C What does the colon after a constructor mean

Question

Possible Duplicates    Variables After the Colon in a Constructor   C   constructor syntax question  noob        I have some C   code here   class demo    private      unsigned char len   dat   public      demo unsigned char le   5  unsigned char default    len le                  dat   new char len                                                 for  int i   0  i  lt   le  i                                             dat i    default             void  demo void                                                             delete     dat                                                       class newdemo   public demo    private      int  dat1   public      newdemo void    demo 0  0              dat1   0                                                          return 0                                                                 My question is  what are the   len le  and   demo 0  0  called    Is it something to do with inheritance

User · Answer

This is called an initialization list   It is for passing arguments to the constructor of a parent class   Here is a good link explaining it  Initialization Lists in C

User · Answer

It means that len is not set using the default constructor  while the demo class is being constructed   For instance   class Demo      int foo  public      Demo    foo   1        Would first place a value in foo before setting it to 1   It s slightly faster and more efficient

User · Answer

You are calling the constructor of its base class  demo

User · Answer

It s called an initialization list   It initializes members before the body of the constructor executes

User · Answer

As others have said  it s an initialisation list  You can use it for two things    Calling base class constructors Initialising member variables before the body of the constructor executes    For case  1  I assume you understand inheritance  if that s not the case  let me know in the comments   So you are simply calling the constructor of your base class   For case  2  the question may be asked   Why not just initialise it in the body of the constructor   The importance of the initialisation lists is particularly evident for const members  For instance  take a look at this situation  where I want to initialise m val based on the constructor parameter   class Demo       Demo int amp  val                   m val   val         private      const int amp  m val       By the C   specification  this is illegal  We cannot change the value of a const variable in the constructor  because it is marked as const  So you can use the initialisation list   class Demo       Demo int amp  val    m val val                private      const int amp  m val       That is the only time that you can change a const member variable  And as Michael noted in the comments section  it is also the only way to initialise a reference that is a class member   Outside of using it to initialise const member variables  it seems to have been generally accepted as  the way  of initialising variables  so it s clear to other programmers reading your code

User · Answer

It s called an initialization list  An initializer list is how you pass arguments to your member variables  constructors and for passing arguments to the parent class s constructor   If you use   to assign in the constructor body  first the default constructor is called  then the assignment operator is called  This is a bit wasteful  and sometimes there s no equivalent assignment operator

[c++] C++, What does the colon after a constructor mean?

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