Home > Articles > Programming > C/C++

  • Print
  • + Share This
This chapter is from the book

18.6 Arrays

round-b.jpg

For a while, we have used array to refer to a sequence of objects allocated on the free store. We can also allocate arrays elsewhere as named variables. In fact, they are common

  • As global variables (but global variables are most often a bad idea)
  • As local variables (but arrays have serious limitations there)
  • As function arguments (but an array doesn’t know its own size)
  • As class members (but member arrays can be hard to initialize)
round-g.jpg

Now, you might have detected that we have a not-so-subtle bias in favor of vectors over arrays. Use std::vector where you have a choice — and you have a choice in most contexts. However, arrays existed long before vectors and are roughly equivalent to what is offered in other languages (notably C), so you must know arrays, and know them well, to be able to cope with older code and with code written by people who don’t appreciate the advantages of vector.

round-b.jpg

So, what is an array? How do we define an array? How do we use an array? An array is a homogeneous sequence of objects allocated in contiguous memory; that is, all elements of an array have the same type and there are no gaps between the objects of the sequence. The elements of an array are numbered from 0 upward. In a declaration, an array is indicated by “square brackets”:

const int max = 100;
int gai[max];               // a global array (of 100 ints); “lives forever”

void f(int n)
{
     char lac[20];        // local array; “lives” until the end of scope
     int lai[60];
     double lad[n];      // error: array size not a constant
     // . . .
}

Note the limitation: the number of elements of a named array must be known at compile time. If you want the number of elements to be a variable, you must put it on the free store and access it through a pointer. That’s what vector does with its array of elements.

Just like the arrays on the free store, we access named arrays using the subscript and dereference operators ([ ] and *). For example:

void f2()
{
     char lac[20];      // local array; “lives” until the end of scope

     lac[7] = 'a';
     *lac = 'b';       // equivalent to lac[0]='b'

     lac[–2] = 'b';   // huh?
     lac[200] = 'c';  // huh?
}
round-r.jpg

This function compiles, but we know that “compiles” doesn’t mean “works correctly.” The use of [ ] is obvious, but there is no range checking, so f2() compiles, and the result of writing to lac[–2] and lac[200] is (as for all out-of-range access) usually disastrous. Don’t do it. Arrays do not range check. Again, we are dealing directly with physical memory here; don’t expect “system support.”

round-r.jpg

But couldn’t the compiler see that lac has just 20 elements so that lac[200] is an error? A compiler could, but as far as we know no production compiler does. The problem is that keeping track of array bounds at compile time is impossible in general, and catching errors in the simplest cases (like the one above) only is not very helpful.

18.6.1 Pointers to array elements

A pointer can point to an element of an array. Consider:

double ad[10];
double* p = &ad[5];            // point to ad[5]

We now have a pointer p to the double known as ad[5]:

We can subscript and dereference that pointer:

*p =7;
p[2] = 6;
p[–3] = 9;

We get

That is, we can subscript the pointer with both positive and negative numbers. As long as the resulting element is in range, all is well. However, access outside the range of the array pointed into is illegal (as with free-store-allocated arrays; see §17.4.3). Typically, access outside an array is not detected by the compiler and (sooner or later) is disastrous.

Once a pointer points into an array, addition and subscripting can be used to make it point to another element of the array. For example:

p += 2;               // move p 2 elements to the right

We get

And

p –= 5;            // move p 5 elements to the left

We get

round-b.jpg

Using +, , +=, and –= to move pointers around is called pointer arithmetic. Obviously, if we do that, we have to take great care to ensure that the result is not a pointer to memory outside the array:

p += 1000;                  // insane: p points into an array with just 10 elements
double d = *p;              // illegal: probably a bad value
                           // (definitely an unpredictable value)
*p = 12.34;               // illegal: probably scrambles some unknown data

Unfortunately, not all bad bugs involving pointer arithmetic are that easy to spot. The best policy is usually simply to avoid pointer arithmetic.

The most common use of pointer arithmetic is incrementing a pointer (using ++) to point to the next element and decrementing a pointer (using ––) to point to the previous element. For example, we could print the value of ad’s elements like this:

for (double* p = &ad[0]; p<&ad[10]; ++p) cout << *p << '\n';

Or backward:

for (double* p = &ad[9]; p>=&ad[0]; ––p) cout << *p << '\n';

This use of pointer arithmetic is not uncommon. However, we find the last (“backward”) example quite easy to get wrong. Why &ad[9] and not &ad[10]? Why >= and not >? These examples could equally well (and equally efficiently) be done using subscripting. Such examples could be done equally well using subscripting into a vector, which is more easily range checked.

Note that most real-world uses of pointer arithmetic involve a pointer passed as a function argument. In that case, the compiler doesn’t have a clue how many elements are in the array pointed into: you are on your own. That is a situation we prefer to stay away from whenever we can.

Why does C++ have (allow) pointer arithmetic at all? It can be such a bother and doesn’t provide anything new once we have subscripting. For example:

double* p1 = &ad[0];
double* p2 = p1+7;
double* p3 = &p1[7];
if (p2 != p3) cout << "impossible!\n";
round-b.jpg

Mainly, the reason is historical. These rules were crafted for C decades ago and can’t be removed without breaking a lot of code. Partly, there can be some convenience gained by using pointer arithmetic in some important low-level applications, such as memory managers.

18.6.2 Pointers and arrays

round-b.jpg

The name of an array refers to all the elements of the array. Consider:

char ch[100];

The size of ch, sizeof(ch), is 100. However, the name of an array turns into (“decays to”) a pointer with the slightest excuse. For example:

char* p = ch;

Here p is initialized to &ch[0] and sizeof(p) is something like 4 (not 100).

This can be useful. For example, consider a function strlen() that counts the number of characters in a zero-terminated array of characters:

int strlen(const char* p)        // similar to the standard library strlen()
{
       int count = 0;
       while (*p) { ++count; ++p; }
       return count;
}

We can now call this with strlen(ch) as well as strlen(&ch[0]). You might point out that this is a very minor notational advantage, and we’d have to agree.

One reason for having array names convert to pointers is to avoid accidentally passing large amounts of data by value. Consider:

int strlen(const char a[])       // similar to the standard library strlen()
{
       int count = 0;
       while (a[count]) { ++count; }
       return count;
}

char lots [100000];

void f()
{
       int nchar = strlen(lots);
       // . . .
}

Naively (and quite reasonably), you might expect this call to copy the 100,000 characters specified as the argument to strlen(), but that’s not what happens. Instead, the argument declaration char p[] is considered equivalent to char* p, and the call strlen(lots) is considered equivalent to strlen(&lots[0]). This saves you from an expensive copy operation, but it should surprise you. Why should it surprise you? Because in every other case, when you pass an object and don’t explicitly declare an argument to be passed by reference (§8.5.3–6), that object is copied.

Note that the pointer you get from treating the name of an array as a pointer to its first element is a value and not a variable, so you cannot assign to it:

char ac[10];
ac = new char [20];              // error: no assignment to array name
&ac[0] = new char [20];          // error: no assignment to pointer value

Finally! A problem that the compiler will catch!

As a consequence of this implicit array-name-to-pointer conversion, you can’t even copy arrays using assignment:

int x[100];
int y[100];
// . . .
x = y;                           // error
int z[100] = y;                  // error

This is consistent, but often a bother. If you need to copy an array, you must write some more elaborate code to do so. For example:

for (int i=0; i<100; ++i) x[i]=y[i];    // copy 100 ints
memcpy(x,y,100*sizeof(int));           // copy 100*sizeof(int) bytes
copy(y,y+100, x);                      // copy 100 ints

Note that the C language doesn’t support anything like vector, so in C, you must use arrays extensively. This implies that a lot of C++ code uses arrays (§27.1.2). In particular, C-style strings (zero-terminated arrays of characters; see §27.5) are very common.

If we want assignment, we have to use something like the standard library vector. The vector equivalent to the copying code above is

vector<int> x(100);
vector<int> y(100);
// . . .
x = y;                         // copy 100 ints

18.6.3 Array initialization

round-b.jpg

An array of chars can be initialized with a string literal. For example:

char ac[] = "Beorn";               // array of 6 chars

Count those characters. There are five, but ac becomes an array of six characters because the compiler adds a terminating zero character at the end of a string literal:

18fig13.jpg

A zero-terminated string is the norm in C and many systems. We call such a zero-terminated array of characters a C-style string. All string literals are C-style strings. For example:

char* pc = "Howdy";               // pc points to an array of 6 chars

Graphically:

18fig14.jpg

Note that the char with the numeric value 0 is not the character '0' or any other letter or digit. The purpose of that terminating zero is to allow functions to find the end of the string. Remember: An array does not know its size. Relying on the terminating zero convention, we can write

int strlen(const char* p)              // similar to the standard library strlen()
{
       int n = 0;
       while (p[n]) ++n;
       return n;
}

Actually, we don’t have to define strlen() because it is a standard library function defined in the <string.h> header (§27.5, §B.11.3). Note that strlen() counts the characters, but not the terminating 0; that is, you need n+1 chars to store n characters in a C-style string.

Only character arrays can be initialized by literal strings, but all arrays can be initialized by a list of values of their element type. For example:

int ai[] = { 1, 2, 3, 4, 5, 6 };          // array of 6 ints
int ai2[100] = {0,1,2,3,4,5,6,7,8,9};     // the last 90 elements are initialized to 0
double ad[100] = { };                     // all elements initialized to 0.0
char chars[] = {'a', 'b', 'c'};          // no terminating 0!

Note that the number of elements of ai is six (not seven) and the number of elements for chars is three (not four) — the “add a 0 at the end” rule is for literal character strings only. If an array isn’t given a size, that size is deduced from the initializer list. That’s a rather useful feature. If there are fewer initializer values than array elements (as in the definitions of ai2 and ad), the remaining elements are initialized by the element type’s default value.

18.6.4 Pointer problems

Like arrays, pointers are often overused and misused. Often, the problems people get themselves into involve both pointers and arrays, so we’ll summarize the problems here. In particular, all serious problems with pointers involve trying to access something that isn’t an object of the expected type, and many of those problems involve access outside the bounds of an array. Here we will consider

  • Access through the null pointer
  • Access through an uninitialized pointer
  • Access off the end of an array
  • Access to a deallocated object
  • Access to an object that has gone out of scope

In all cases, the practical problem for the programmer is that the actual access looks perfectly innocent; it is “just” that the pointer hasn’t been given a value that makes the use valid. Worse (in the case of a write through the pointer), the problem may manifest itself only a long time later when some apparently unrelated object has been corrupted. Let’s consider examples:

round-r.jpg

Don’t access through the null pointer:

int* p = nullptr;
*p = 7;            // ouch!

Obviously, in real-world programs, this typically occurs when there is some code in between the initialization and the use. In particular, passing p to a function and receiving it as the result from a function are common examples. We prefer not to pass null pointers around, but if you have to, test for the null pointer before use:

int* p = fct_that_can_return_a_nullptr();
if (p == nullptr) {
       // do something
}
else {
       // use p
       *p = 7;
}

and

void fct_that_can_receive_a_nullptr(int* p)
{
       if (p == nullptr) {
       // do something
       }
       else {
                 // use p
                 *p = 7;
          }
}

Using references (§17.9.1) and using exceptions to signal errors (§5.6 and §19.5) are the main tools for avoiding null pointers.

Do initialize your pointers:

round-g.jpg
int* p;
*p = 9;            // ouch!

In particular, don’t forget to initialize pointers that are class members.

Don’t access nonexistent array elements:

round-r.jpg
int a[10];
int* p = &a[10];
*p = 11;           // ouch!
a[10] = 12;       // ouch!

Be careful with the first and last elements of a loop, and try not to pass arrays around as pointers to their first elements. Instead use vectors. If you really must use an array in more than one function (passing it as an argument), then be extra careful and pass its size along.

Don’t access through a deleted pointer:

round-r.jpg
int* p = new int{7};
// . . .
delete p;
// . . .
*p = 13;         // ouch!

The delete p or the code after it may have scribbled all over *p or used it for something else. Of all of these problems, we consider this one the hardest to systematically avoid. The most effective defense against this problem is not to have “naked” news that require “naked” deletes: use new and delete in constructors and destructors or use a container, such as Vector_ref (§E.4), to handle deletes.

Don’t return a pointer to a local variable:

round-r.jpg
int* f()
{
     int x = 7;
     // . . .
      return &x;
}
// . . .
int* p = f();
// . . .
*p = 15;            // ouch!

The return from f() or the code after it may have scribbled all over *p or used it for something else. The reason for that is that the local variables of a function are allocated (on the stack) upon entry to the function and deallocated again at the exit from the function. In particular, destructors are called for local variables of classes with destructors (§17.5.1). Compilers could catch most problems related to returning pointers to local variables, but few do.

Consider a logically equivalent example:

vector& ff()
{
    vector x(7);     // 7 elements
    // . . .
    return x;
}        // the vector x is destroyed here
// . . .

vector& p = ff();
// . . .
p[4] = 15;           // ouch!

Quite a few compilers catch this variant of the return problem.

round-g.jpg

It is common for programmers to underestimate these problems. However, many experienced programmers have been defeated by the innumerable variations and combinations of these simple array and pointer problems. The solution is not to litter your code with pointers, arrays, news, and deletes. If you do, “being careful” simply isn’t enough in realistically sized programs. Instead, rely on vectors, RAII (“Resource Acquisition Is Initialization”; see §19.5), and other systematic approaches to the management of memory and other resources.

  • + Share This
  • 🔖 Save To Your Account

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