organize the compilation units as we want. We can implement several classes in the same .cpp file, or spread the implementation of a class across several .cpp files, and we can give the source files any names we like. When we make a change in one particular .cpp file, we need to recompile only that file and then relink the application to create a new executable.

Before we go further, let's quickly review the source code of a trivial C++ program that computes the square of an integer. The program consists of two compilation units: main.cpp and square.cpp.

Here's square.cpp:

1 double square(double n)
2 {
3     return n * n;
4 }

This file simply contains a global function called square() that returns the square of its parameter.

Here's main.cpp:

1 #include <cstdlib>
 2 #include <iostream>

 3 double square(double);

 4 int main(int argc, char *argv[])
 5 {
 6     if (argc != 2) {
 7         std::cerr << "Usage: square <number>" << std::endl;
 8         return 1;
 9     }

10     double n = std::strtod(argv[1], 0);
11     std::cout << "The square of " << argv[1] << " is "
12               << square(n) << std::endl;
13     return 0;
14 }

The main.cpp source file contains the main() function's definition. In C++, this function takes an int and a char * array (an array of character strings) as parameters. The program's name is available as argv[0] and the command-line arguments as argv[1], argv[2], ..., argv[argc - 1]. The parameter names argc ("argument count") and argv ("argument values") are conventional. If the program doesn't access the command-line arguments, we can define main() with no parameters.

The main() function uses strtod() ("string to double"), cout (C++'s standard output stream), and cerr (C++'s standard error stream) from the Standard C++ library to convert the command-line argument to a double and to print text to the console. Strings, numbers, and end-of-line markers (endl) are output using the << operator, which is also used for bit-shifting. To access this standard functionality, we need the #include directives on lines 1 and 2.

All the functions and most other items in the Standard C++ library are in the std namespace. One way to access an item in a namespace is to prefix its name with the namespace's name using the :: operator. In C++, the :: operator separates the components of a complex name. Namespaces make large multi-person projects easier because they help avoid name conflicts. We cover them later in this appendix.

The declaration on line 3 is a function prototype. It tells the compiler that a function exists with the given parameters and return value. The actual function can be located in the same compilation unit or in another compilation unit. Without the function prototype, the compiler wouldn't let us call the function on line 12. Parameter names in function prototypes are optional.

The procedure to compile the program varies from platform to platform. For example, to compile on Solaris with the Sun C++ compiler, we would type the following commands:

CC -c main.cpp
CC -c square.cpp
CC main.o square.o -o square

The first two lines invoke the compiler to generate .o files for the .cpp files. The third line invokes the linker and generates an executable called square, which we can run as follows:

./square 64

This run of the program outputs the following message to the console:

The square of 64 is 4096

To compile the program, you probably want to get help from your local C++ guru. Failing this, you can still read the rest of this appendix without compiling anything and follow the instructions in Chapter 1 to compile your first C++/Qt application. Qt provides tools that make it easy to build applications on all platforms.

Back to our program: In a real-world application, we would normally put the square() function prototype in a separate file and include that file in all the compilation units where we need to call the function. Such a file is called a header file and usually has a .h extension (.hh, .hpp, and .hxx are also common). If we redo our example using the header file approach, we would create a file called square.h with the following contents:

1 #ifndef SQUARE_H
2 #define SQUARE_H

3 double square(double);

4 #endif

The header file is bracketed by three preprocessor directives (#ifndef, #define, and #endif). These directives ensure that the header file is processed only once, even if the header file is included several times in the same compilation unit (a situation that can arise when header files include other header files). By convention, the preprocessor symbol used to accomplish this is derived from the file name (in our example, SQUARE_H). We will come back to the preprocessor later in this appendix.

The new main.cpp file looks like this:

1 #include <cstdlib>
 2 #include <iostream>

 3 #include "square.h"

 4 int main(int argc, char *argv[])
 5 {
 6     if (argc != 2) {
 7         std::cerr << "Usage: square <number>" << std::endl;
 8         return 1;
 9     }

10     double n = std::strtod(argv[1], 0);
11     std::cout << "The square of " << argv[1] << " is "
12               << square(n) << std::endl;
13     return 0;
14 }

The #include directive on line 3 expands to the contents of the file square.h. Directives that start with a # are picked up by the C++ preprocessor before the compilation proper takes place. In the old days, the preprocessor was a separate program that the programmer invoked manually before running the compiler. Modern compilers handle the preprocessor step implicitly.

The #include directives on lines 1 and 2 expand to the contents of the cstdlib and iostream header files, which are part of the Standard C++ library. Standard header files have no .h suffix. The angle brackets around the file names indicate that the header files are located in a standard location on the system, and double quotes tell the compiler to look in the current directory. Includes are normally gathered at the top of a .cpp file.

Unlike .cpp files, header files are not compilation units in their own right and do not produce any object files. Header files may only contain declarations that enable different compilation units to communicate with each other. Consequently, it would be inappropriate to put the square() function's implementation in a header file. If we did so in our example, nothing bad would happen, because we include square.h only once, but if we included square.h from several .cpp files, we would get multiple implementations of the square() function (one per .cpp file that includes it). The linker would then complain about multiple (identical) definitions of square() and refuse to generate an executable. Conversely, if we declare a function but never implement it, the linker complains about an "unresolved symbol".

So far, we have assumed that an executable consists exclusively of object files. In practice, it often also links against libraries that implement ready-made functionality. There are two main types of libraries:

  • Static libraries are put directly into the executable, as though they were object files. This ensures that the library cannot get lost but increases the size of the executable.
  • Dynamic libraries (also called shared libraries or DLLs) are located at a standard location on the user's machine and are automatically loaded at application startup.

For the square program, we link against the Standard C++ library, which is implemented as a dynamic library on most platforms. Qt itself is a collection of libraries that can be built either as static or as dynamic libraries (the default is dynamic).

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