Home > Articles

This chapter is from the book

This chapter is from the book

12.2 vector

The most useful standard-library container is vector. A vector is a sequence of elements of a given type. The elements are stored contiguously in memory. A typical implementation of vector (§5.2.2, §6.2) will consist of a handle holding pointers to the first element, one-past-the-last element, and one-past-the-last allocated space (§13.1) (or the equivalent information represented as a pointer plus offsets):

In addition, it holds an allocator (here, alloc), from which the vector can acquire memory for its elements. The default allocator uses new and delete to acquire and release memory (§12.7). Using a slightly advanced implementation technique, we can avoid storing any data for simple allocators in a vector object.

We can initialize a vector with a set of values of its element type:

vector<Entry> phone_book = {
         {"David Hume",123456},
         {"Karl Popper",234567},
         {"Bertrand Arthur William Russell",345678}

Elements can be accessed through subscripting. So, assuming that we have defined << for Entry, we can write:

void print_book(const vector<Entry>& book)
        for (int i = 0; i!=book.size(); ++i)
                 cout << book[i] << '\n';

As usual, indexing starts at 0 so that book[0] holds the entry for David Hume. The vector member function size() gives the number of elements.

The elements of a vector constitute a range, so we can use a range-for loop (§1.7):

void print_book(const vector<Entry>& book)
        for (const auto& x : book)        // for "auto" see §1.4
                cout << x << '\n';

When we define a vector, we give it an initial size (initial number of elements):

vector<int> v1 = {1, 2, 3, 4};            // size is 4
vector<string> v2;                           // size is 0
vector<Shape*> v3(23);                  // size is 23; initial element value: nullptr
vector<double> v4(32,9.9);            // size is 32; initial element value: 9.9

An explicit size is enclosed in ordinary parentheses, for example, (23), and by default, the elements are initialized to the element type’s default value (e.g., nullptr for pointers and 0 for numbers). If you don’t want the default value, you can specify one as a second argument (e.g., 9.9 for the 32 elements of v4).

The initial size can be changed. One of the most useful operations on a vector is push_back(), which adds a new element at the end of a vector, increasing its size by one. For example, assuming that we have defined >> for Entry, we can write:

void input()
        for (Entry e; cin>>e; )

This reads Entrys from the standard input into phone_book until either the end-of-input (e.g., the end of a file) is reached or the input operation encounters a format error.

The standard-library vector is implemented so that growing a vector by repeated push_back()s is efficient. To show how, consider an elaboration of the simple Vector from Chapter 5 and Chapter 7 using the representation indicated in the diagram above:

template<typename T>
class Vector {
       allocator<T> alloc;    // standard-library allocator of space for Ts
       T* elem;                     // pointer to first element
       T* space;                   // pointer to first unused (and uninitialized) slot
       T* last;                       // pointer to last slot
       // ...
       int size() const { return space-elem; }            // number of elements
       int capacity() const { return last-elem; }        // number of slots available for elements
       // ...
       void reserve(int newsz);                         // increase capacity() to newsz
       // ...
       void push_back(const T& t);                 // copy t into Vector
       void push_back(T&& t);                         // move t into Vector

The standard-library vector has members capacity(), reserve(), and push_back(). The reserve() is used by users of vector and other vector members to make room for more elements. It may have to allocate new memory and when it does, it moves the elements to the new allocation. When reserve() moves elements to a new and larger allocation, any pointers to those elements will now point to the wrong location; they have become invalidated and should not be used.

Given capacity() and reserve(), implementing push_back() is trivial:

template<typename T>
void Vector<T>::push_back(const T& t)
        if (capacity()<=size())                         // make sure we have space for t
                 reserve(size()==0?8:2*size());  // double the capacity
        construct_at(space,t);                       // initialize *space to t ("place t at space")

Now allocation and relocation of elements happens only infrequently. I used to use reserve() to try to improve performance, but that turned out to be a waste of effort: the heuristic used by vector is on average better than my guesses, so now I only explicitly use reserve() to avoid reallocation of elements when I want to use pointers to elements.

A vector can be copied in assignments and initializations. For example:

vector<Entry> book2 = phone_book;

Copying and moving vectors are implemented by constructors and assignment operators as described in §6.2. Assigning a vector involves copying its elements. Thus, after the initialization of book2, book2 and phone_book hold separate copies of every Entry in the phone book. When a vector holds many elements, such innocent-looking assignments and initializations can be expensive. Where copying is undesirable, references or pointers (§1.7) or move operations (§6.2.2) should be used.

The standard-library vector is very flexible and efficient. Use it as your default container; that is, use it unless you have a solid reason to use some other container. If you avoid vector because of vague concerns about “efficiency,” measure. Our intuition is most fallible in matters of the performance of container uses.

12.2.1 Elements

Like all standard-library containers, vector is a container of elements of some type T, that is, a vector<T>. Just about any type qualifies as an element type: built-in numeric types (such as char, int, and double), user-defined types (such as string, Entry, list<int>, and Matrix<double,2>), and pointers (such as const char*, Shape*, and double*). When you insert a new element, its value is copied into the container. For example, when you put an integer with the value 7 into a container, the resulting element really has the value 7. The element is not a reference or a pointer to some object containing 7. This makes for nice, compact containers with fast access. For people who care about memory sizes and run-time performance this is critical.

If you have a class hierarchy (§5.5) that relies on virtual functions to get polymorphic behavior, do not store objects directly in a container. Instead store a pointer (or a smart pointer; §15.2.1). For example:

vector<Shape> vs;                                  // No, don't - there is no room for a Circle or a Smiley (§5.5)
vector<Shape*> vps;                              // better, but see §5.5.3 (don't leak)
vector<unique_ptr<Shape>> vups;      // OK

12.2.2 Range Checking

The standard-library vector does not guarantee range checking. For example:

void silly(vector<Entry>& book)
         int i = book[book.size()].number;            // book.size() is out of range
         // ...

That initialization is likely to place some random value in i rather than giving an error. This is undesirable, and out-of-range errors are a common problem. Consequently, I often use a simple range-checking adaptation of vector:

template<typename T>
struct Vec : std::vector<T> {
         using vector<T>::vector;                          // use the constructors from vector (under the name Vec)

         T& operator[](int i) { return vector<T>::at(i); }                           // range check
         const T& operator[](int i) const { return vector<T>::at(i); }      // range check const objects; §5.2.1

         auto begin() { return Checked_iter<vector<T>>{*this}; }          // see §13.1
         auto end() { return Checked_iter<vector<T>>{*this, vector<T>::end()};}

Vec inherits everything from vector except for the subscript operations that it redefines to do range checking. The at() operation is a vector subscript operation that throws an exception of type out_of_range if its argument is out of the vector’s range (§4.2).

For Vec, an out-of-range access will throw an exception that the user can catch. For example:

void checked(Vec<Entry>& book)
        try {
                  book[book.size()] = {"Joe",999999};       // will throw an exception
                  // ...
        catch (out_of_range&) {
                 cerr << "range error\n";

The exception will be thrown, and then caught (§4.2). If the user doesn’t catch an exception, the program will terminate in a well-defined manner rather than proceeding or failing in an undefined manner. One way to minimize surprises from uncaught exceptions is to use a main() with a try-block as its body. For example:

int main()
try {
          // your code

catch (out_of_range&) {
         cerr << "range error\n";
catch (...) {
         cerr << "unknown exception thrown\n";

This provides default exception handlers so that if we fail to catch some exception, an error message is printed on the standard error-diagnostic output stream cerr (§11.2).

Why doesn’t the standard guarantee range checking? Many performance-critical applications use vectors and checking all subscripting implies a cost on the order of 10%. Obviously, that cost can vary dramatically depending on hardware, optimizers, and an application’s use of subscripting. However, experience shows that such overhead can lead people to prefer the far more unsafe builtin arrays. Even the mere fear of such overhead can lead to disuse. At least vector is easily range checked at debug time and we can build checked versions on top of the unchecked default.

A range-for avoids range errors at no cost by implicitly accessing all elements in the range. As long as their arguments are valid, the standard-library algorithms do the same to ensure the absence of range errors.

If you use vector::at() directly in your code, you don’t need my Vec workaround. Furthermore, some standard libraries have range-checked vector implementations that offer more complete checking than Vec.

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.


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.


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.


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.


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


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


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.


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.


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