How do C class members get initialized if I don t do it explicitly

Question

Suppose I have a class with private memebers ptr  name  pname  rname  crname and age   What happens if I don t initialize them myself  Here is an example   class Example       private          int  ptr          string name          string  pname          string  amp rname          const string  amp crname          int age       public          Example           And then I do   int main         Example ex      How are the members initialized in ex  What happens with pointers  Do string and int get 0-intialized with default constructors string   and int    What about the reference member  Also what about const references   What else should I know about   Does anyone know a tutorial that covers these cases  Maybe in some books  I have access in university s library to a lot of C   books   I d like to learn it so I can write better  bug free  programs  Any feedback would help

User · Answer

Uninitialized non-static members will contain random data. Actually, they will just have the value of the memory location they are assigned to.

Of course for object parameters (like string) the object's constructor could do a default initialization.

In your example:

int *ptr; // will point to a random memory location
string name; // empty string (due to string's default costructor)
string *pname; // will point to a random memory location
string &rname; // it would't compile
const string &crname; // it would't compile
int age; // random value

User · Answer

In lieu of explicit initialization  initialization of members in classes works identically to initialization of local variables in functions   For objects  their default constructor is called  For example  for std  string  the default constructor sets it to an empty string  If the object s class does not have a default constructor  it will be a compile error if you do not explicitly initialize it   For primitive types  pointers  ints  etc   they are not initialized -- they contain whatever arbitrary junk happened to be at that memory location previously   For references  e g  std  string amp    it is illegal not to initialize them  and your compiler will complain and refuse to compile such code  References must always be initialized   So  in your specific case  if they are not explicitly initialized       int  ptr      Contains junk     string name      Empty string     string  pname      Contains junk     string  amp rname      Compile error     const string  amp crname      Compile error     int age      Contains junk

User · Answer

First  let me explain what a mem-initializer-list is  A mem-initializer-list is a comma-separated list of mem-initializers  where each mem-initializer is a member name followed by    followed by an expression-list  followed by a    The expression-list is how the member is constructed  For example  in  static const char s str      bodacydo   class Example   private      int  ptr      string name      string  pname      string  amp rname      const string  amp crname      int age   public      Example             name s str  s str   8   rname name   crname name   age -4                   the mem-initializer-list of the user-supplied  no-arguments constructor is name s str  s str   8   rname name   crname name   age -4   This mem-initializer-list means that the name member is initialized by the std  string constructor that takes two input iterators  the rname member is initialized with a reference to name  the crname member is initialized with a const-reference to name  and the age member is initialized with the value -4   Each constructor has its own mem-initializer-list  and members can only be initialized in a prescribed order  basically the order in which the members are declared in the class   Thus  the members of Example can only be initialized in the order  ptr  name  pname  rname  crname  and age   When you do not specify a mem-initializer of a member  the C   standard says      If the entity is a nonstatic data member     of class type      the entity is default-initialized  8 5       Otherwise  the entity is not initialized    Here  because name is a nonstatic data member of class type  it is default-initialized if no initializer for name was specified in the mem-initializer-list  All other members of Example do not have class type  so they are not initialized   When the standard says that they are not initialized  this means that they can have any value  Thus  because the above code did not initialize pname  it could be anything   Note that you still have to follow other rules  such as the rule that references must always be initialized  It is a compiler error to not initialize references

User · Answer

You can also initialize data members at the point you declare them   class another example  public      another example         another example    private      int m iInteger 10      double m dDouble 10 765       I use this form pretty much exclusively  although I have read some people consider it  bad form   perhaps because it was only recently introduced - I think in C  11   To me it is more logical   Another useful facet to the new rules is how to initialize data-members that are themselves classes   For instance suppose that CDynamicString is a class that encapsulates string handling   It has a constructor that allows you specify its initial value CDynamicString wchat t  pstrInitialString    You might very well use this class as a data member inside another class - say a class that encapsulates a windows registry value which in this case stores a postal address   To  hard code  the registry key name to which this writes you use braces   class Registry Entry  public      Registry Entry         Registry Entry        Commit     Writes data to registry      Retrieve     Reads data from registry  private      CDynamicString m cKeyName L Postal Address        CDynamicString m cAddress       Note the second string class which holds the actual postal address does not have an initializer so its default constructor will be called on creation - perhaps automatically setting it to a blank string

User · Answer

If you example class is instantiated on the stack  the contents of uninitialized scalar members is random and undefined   For a global instance  uninitialized scalar members will be zeroed   For members which are themselves instances of classes  their default constructors will be called  so your string object will get initialized     int  ptr    uninitialized pointer  or zeroed if global  string name    constructor called  initialized with empty string string  pname    uninitialized pointer  or zeroed if global  string  amp rname    compilation error if you fail to initialize this const string  amp crname    compilation error if you fail to initialize this int age    scalar value  uninitialized and random  or zeroed if global

User · Answer

Members with a constructor will have their default constructor called for initialisation   You cannot depend on the contents of the other types

User · Answer

It depends on how the class is constructed  Answering this question comes understanding a huge switch case statement in the C   language standard  and one which is hard for mere mortals to get intuition about   As a simple example of how difficult thing are   main cpp   include  lt cassert gt   int main         struct C   int i             This syntax is called  default initialization      C a         i undefined         This syntax is called  value initialization      C b        assert b i    0       In default initialization you would start from  https   en cppreference com w cpp language default initialization we go to the part  The effects of default initialization are  and start the case statement     if T is a non-POD   no  the definition of POD is in itself a huge switch statement   if T is an array type   no  otherwise  nothing is done   therefore it is left with an undefined value   Then  if someone decides to value initialize we go to https   en cppreference com w cpp language value initialization  The effects of value initialization are  and start the case statement     if T is a class type with no default constructor or with a user-provided or deleted default constructor   not the case  You will now spend 20 minutes Googling those terms    we have an implicitly defined default constructor  in particular because no other constructor was defined  it is not user-provided  implicitly defined  it is not deleted    delete    if T is a class type with a default constructor that is neither user-provided nor deleted   yes    the object is zero-initialized and then it is default-initialized if it has a non-trivial default constructor   no non-trivial constructor  just zero-initialize  The definition of  zero-initialize  at least is simple and does what you expect  https   en cppreference com w cpp language zero initialization    This is why I strongly recommend that you just never rely on  implicit  zero initialization  Unless there are strong performance reasons  explicitly initialize everything  either on the constructor if you defined one  or using aggregate initialization  Otherwise you make things very very risky for future developers

User · Answer

If it is on the stack  the contents of uninitialized members that don t have their own constructor will be random and undefined  Even if it is global  it would be a bad idea to rely on them being zeroed out  Whether it is on the stack or not  if a member has its own constructor  that will get called to initialize it   So  if you have string  pname  the pointer will contain random junk  but for string name  the default constructor for string will be called  giving you an empty string  For your reference type variables  I m not sure  but it ll probably be a reference to some random chunk of memory

[c++] How do C++ class members get initialized if I don't do it explicitly?

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