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4.7 — Structs

There are many instances in programming where we need more than one variable in order to represent an object. For example, to represent yourself, you might want to store your name, your birthday, your height, your weight, or any other number of characteristics about yourself. You could do so like this:

However, you now have 6 independent variables that are not grouped in any way. If you wanted to pass information about yourself to a function, you’d have to pass each variable individually. Furthermore, if you wanted to store information about someone else, you’d have to declare 6 more variables for each additional person! As you can see, this can quickly get out of control.

Fortunately, C++ allows us to create our own user-defined aggregate data types. An aggregate data type is a data type that groups multiple individual variables together. One of the simplest aggregate data types is the struct. A struct (short for structure) allows us to group variables of mixed data types together into a single unit.

Declaring and defining structs

Because structs are user-defined, we first have to tell the compiler what our struct looks like before we can begin using it. To do this, we declare our struct using the struct keyword. Here is an example of a struct declaration:

This tells the compiler that we are defining a struct named Employee. The Employee struct contains 3 variables inside of it: a short named id, an int named age, and a double named wage. These variables that are part of the struct are called members (or fields). Keep in mind that Employee is just a declaration -- even though we are telling the compiler that the struct will have member variables, no memory is allocated at this time. By convention, struct names start with a capital letter to distinguish them from variable names.

Warning: One of the easiest mistakes to make in C++ is to forget the semicolon at the end of a struct declaration. This will cause a compiler error on the next line of code. Modern compilers like Visual Studio 2010 will give you an indication that you may have forgotten a semicolon, but older or less sophisticated compilers may not, which can make the actual error hard to find.

In order to use the Employee struct, we simply declare a variable of type Employee:

This defines a variable of type Employee named joe. As with normal variables, defining a struct variable allocates memory for that variable.

It is possible to define multiple variables of the same struct type:

Accessing struct members

When we define a variable such as Employee joe, joe refers to the entire struct (which contains the member variables). In order to access the individual members, we use the member selection operator (which is a period). Here is an example of using the member selection operator to initialize each member variable:

As with normal variables, struct member variables are not initialized, and will typically contain junk. We must initialize them manually.

In the above example, it is very easy to tell which member variables belong to Joe and which belong to Frank. This provides a much higher level of organization than individual variables would. Furthermore, because Joe’s and Frank’s members have the same names, this provides consistency across multiple variables of the same struct type.

Struct member variables act just like normal variables, so it is possible to do normal operations on them:

Initializing structs

Initializing structs by assigning values member by member is a little cumbersome, so C++ supports a faster way to initialize structs using an initializer list. This allows you to initialize some or all the members of a struct at declaration time.

In C++11, we can also use uniform initialization:

If the initializer list does not contain an initializer for some elements, those elements are initialized to a default value (that generally corresponds to the zero state for that type). In the above example, we see that frank.wage gets default initialized to 0.0 because we did not specify an explicit initialization value for it.

C++11/14: Non-static member initialization

Starting with C++11, it’s possible to give non-static (normal) struct members a default value:

Unfortunately, in C++11, the non-static member initialization syntax is incompatible with the initializer list and uniform initialization syntax. For example, in C++11, the following program won’t compile:

Consequently, in C++11, you’ll have to decide whether you want to use non-static member initialization or uniform initialization. Uniform initialization is more flexible, so we recommend sticking with that one.

However, in C++14, this restriction was lifted and both can be used. If both are provided, the initializer list/uniform initialization syntax takes precedence. In the above example, Rectangle x would be initialized with length and width 2.0. In C++14, using both should be preferred, as it allows you to declare a struct with or without initialization parameters and ensure the members are initialized.

We talk about what static members are in chapter 8. For now, don’t worry about them.

Assigning to structs

Prior to C++11, if we wanted to assign values to the members of structs, we had to do so individually:

This is a pain, particularly for structs with many members. In C++11, you can now assign values to structs members using an initializer list:

Structs and functions

A big advantage of using structs over individual variables is that we can pass the entire struct to a function that needs to work with the members:

In the above example, we pass an entire Employee struct to printInformation(). This prevents us from having to pass each variable individually. Furthermore, if we ever decide to add new members to our Employee struct, we will not have to change the function declaration or function call!

The above program outputs:

ID:   14
Age:  32
Wage: 24.15

ID:   15
Age:  28
Wage: 18.27

A function can also return a struct, which is one of the few ways to have a function return multiple variables.

This prints:

The point is zero

Nested structs

Structs can contain other structs. For example:

In this case, if we wanted to know what the CEO’s salary was, we simply use the member selection operator twice: myCompany.CEO.wage;

This selects the CEO member from myCompany, and then selects the wage member from within CEO.

You can use nested initializer lists for nested structs:

Struct size and data structure alignment

Typically, the size of a struct is the sum of the size of all its members, but not always!

Consider the Employee struct. On many platforms, a short is 2 bytes, an int is 4 bytes, and a double is 8 bytes, so we’d expect Employee to be 2 + 4 + 8 = 14 bytes. To find out the exact size of Employee, we can use the sizeof operator:

On the author’s machine, this prints:

The size of Employee is 16

It turns out, we can only say that the size of a struct will be at least as large as the size of all the variables it contains. But it could be larger! For performance reasons, the compiler will sometimes add gaps into structures (this is called padding).

In the Employee struct above, the compiler is invisibly adding 2 bytes of padding after member id, making the size of the structure 16 bytes instead of 14. The reason it does this is beyond the scope of this tutorial, but readers who want to learn more can read about data structure alignment on Wikipedia. This is optional reading and not required to understand structures or C++!

Accessing structs across multiple files

Because struct declarations do not take any memory, if you want to share a struct declaration across multiple files (so you can instantiate variables of that struct type in multiple files), put the struct declaration in a header file, and #include that header file anywhere you need it.

Struct variables are subject to the same rules as normal variables. Consequently, to make a struct variable accessible across multiple files, you can use the extern keyword to do so.

Final notes on structs

Structs are very important in C++, as understanding structs is the first major step towards object-oriented programming! Later on in these tutorials, you’ll learn about another aggregate data type called a class, which is built on top of structs. Understanding structs well will help make the transition to classes that much easier.

The structs introduced in this lesson are sometimes called plain old data structs (or POD structs) since the members are all data (variable) members. In the future (when we discuss classes) we’ll talk about other kinds of members.

Quiz

1) You are running a website, and you are trying to keep track of how much money you make per day from advertising. Declare an advertising struct that keeps track of how many ads you’ve shown to readers, what percentage of ads were clicked on by users, and how much you earned on average from each ad that was clicked. Read in values for each of these fields from the user. Pass the advertising struct to a function that prints each of the values, and then calculates how much you made for that day (multiply all 3 fields together).

2) Create a struct to hold a fraction. The struct should have an integer numerator and an integer denominator member. Declare 2 fraction variables and read them in from the user. Write a function called multiply that takes both fractions, multiplies them together, and prints the result out as a decimal number. You do not need to reduce the fraction to its lowest terms.

Quiz Answers

1) Show Solution

2) Show Solution

4.8 -- The auto keyword
Index
4.6 -- Typedefs and type aliases

326 comments to 4.7 — Structs

  • Sean Kelly

    Hello Alex, I wanted to thank you for these great tutorials and to ask you a question that is scratching my brain!

    When it comes to the answer of the first question involving the quiz, you print out:

    Now I am just wondering about operator precedence, since * and / are on the same level of precedence and the associativity is from left to right, why does this compile the way you would think it would? I would of thought it would of done ad.adsShown * ad.clickThroughRatePercentage and then do / 100? If you could explain my error that would be awesome!

    • Alex

      You are correct about the precedence and associativity. So this evaluates each term from left to right. First, ad.adsShown is multiplied by ad.clickThroughRatePercentage. Then that's divided by 100. Then ad.averageEarningsPerClick is multiplied in.

      It's not clear to me what you were thinking it should actually do instead?

      • Sean Kelly

        I see why I was confused, here is what I was thinking.

        Say you have shown 100 adds but only 10 of them are actually clicked on with each add giving an average of 1$ per click.
        100 * 10 / 100 * 1
        100 * 0.1 * 1
        10 * 1 = $10.

        So when I did my program I wrote it like this:

        I guess it does not matter whether you have it follow precedence or effect the path in the way that I did, the output is always the same.

  • A. Lousseief

    //
    // This is a simple program to use struct in C++
    // on October 2015 by A. Lousseief
    //
    #include <iostream>
    using namespace std;

    // Definition of struct to create fractions
    struct Fraction {
        int numerator;
        int denominator;
    }f1, f2;

    // function to multiply two fractions i.e f1*f2
    void multiply( Fraction f1, Fraction f2){
    //        cout << static_cast<float>(f1.numerator / f1.denominator)*(f2.numerator / f2.denominator);
            cout <<"\n\t\t(" <<f1.numerator <<"/"<<f1.denominator <<")*("<< f2.numerator <<"/"<< f2.denominator <<") = "     
                 <<"("<<f1.numerator<<"*"<<f2.numerator <<")/("<< f1.denominator<<"*"<<f2.denominator <<") = "    
                 <<"("<< f1.numerator*f2.numerator <<"/"<< f1.denominator*f2.denominator <<")\n\n";    
    }

    int main() {
            // Allocate the first fraction f1
            cout << "Enter the numerator of the first fraction f1: ";
            cin >> f1.numerator;
            cout << "Enter the denominator of the first fraction f1: ";
            cin >> f1.denominator;
            // Allocate the second fraction f2
            cout << "Enter the numerator of the second fraction f2: ";
            cin >> f2.numerator;
            cout << "Enter the denominator of thesecond fraction f2: ";
            cin >> f2.denominator;

            cout <<"\n\t\tThe product of f1 and f2 is: ";

            multiply(f1,f2);

            return 0;
    }

  • Gopal

    Thanks Alex,

    In this case if i want my struct variable as local, i have to have declare/define Structure inside the function (Main or any other user defined function).

    I hope my understanding is clear.

    • Alex

      Not quite. You can declare a struct anywhere. It's just a declaration, so it doesn't matter where you do it. It's common to declare structs in header files and #include them where you need them.

      What matter is where you _define_ your struct variable. If you define the struct variable in global space, your struct is global. If you define it inside a function, it's local.

  • Gopal

    Hey Alex,

    Nice tutorial. You are doing great job.

    I have a question here. It looks like struct variables (member)act as global variable, whether this may result in evil thing?

    • Alex

      > It looks like struct variables (member)act as global variable, whether this may result in evil thing?

      Why do you say this?

      Struct member variables are part of the struct itself. If the struct is global, they are global. If the struct is local, they are local.

  • Rob

    Will there be a tutorial on Union types? I've come across them while trying SFML Events and I'm not sure if I entirely understand them

  • bio

    thanks mate !!!you are awesome!

  • bio

    2QUIZ:when i try to compile ,i get this msg:    
        [Error] reference to 'multiplies' is ambiguous

    • Alex

      The problem is this line:

      It turns out that there's a std::multiplies function, and the code isn't sure whether you mean your version of multiplies() or std::multiplies.

      You can fix this by changing the call to multiplies like this:

  • Gayan

  • begginner

    #include<iostream>
    #include<string>
    using namespace std;
    struct advertise
    {

        int no_of_ads;
        int no_of_users;
        int average_per_click;
        };

    getreport(advertise pa)
    {
        cout<<"enter no af ads no of user and avg per click"<<endl;
        cin>>pa.no_of_ads;
        cin>>pa.no_of_users;
        cin>>pa.average_per_click;
        cout<<"no of ads are"<<pa.no_of_ads<<endl;
    cout<<"no of users are"<<pa.no_of_users<<endl;
        cout<<"avaerage per click"<<pa.average_per_click<<endl;
        cout<<"money per day is"<<pa.no_of_ads*pa.no_of_users*pa.average_per_click;
    }

    main(){
        advertise a;
        getreport( a);
        

    }

  • Thnx Alex. You are great.
    Still confused, what is 32767 here and should I use cin.ignore. In both the cases where getline (cin, ) and simple cin are used, or only when getline is used. Why this only messes up with getline and not with cin?

    • Alex

      32767 is just a large integer that tells cin.ignore how many characters to look through for a '\n'. In this case, you could use a low number since the '\n' is likely going to be the first character (unless the user entered invalid input when asked for an integer).

      Unfortunately, getline and cin just work different with regards to how they handle input (particularly whitespace). They're a pain, and their error handling capabilities are limited. I personally find it easier to read in everything as a string and then convert the string to an integer when that's appropriate. That helps avoid unexpected characters in the streams.

  • I m trying to write a program that prints a company's information (e.g. company name, name of CEO etc.). I am using struct to group all the related variables. My struct looks like:

    I defined a void printInfo (Company x) to print information of a company. My main () takes data from user and passes that to printInfo(). Rest of the work is done by printInfo (). My problem is with main () that is following:

    The above code compiles and links fine (not including the void printInfo (), nothing wrong with that). On execution, the program starts from taking input for x.NameOfCompany. That's okay. But all the other getline () puzzled me. Program never asks for input for one (or both when i change the order of getlines, e.g. putting BranchesIn before NameOfCeo) variables which is set to take input through getline method. What's the problem. Does getline has any limitations?

    If the code has any syntax or typographical error, please ignore. I swear that was linked and compiled fine.
    Thanx Alex 🙂

    • Alex

      The problem is the way cin works. When it asks for total revenue, and you enter "45" and hit enter, this gets input as "45\n". The 45 gets extracted to x.Revenue, but the "\n" stays in the input stream. This messes up the next call to getline() (because it sees the '\n' and says, "I'm done!").

      To fix this, after each call to cin to read in a number, you can use the following to get rid of the extra '\n' stuck in the input stream:

  • Jacob

    In this segment of code, if Joe and Frank have the same wage, for example, if Joe and Frank both make 24.15, (joe.wage > frank.wage) will evaluate to false, and it will say "Joe makes less than frank"

  • "Structs can contain other structs"

    If you want to tell what struct can contain, you should clear everything.  When I finished reading this lesson and moved to comprehensive quiz, I found that you have nested an enumerated  type as a struct member in your solution. Before that, I was believing struct can only contain structs. It is still unclear to me what other kinds of objects a struct can contain.

    One question: I want to create great programs using C++. What should I do after completing your tutorials? How much time would it take to reach there?
    Thanks... 🙂

  • thanks all
    this note is very necessary

  • techsavvy....aye

    But I took the example of- Input the first numerator:12

                               Input the first denominator:4

                               Input the second numerator:12

                               Input the second denominator:3

    Which does not include any fractional division but still according to the problem which I had it is giving 4 instead of 12.

    Sorry!!!....to keep bothering you off topic.

    • Alex

      Oh.

      Should be:

  • techsavvy....aye

    Just by changing the signs in the void multiply part of the quiz question 2. Why does it give a different answer.

    • Alex

      static_cast(f1.numerator / f1.denominator) does an integer division (dropping any fractional part) and then casts the result to a float (which isn't useful because we've already lost any fractional result).

      The quiz example does the integer multiplication first, casts that to a float, and then does a floating point division (so as not to lose the fractional component).

  • techsavvy....aye

    A typo (maybe)

    everywhere you have mentioned joe's age to be 32 but here "Initializer lists" it is 42.

    Though it isn't an issue. Just to let you know.

    😛

  • mehdi

    Thanks for your excellent tutorial.
    When working with multiple files, do we have to use keyword extern to declare a struct?
    Another question: How can one combine arrays with structs? For example when instead of only two employees (Joe and Frank), we have 1000 employees.

    • Alex

      Declaring a struct defines a new data type. If you want to share a struct declaration across multiple files (so you can instantiate variables of that struct type in multiple files), put the struct declaration in a header file, and #include that header file anywhere you need it.

      If you instantiate a struct variable, and you want to share that variable across multiple files, you can extern that variable.

      Making an array of structs is just like making an array of any other data type. Just use the structs name as the data type. e.g.

      Employee myEmployees[10];

  • Todd

    Typo.

    "The struct should have a (an) integer numerator and a (an) integer denominator member."

    I really enjoyed this section! Structs seem very helpful.

    • Alex

      Thanks for the correction.

      Structs are super useful! However, somewhat comically, they are rarely used in C++, because classes are even MORE useful.

      The good news is that classes build directly on top of structs, so the knowledge you gained here will still be put to good use.

  • Denis

    Hello Alex,

    The C++ language (actually its predecessor C) introduces structs as a mean of aggregating the variables into a one logical concept,
    which is a good thing.
    But isn't it quite redundant as C++ has also got namespaces, which can as well serve us the same purpose?

    Moreover, we can also aggregate functions (behavior) under the same roof along with variables using the concept namespaces.

    What is the advantage of structs then (except for being able to pass and return multiple vars to functions) ?

    Thank you in advance.
    Denis

    • Alex

      Good question.

      Structs define a new data type and allow you to instantiate as many variables of that data type as you like.

      Namespaces do not define a new data type, are intended to provide a way to group related bits of code (functions, data types, etc...) together to avoid naming collisions.

      Although structs and namespaces can both be used to "collect" a group of related variables, I'd argue that structs do it better:
      * Structs allow you to set up a struct once and then instantiate as many of them as you want. With the namespace method, you'd have to define a whole new namespace every time you wanted a new "instance" of your group.
      * As you've already noted, you can pass structs into a function in their entirety, whereas with namespaces you'd have to pass each element individually.
      * Structs enforce a consistent naming convention for objects of that type (a good thing). This also makes them easier to debug.

  • vinay

    In the quiz question no.1 , there is a below type in the solution

    [   cout << "Total Earnings: $" <<
            (ad.adsShown * ad.clickThroughRate / 100 * ad.averageEarningsPerClick) << endl;]

    please correct the above line from ad.clickThroughRate to ad.clickThroughpercentage as declared in the advertising structure.

  • Joseph

    Hi, I don't get what the point is in using a struct in example 1, can someone explain?

    • Alex

      You mean in quiz 1? Generally structs are useful for keeping related variables together in one place, both for organizational purposes, and because it makes it easier to code.

      Any time you have two or more related non-trivial variables, you should consider putting them in a struct.

  • Jeremy

    Are structure types generally defined outside all blocks (like a global variable)? This is the only thing I can't seem to find an answer to. You obviously can declare them in blocks or functions, but is that bad style?

    • Janez

      Yes, they are (usually) defined outside all blocks. Since they are (user-defined) types, you want to access them from multiple functions (like you want to be able to declare variable of type int in multiple blocks).

  • Kevin

    I completed question 2 slightly differently, it works fine and gives the correct results. Have I done anything that would be considered bad practice or is this OK?  

    [#include "stdafx.h"
    #include <iostream>

    struct FRACTION
    {
        int numer_one;
        int numer_two;
        int denom_one;
        int denom_two;
    };

    void fractionMultiplied(FRACTION fraction)
    {
        using namespace std;
        // fraction multiplication
        float numer_sum = fraction.numer_one * fraction.numer_two;
        float denom_sum = fraction.denom_one * fraction.denom_two;
        float decemal_output = numer_sum / denom_sum;

        cout << "The decimal value of the sum of both of your fractions is: " << decemal_output;
    }

    int main()
    {
        using namespace std;
        FRACTION fraction;
        
        cout << "Enter two fractions to have them multiplied together:" << endl;
        // get fraction numerators and denominators
        cin >> fraction.numer_one;

        cout << " / ";

        cin >> fraction.denom_one;

        cout << " nAnd the other: n";

        cin >> fraction.numer_two;

        cout << " / ";

        cin >> fraction.denom_two;
        // call a function to output the sum of both fractions
        fractionMultiplied(fraction);

        return 0;

    }

    /* END OF PROGRAMME */]

  • Adi

    Struct in c++?

    Aren't we supposed to be using Classes.  Doesn't Class supercede Struct?

    • Kevin

      Classes have not yet been covered in the tutorial as are in a later chapter.

    • Janez

      Structure members are public by default and classes have their members private by default. There are some instances where you would use struct instead of class. Search for structs vs. classes for more thorough explanation (answers on stackoverflow are usually really good).

      • Alex

        Structs are simple aggregate types, and their use is appropriate in C++ when you don't need any member functions or access controls.

  • Josh

    I'm having a bit of trouble with this.

    Stats sVortex = {0, 0, 0, 0, 0};

    i've typed this in, making sure to set sVortext to struct Stats (yes, i have already declared Stats as a struct in my header). This doesn't bring up any problems/compiler errors. However, once I try to reference it: Stats.sVortex.nDex; it says that: class "Stats" has no member "sVortex". No matter what I do, nothing seems to fix it.

    Please help,
    -Josh

  • Sind

    Can C++ structures include functions inside structure declaration?

  • Edward

    I managed to get my solution to question 2 correct. Initially though I had this code inside my multiply function.

    cout << static_cast((sF1.numerator)/(sF1.denominator))*((sF2.numerator)/(sF2.denominator));

    The program ran and compiled fine but performed integer division when I entered 1/5 * 1/1 in order to test the program, giving an answer of 0 instead of 0.2.

    When I re-ordered that line of code to

    cout << static_cast((sF1.numerator)*(sF2.numerator))/((sF1.denominator)*(sF2.denominator));

    the program works fine and performs the division properly, giving the correct answer (0.2).

    Why is this, to me the functions perform the same operations, am I missing something obvious?

    • Alex

      In the first line, you're doing integer division and casting the integer result to a float (which is useless).

      In the second line, you're casting the first operand to a float, which causes the division operator to convert the second operand to a float as well, and do floating point division.

  • Leolas

    Hi, this is my code for the question 2; i made it slightly different, so i would like to your opinions/corrections 🙂 . However, i forget the cast for the float, so i think that my code is doing integer division :s, well, i would very appreciate your advices or something 🙂

    #include
    using namespace std;

    struct Fraction
    {
    int nNumerator1;
    int nDenominator1;
    int nNumerator2;
    int nDenominator2;
    };

    void Multiply(Fraction Elements)
    {
    cout << "The product of the two fractions is: " << Elements.nNumerator1 / Elements.nDenominator1 * Elements.nNumerator2 / Elements.nDenominator2;
    }

    int main()
    {
    Fraction Elements;
    cout <> Elements.nNumerator1;
    cout <> Elements.nDenominator1;
    cout <> Elements.nNumerator2;
    cout <> Elements.nDenominator2;

    Multiply(Elements);
    return 0;
    }

  • Arthur_

    Why do you (for the second question) declare the numerator and denominators as integers when you know you are going to need them as floats later? Wouldn't it be more logical to declare them as floats or longs and then you don't have to static_cast them later?

    • Alex

      In standard mathematics, the numerators and denominators in fractions are always defined as integers. It's better to have your code represent mathematical convention and do the conversions when necessary than to do something non-standard for the sake of avoiding a couple of static_casts.

      Plus using floating point numbers would allow you to define fractions like 3.4 / 10.2. Although technically this works (and is equivalent to 1 / 3), we don't fraction that way. 🙂

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