从 POD 结构继承的类中默认构造函数的奇怪行为

Question

This question relates to this one .

As I mentioned in previous question I've decided to inherit my class from Win structure BITMAP to provide some extended functionality.

I've noticed interest detail in compiled program behavior. First I have defined default constructor for my class like below:

CPreviewFrame::CPreviewFrame():
   m_bufferSize( 0 )
{
   bmBits = NULL; //ensure that compiler in debug won't init it with 0xccccc... and delete[] will do the job
}

In idea compiler had to generate code which calls default constructor for base type even if it wasn't called manually in init list. But while debugging I noticed that BITMAP's data members are not initialized! I added manual initialize for BITMAP and it worked - all data members were initialized by zeros:

CPreviewFrame::CPreviewFrame():
   BITMAP( ),
   m_bufferSize( 0 )
{
   //bmBits = NULL; //it's not needed anymore probably
}

Why does it happen? Isn't compiler obligated to call default constructor or it's applied to classes only? (it can't be so I think - only difference is in default access qualifiers for members and for inheritance)

Answer 1

If you do not provide an explicit initializer for a POD-struct, then the object has an indeterminate initial value per Section 8.5/9 of the C++ standard. Adding an initializer for BITMAP that is an empty set of parenthesis to the initializer list of your CPreviewFrame constructor value-initializes the BITMAP object per Section 8.5/7. According to Section 8.5/5, that will mean all the non-static members of BITMAP will be zero-initialized since they are not arrays, unions, or class-types.

In your initial example though, you only initialized the bmBits member of the BITMAP structure in the actual body of the CPreviewFrame constructor ... that leaves the rest of the data-members of BITMAP with values that are indeterminate since no initializer was specified for the BITMAP structure itself. Since each non-static data-member of a class is initialized before the actual body of the constructor is called, the lack of an explicit initializer for BITMAP , which is a non-static POD-struct data-member of your CPreviewFrame class, means that the behavior described in 8.5/9, where the values are set to an indeterminate initial value, takes effect.