What is this weird colon-member syntax in the constructor

Question

Recently I ve seen an example like the following    include  lt iostream gt   class Foo   public    int bar    Foo int num   bar num          int main void      std  cout  lt  lt  Foo 42  bar  lt  lt  std  endl    return 0      What does this strange   bar num  mean  It somehow seems to initialize the member variable but I ve never seen this syntax before  It looks like a function constructor call but for an int   Makes no sense for me  Perhaps someone could enlighten me  And  by the way  are there any other esoteric language features like this  you ll never find in an ordinary C   book

User · Answer

It s an initialization list for the constructor  Instead of default constructing x  y and z and then assigning them the values received in the parameters  those members will be initialized with those values right off the bat  This may not seem terribly useful for floats  but it can be quite a timesaver with custom classes that are expensive to construct

User · Answer

You are correct  this is indeed a way to initialize member variables   I m not sure that there s much benefit to this  other than clearly expressing that it s an initialization   Having a  bar num  inside the code could get moved around  deleted  or misinterpreted much more easily

User · Answer

This is called an initialization list   It is a way of initializing class members   There are benefits to using this instead of simply assigning new values to the members in the body of the constructor  but if you have class members which are constants or references they must be initialized

User · Answer

This is an initialization list  It ll initialize the members before the constructor body is run  Consider  class Foo    public     string str     Foo string  amp p             str   p              vs   class Foo   public    string str    Foo string  amp p   str p           In the first example  str will be initialized by its no-argument constructor  string      before the body of the Foo constructor  Inside the foo constructor  the  string amp  operator   const string amp  s      will be called on  str  as you do   str   p   Wheras in the second example  str will be initialized directly by calling its constructor  string  const string amp  s      with  p  as an argument

User · Answer

I don t know how you could miss this one  it s pretty basic  That s the syntax for initializing member variables or base class constructors   It works for plain old data types as well as class objects

User · Answer

Not mentioned yet on this thread  since C  11  the member initializer list can use list-initialization   aka   uniform initialization    braced initialization     Foo int num   bar num       which has the same semantics as list-initialization in other contexts

User · Answer

Foo int num   bar num       This construct is called a Member Initializer List in C    Simply said  it initializes your member bar to a value num   What is the difference between Initializing and Assignment inside a constructor  Member Initialization  Foo int num   bar num       Member Assignment  Foo int num       bar   num     There is a significant difference between Initializing a member using Member initializer list and assigning it an value inside the constructor body  When you initialize fields via Member initializer list the constructors will be called once and the object will be constructed and initialized in one operation  If you use assignment then the fields will be first initialized with default constructors and then reassigned  via assignment operator  with actual values  As you see there is an additional overhead of creation  amp  assignment in the latter  which might be considerable for user defined classes  Cost of Member Initialization   Object Construction  Cost of Member Assignment   Object Construction   Assignment  The latter is actually equivalent to  Foo int num    bar    bar   num    While the former is equivalent to just  Foo int num   bar num     For an inbuilt  your code example  or POD class members there is no practical overhead   When do you HAVE TO use Member Initializer list  You will have rather forced  to use a Member Initializer list if   Your class has a reference member Your class has a non static const member or Your class member doesn t have a default constructor or For initialization of base class members or When constructor   s parameter name is same as data member this is not really a MUST    A code example  class MyClass   public       Reference member  has to be Initialized in Member Initializer List   int  amp i    int b       Non static const member  must be Initialized in Member Initializer List   const int k        Constructor   s parameter name b is same as class data member      Other way is to use this- gt b to refer to data member   MyClass int a  int b  int c    i a   b b   k c           Without Member Initializer        this- gt b   b          class MyClass2   public MyClass   public    int p    int q    MyClass2 int x  int y  int z  int l  int m    MyClass x  y  z   p l   q m         int main       int x   10    int y   20    int z   30    MyClass obj x  y  z      int l   40    int m   50    MyClass2 obj2 x  y  z  l  m      return 0      MyClass2 doesn t have a default constructor so it has to be initialized through member initializer list  Base class MyClass does not have a default constructor  So to initialize its member one will need to use Member Initializer List    Important points to Note while using Member Initializer Lists  Class Member variables are always initialized in the order in which they are declared in the class  They are not initialized in the order in which they are specified in the Member Initializer List  In short  Member initialization list does not determine the order of initialization  Given the above it is always a good practice to maintain the same order of members for Member initialization as the order in which they are declared in the class definition  This is because compilers do not warn if the two orders are different but a relatively new user might confuse member Initializer list as the order of initialization and write some code dependent on that

User · Answer

The other already explained to you that the syntax that you observe is called  constructor initializer list   This syntax lets you to custom-initialize base subobjects and member subobjects of the class  as opposed to allowing them to default-initialize or to remain uninitialized    I just want to note that the syntax that  as you said   looks like a constructor call   is not necessarily a constructor call  In C   language the    syntax is just one standard form of initialization syntax  It is interpreted differently for different types  For class types with user-defined constructor it means one thing  it is indeed a constructor call   for class types without user-defined constructor it means another thing  so called value initialization   for empty     and for non-class types it again means something different  since non-class types have no constructors    In your case the data member has type int  int is not a class type  so it has no constructor  For type int this syntax means simply  initialize bar with the value of num  and that s it  It is done just like that  directly  no constructors involved  since  once again  int is not a class type of therefore it can t have any constructors

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there is another  benefit   if the member variable type does not support null initialization or if its a reference  which cannot be null initialized  then you have no choice but to supply an initialization list

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This is not obscure  it s the C   initialization list syntax  Basically  in your case  x will be initialized with  x  y with  y  z with  z

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That s constructor initialisation   It is the correct way to initialise members in a class constructor  as it prevents the default constructor being invoked   Consider these two examples      Example 1 Foo Bar b       bar   b        Example 2 Foo Bar b       bar b        In example 1   Bar bar      default constructor bar   b      assignment   In example 2   Bar bar b     copy constructor   It s all about efficiency

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It s a member initialization list   You should find information about it in any good C   book   You should  in most cases  initialize all member objects in the member initialization list  however  do note the exceptions listed at the end of the FAQ entry      The takeaway point from the FAQ entry is that      All other things being equal  your code will run faster if you use initialization lists rather than assignment

[c++] What is this weird colon-member (" : ") syntax in the constructor?

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