11.7 — Non-static member initialization

By Alex on February 12th, 2016 | last modified by nascardriver on January 6th, 2021

When writing a class that has multiple constructors (which is most of them), having to specify default values for all members in each constructor results in redundant code. If you update the default value for a member, you need to touch each constructor.

It’s possible to give normal class member variables (those that don’t use the static keyword) a default initialization value directly:

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#include <iostream>   class Rectangle { private:     double m_length{ 1.0 }; // m_length has a default value of 1.0     double m_width{ 1.0 }; // m_width has a default value of 1.0   public:     void print()     {         std::cout << "length: " << m_length << ", width: " << m_width << '\n';     } }; int main() {     Rectangle x{}; // x.m_length = 1.0, x.m_width = 1.0     x.print();     return 0; }

This program produces the result:

length: 1.0, width: 1.0

Non-static member initialization (also called in-class member initializers) provides default values for your member variables that your constructors will use if the constructors do not provide initialization values for the members themselves (via the member initialization list).

However, note that constructors still determine what kind of objects may be created. Consider the following case:

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#include <iostream>   class Rectangle { private:     double m_length{ 1.0 };     double m_width{ 1.0 };   public:       // note: No default constructor provided in this example       Rectangle(double length, double width)         : m_length{ length },           m_width{ width }     {         // m_length and m_width are initialized by the constructor (the default values aren't used)     }       void print()     {         std::cout << "length: " << m_length << ", width: " << m_width << '\n';     }   }; int main() {     Rectangle x{}; // will not compile, no default constructor exists, even though members have default initialization values     return 0; }

Even though we’ve provided default values for all members, no default constructor has been provided, so we are unable to create Rectangle objects with no arguments.

If a default initialization value is provided and the constructor initializes the member via the member initializer list, the member initializer list will take precedence. The following example shows this:

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#include <iostream>   class Rectangle { private:     double m_length{ 1.0 };     double m_width{ 1.0 };   public:       Rectangle(double length, double width)         : m_length{ length },           m_width{ width }     {         // m_length and m_width are initialized by the constructor (the default values aren't used)     }       Rectangle(double length)         : m_length{ length }     {         // m_length is initialized by the constructor.         // m_width's default value (1.0) is used.     }       void print()     {         std::cout << "length: " << m_length << ", width: " << m_width << '\n';     }   }; int main() {     Rectangle x{ 2.0, 3.0 };     x.print();       Rectangle y{ 4.0 };     y.print();     return 0; }

This prints:

length: 2.0, width: 3.0
length: 4.0, width: 1.0

Note that initializing members using non-static member initialization requires using either an equals sign, or a brace (uniform) initializer -- the direct initialization form doesn’t work here.

Rule

Favor use of non-static member initialization to give default values for your member variables.

Quiz time

Question #1

Update the following program to use non-static member initialization and member initializer lists.

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#include <string> #include <iostream>   class Ball { private: std::string m_color; double m_radius;   public: // Default constructor with no parameters Ball() { m_color = "black"; m_radius = 10.0; }   // Constructor with only color parameter (radius will use default value) Ball(const std::string &color) { m_color = color; m_radius = 10.0; }   // Constructor with only radius parameter (color will use default value) Ball(double radius) { m_color = "black"; m_radius = radius; }   // Constructor with both color and radius parameters Ball(const std::string &color, double radius) { m_color = color; m_radius = radius; }   void print() { std::cout << "color: " << m_color << ", radius: " << m_radius << '\n'; } };   int main() { Ball def; def.print();   Ball blue{ "blue" }; blue.print(); Ball twenty{ 20.0 }; twenty.print(); Ball blueTwenty{ "blue", 20.0 }; blueTwenty.print();   return 0; }

This program should produce the result:

color: black, radius: 10
color: blue, radius: 10
color: black, radius: 20
color: blue, radius: 20

Show Hint

You only need 2 constructors if you use default arguments.

Show Solution

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#include <iostream> #include <string> class Ball {   private:     std::string m_color{ "black" };     double m_radius{}; // The radius always gets initialized by the constructors.     public:     // Constructor with only radius parameter (color will use default value)     Ball(double radius)         : m_radius{ radius }     {     }       // Constructor with both color and radius parameters     Ball(const std::string& color = "black", double radius = 10.0)         : m_color{ color },           m_radius{ radius }     {     }       void print()     {         std::cout << "color: " << m_color << ", radius: " << m_radius << '\n';     } };   int main() {     Ball def{};     def.print();       Ball blue{ "blue" };     blue.print();       Ball twenty{ 20.0 };     twenty.print();       Ball blueTwenty{ "blue", 20.0 };     blueTwenty.print();       return 0; }

Question #2

Why don’t we need to declare a constructor with no parameters in the program above, even though we’re constructing def without arguments?

Show Solution

“Ball def{};” uses the second Ball constructor which has a default argument for every parameter.

11.8 -- Overlapping and delegating constructors

Index

11.6 -- Constructor member initializer lists

C++ Programming |  Print This Post

128 comments to 11.7 — Non-static member initialization

• Kacper Bazan

  May 26, 2021 at 5:56 pm · Reply

  I'm struggling to see why it is better to include member declarations outside of constructors. In the example with the Rectangle Class, I would need to make three constructors if I plan on initializing Rectangles in these ways:

  1 2 3

  Rectangle(double, double) //uses no default values Rectangle(double) //uses the second default value Rectangle() //uses both default values

  I understand that it is helpful to have default initializations in one place next to member variables, but wouldn't you need to add many more constructors?

  Using previous lessons as a guide I'd make a constructor like this:

  1 2	Rectangle(double length=1.0, double width=1.0) :m_length{length}, m_width{width}{}

  What am I missing?

  • Alex

    May 30, 2021 at 9:57 am · Reply

    Multiple Rectangle constructors are provided just to show off the mechanics (having default arguments would increase the complexity of the example). If you were writing non-academic code, you'd likely do as you suggest.

  • Haldhar Patel

    May 30, 2021 at 11:03 am · Reply

    But what if I say we want to use only length default value and width value from the user from your second method of constructor?
    It turns out that you need two constructors from the second method. But still it is less than the first method

    > But what if you have an array member{or some other complex ones} in your class and you need some default values for that array , how will you make an optional parameter for that , I think it might be complicated that way , but easy to give the array default values by non-static member initialization and use that default values in constructors

    > Also it is a good practice to initialize variables on creation so that you might not use the garbage value in it by mistake.
    I am not able to come up with good reasons{because I am a beginner , also I might be wrong too} but they are for sure of great use , which we may find out later , so keep moving :)

• Mohammad Ali

  April 10, 2021 at 11:49 am · Reply

  Hi,

  If the idea for initializing private members is to avoid having to specify a default value for them in multiple constructors, how does assigning that value via non-static member initialization help? Because even though they have a default value initialized, we still need to specify that default value for the constructor parameters to make them optional. This in turn means we still need to provide the default value at multiple places. What am I missing?

  Thanks.

  • Alex

    April 12, 2021 at 9:33 am · Reply

    The idea for initializing private members is to ensure they have a known, consistent value.

    Without non-static member initialization, we'd have to initialize every member in every constructor (or use delegating constructors), which leads to redundancy. This allows us to set our default values in one place, and then the constructors can provide other initialization values if those are preferred.

    > Because even though they have a default value initialized, we still need to specify that default value for the constructor parameters to make them optional.

    I'm not sure I understand what you mean by this.

• J34NP3T3R

  January 29, 2021 at 1:40 am · Reply

  so im trying to use the default values as default values in the parameters but it gives me an error
  "a nonstatic member reference must be relative to a specific object"

  reason im testing if i can do this is because if i wanted to change the default values i was hoping i could change them in the default values section only
  and not have to change it on each constructor as well. ALSO it could add a benefit wherein it could work for no parameters and both parameters.
  but it seems i cant do this. would like to understand why

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  class Ball {   private:     std::string m_color{ "black" };     double m_radius{}; // The radius always gets initialized by the constructors.   public:     // test Constructor to receive no paramaters and both parameters at the same time     Ball(const std::string& color = m_color, double radius = m_radius)  :  m_color{ color }, m_radius{ radius }     {           // color = m_color and radius = m_radius   gives the error     }       void print()     {         std::cout << "color: " << m_color << ", radius: " << m_radius << '\n';     } };

  • nascardriver

    January 30, 2021 at 9:12 am · Reply

    The object doesn't yet exist when the constructor get's called. `m_color` doesn't have a value until you initialize it. You can add `static` member variables and use them as default values.

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    static constexpr std::string_view default_color{ "black" };   std::string m_color{ default_color };   Ball(const std::string& color = default_color, /* ... */

• Mike

  January 20, 2021 at 5:56 am · Reply

  Hi guys, I love the site. I am learning a great deal about C++.

  I wanted to say that I appreciated quiz #1 very much for this lesson.  I was unable to solve it (I wish that I had) and when I looked at the solution I was happy and impressed with its simplicity.  Thank you for making my brain work!  It will help me remember the proper way to do things in the future.

• Phil

  January 12, 2021 at 7:22 am · Reply

  Hi all,
  please consider a slightly modified example from the learning material:

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  #include <iostream> class Rectangle { private:     double m_length{ 1.0 };     double m_width{ 1.0 }; public:     // note: No default constructor provided in this example     Rectangle(double length, double width)         : m_length{ length },           m_width{ width }     {         // m_length and m_width are initialized by the constructor (the default values aren't used)     }     void print()     {         std::cout << "length: " << m_length << ", width: " << m_width << '\n';     } }; int main() {     double temp{ 0 };     Rectangle x{ 1, 2 }; // I want a compiler error here too     return 0; }

  As I learned in the previous chapters, uniform initialization does type checking for fundamental datatypes, so I cannot initialize a double with an int.
  Is there a way to get a similar type checking when a class is created? I assume that implicit type conversions are happening in this example and I would like to catch them.
  Thank you very much for your help in advance!

  • Phil

    January 12, 2021 at 8:41 am · Reply

    I just realized by now, that uniform initialization will throw an error only if narrowing conversion is being made, that's why I have not got any errors. Sorry for spamming!

• Wake

  January 7, 2021 at 7:58 pm · Reply

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

  #include <string> #include <iostream>   class Ball { private:     std::string m_color{ "black" };     double m_radius{ 10 };   public:       Ball(double radius)         : m_radius{ radius }     {     }     Ball(std::string color= "black", double radius=10)         : m_radius{ radius }     {     }       void print()     {         std::cout << "color: " << m_color << ", radius: " << m_radius << '\n';     } };   int main() {     Ball def;     def.print();       Ball blue{ "blue" };     blue.print();       Ball twenty{ 20.0 };     twenty.print();       Ball blueTwenty{ "blue", 20.0 };     blueTwenty.print();       return 0; }

• SlayerSavin

  January 4, 2021 at 12:05 am · Reply

  Thanks Nascardriver for previous replies,please clarify my doubt.

  >In Quiz question #1 what is the use of using non static member intialization,As we did

  1 2 3 4 5

  Ball(const std::string& color = "black", double radius = 10.0)         : m_color{ color },           m_radius{ radius }     {     }

  >When we have multiple constructors,which method should we use for intialization?
  1.Member intializer list with delegating constructors
  2.constructors using a seperate function as in this example

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  class Foo { public:     Foo()     {         init();     }     Foo(int value)     {         init();         // do something with value     }     void init()     {         // code to "initialize" Foo     } };

  >is there any standard way among programmers for using constructors as there are many options.
  Thanks in advance.

  • nascardriver

    January 6, 2021 at 8:03 am · Reply

    `m_color`'s default value is used by the `Ball(double)` constructor

    Use method 1., 2. isn't initialization but assignment. Assignment is more expensive and not always possible.

    > is there any standard way among programmers for using constructors as there are many options.
    I don't understand. Can you provide an example of some of the options?

• goiu

  January 2, 2021 at 2:55 am · Reply

  Hi there!
  Regarding the answer to the second question, isn't the constructor with default arguments for all of its parameters a default constructor?
  If i don't remember bad, it was stated this in the lesson about constructors

  • nascardriver

    January 4, 2021 at 8:07 am · Reply

    Yes that's correct! I've rephrased the question, thanks for pointing it out

• yeokaiwei

  December 9, 2020 at 12:30 am · Reply

  1. Query
  " If you update the default value for a member, you need to touch each constructor."
  Is touch a new keyword? Does it mean call?

  2. Feedback
  For the Rectangle example
  length: 2.0, width: 3.0
  length: 4.0, width: 1.0

  On my laptop, it prints
  length: 2, width: 3
  length: 4, width: 1

  I'm not sure if this is important but I felt you should know.

  3. Quiz 1 Ball
  I feel like there is a significantly better way to create constructors.

  If the input parameters rises to 10 or more, you want to know colour, race, height, weight, age, networth, gender, heart rate, cholesterol level, etc.

  You would need a lot of constructors.

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67

  #include <string> #include <iostream>   class Ball { private:     std::string m_color;     double m_radius;   public:     //Unnecessary (Soln)     // Default constructor with no parameters     /*Ball()     {         m_color = "black";         m_radius = 10.0;     }*/       //Unnecessary (Soln)     //// Constructor with only color parameter (radius will use default value)     //Ball(const std::string& color): m_color{ color }     //{     //    m_color = color;     //    m_radius = 10.0;     //}       // Constructor with only radius parameter (color will use default value)     Ball    (double radius)         : m_radius{ radius }     {         m_color = "black";         m_radius = radius;     }          // Constructor with both color and radius parameters     //Put the defaults in by assignment, not by brace.     //Reduces the need for a default constructor     //Match the member initializer list with the input parameters by indentation     Ball(const std::string& color = "black", double radius = 10.0)                     : m_color{color},        m_radius{radius}     {         m_color = color;         m_radius = radius;     }       void print()     {         std::cout << "color: " << m_color << ", radius: " << m_radius << '\n';     } };   int main() {     Ball def;     def.print();       Ball blue{ "blue" };     blue.print();       Ball twenty{ 20.0 };     twenty.print();       Ball blueTwenty{ "blue", 20.0 };     blueTwenty.print();       return 0; }

  • nascardriver

    December 10, 2020 at 8:31 am · Reply

    "touch" means the same as in real life, just not with your hands but with your cursor. Every constructor needs to be updated.

    Assigning values to members after having initialized them has no effect. Re-read lesson 8.5a

    • yeokaiwei

      December 11, 2020 at 3:02 pm · Reply

      "Assigning values to members after having initialized them has no effect. Re-read lesson 8.5a"

      Sorry, may I ask which line this is?

      • nascardriver

        December 12, 2020 at 1:05 am · Reply

        Both of your constructors. You're initializing the members in the member-initializer-list, then you assign to them in the constructor body

        • yeokaiwei

          December 12, 2020 at 9:58 am · Reply

          Oh, Line 31/32 Line 42/43 are redundant.

          Once I use a member initializer list, I don't have to assign them in the {} anymore.

          Assigning them again is a waste of time.

          I got it.

          Thanks nascar.

          On a side note, why didn't the compiler warn me?

          Assigning is redundant code ie garbage, right?

          • nascardriver

            December 13, 2020 at 4:48 am · Reply

            clang-tidy would have warned you

            1 2 3 4 5 6

            <source>:42:9: warning: 'm_color' should be initialized in a member initializer of the constructor [cppcoreguidelines-prefer-member-initializer]         m_color = color;         ^~~~~~~~~~~~~~~~ <source>:43:9: warning: 'm_radius' should be initialized in a member initializer of the constructor [cppcoreguidelines-prefer-member-initializer]         m_radius = radius;         ^~~~~~~~~~~~~~~~~~

            • yeokaiwei

              December 13, 2020 at 6:19 am · Reply

              Oh, I see.

              Different compilers, different "something".

              1. Should I switch to Clang? Or install a Clang add-on?

              2. With regards to our conversion regarding passing by value or reference &, it seems Prof Bjarne Stroustrup seems to recommend "moving" instead of copying.

              1 2 3 4 5 6 7 8

              class Vector { Vector(const Vector &a); //copy constructor Vector & opertator=(const Vector& a); //copy assignment   Vector(Vector&& a); //move constructor Vector& operator=(Vector&& a); //move assignment };

              && means "rvalue reference".

              According to the book I'm reading, should we be using "move" constructors and assignments in the tutorials instead?

              The site might need a massive rework.

              3. Lastly, could you help me with the 8.15 Quiz on part G. My code works until part F. Thank you.

              • nascardriver

                December 13, 2020 at 6:24 am · Reply

                1. VS has support for clang-tidy I think. I've linked you instructions before.
                2. You need both. Move semantics are covered later.
                3. Post your code in that lesson, explain what you're trying to do and describe you've tried to make that happen and why it didn't work.

                • yeokaiwei

                  December 13, 2020 at 6:38 am · Reply

                  1. I turned on clang-tidy and Code Analysis under Code Analysis -> General for the above Ball Quiz. Clang-tidy had no effect when I uncommented.

                  "m_color = "black"; m_radius = radius;"

                  You can see I used Clang-Tidy but it has no effect.

                  No warnings or errors.

                  1>Target Performance Summary:
                  1>        0 ms  _SelectedFiles                             1 calls
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                  1>
                  1>Task Performance Summary:
                  1>        0 ms  Message                                    2 calls
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                  1>        0 ms  MSBuild                                    1 calls
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                  ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

                  • nascardriver

                    December 13, 2020 at 6:47 am

                    According to https://devblogs.microsoft.com/cppblog/code-analysis-with-clang-tidy-in-visual-studio/
                    there's a "clang-tidy" tab somewhere. I'm not Microsoft Support, you'll have to work through this on your own.

                  • yeokaiwei

                    December 13, 2020 at 7:31 am

                    1. Nascar, there's a bug with clang-tidy

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                    #include <string> #include <iostream>   class Ball { private:     std::string m_color;     double m_radius;   public:          Ball    (double radius)         : m_color{ "black" }, m_radius {radius    }     {     }          Ball(const std::string& color = "black", double radius = 10.0)                     : m_color{color},        m_radius{radius}     {     }       void print()     {         std::cout << "color: " << m_color << ", radius: " << m_radius << '\n';     } };   int main() {     Ball def;     def.print();       Ball blue{ "blue" };     blue.print();       Ball twenty{ 20.0 };     twenty.print();       Ball blueTwenty{ "blue", 20.0 };     blueTwenty.print();       return 0; }

                    color: black, radius: 10
                    color: blue, radius: 10
                    color: black, radius: 20
                    color: blue, radius: 20

                    When you turn on clang-tidy, the answer can turn into

                    color: black, radius: 10
                    color: blue, radius: 10
                    color: , radius: 20
                    color: blue, radius: 20

                • yeokaiwei

                  December 13, 2020 at 6:40 am · Reply

                  2. Got it. Will wait until that tutorial.

                  3. Actually, I posted the code a few days ago with all the mistakes I made. Up to part F, it worked. Then, part G. *bazinga*

• Marcus Holloway

  December 4, 2020 at 7:42 am · Reply

  I was  trying to experiment.

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  #include <string> #include <iostream>   using namespace std;   class Ball {     private:         string m_color;         double m_radius;       public:         Ball(double radius) : m_radius{radius}         {           }           Ball(const string &color = "black", double radius = 10.0)                     : m_color{color}, m_radius{radius}         {           }           void print()         {             std::cout << "color: " << m_color << ", radius: " << m_radius << '\n';         } };   int main() {     Ball def{};     def.print();       Ball blue{"blue"};     blue.print();       Ball twenty{20.0};     twenty.print();       Ball blueTwenty{"blue", 20.0};     blueTwenty.print(); }

  This is what my terminal printed out:
  color: black, radius: 10
  color: blue, radius: 10
  color: , radius: 20
  color: blue, radius: 20

  Why are my radii not double in the terminal?
  Why does twenty.print( ) give me:   color: , radius: 20</iostream></string>

  • nascardriver

    December 5, 2020 at 5:09 am · Reply

    > Why are my radii not double in the terminal?
    `std::cout` doesn't print trailing zeroes.

    > Why does twenty.print( ) give me: color: , radius: 20
    What did you expect it to do?

    • Marcus Holloway

      December 6, 2020 at 3:01 am · Reply

      I meant to say that twenty.print() gives me "color:, radius:20" because Ball twenty{20.0} uses constructor Ball(double radius): m_radius{radius}{}.
      Am i right to say so?

      • nascardriver

        December 6, 2020 at 4:45 am · Reply

        That's right

• Rishi

  November 23, 2020 at 10:52 pm · Reply

  Why non-static member initialization? What about static members?

• Raul N

  November 9, 2020 at 1:08 am · Reply

  Hello,

  In the second example you wrote 'No default constructor is provided'.
  I remember in one of the previous lessons you said that the constructor from below is generated automatically. Did I get something wrong?

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  Rectangle() { }

  • Oleg+Revedzhuk

    November 13, 2020 at 12:31 pm · Reply

    That is only generated if no constructor is provided by the programmer.

    For example if you only define

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    Ball(const std::string& color, double radius)         : m_color{ color },           m_radius{ radius }     {     }

    Then the compiler will assume that Ball should only be created with two parameters. No default constructor will be added, and you will not be able to define a Ball variable with no parameters.

• Tony

  September 28, 2020 at 5:23 am · Reply

  Hi nascardriver. I can't get to understand the solution to quiz 1.

  Why are not we doing it like

  1	double radius{10.0}

  ?

  I can't understand why we're not giving default non static member initialization to both. Thanks!

  • nascardriver

    September 28, 2020 at 6:54 am · Reply

    If you did

    1	double m_radius{ 10.0 };

    the reader would think that `m_radius` gets this value (10.0). But that's not true. There's no way to make the class use the value provided at `m_radius`' declaration, because it always gets overridden by the constructor.

• John

  September 5, 2020 at 4:31 pm · Reply

  I have a question about Question 2. Isn't the second Ball constructor the default constructor? In 8.5, a default constructor was defined as "a constructor that takes no parameters (or has parameters that all have default values)." The second constructor has parameters that all have default values, so wouldn't it be the default constructor?

• Hjw

  August 29, 2020 at 12:27 pm · Reply

  'Note that initializing members using non-static member initialization requires using either an equals sign, or a brace (uniform) initializer -- the direct initialization form doesn’t work here.'

  Don't you mean the opposite? All code above uses direct initialization {}?