How to call a function in a member initialization list?

November 9, 2021

I have the following simple c++ (RAII pattern) wrapper around jpeg_decompress_struct (ijg/libjpeg-turbo):

class Decompress {
  typedef jpeg_decompress_struct *ptr;
  jpeg_decompress_struct srcinfo;

 public:
  Decompress() { jpeg_create_decompress(&srcinfo); }
  ~Decompress() { jpeg_destroy_decompress(&srcinfo); }
  operator ptr() { return &srcinfo; }
  jpeg_decompress_struct *operator->() { return &srcinfo; }
};

I am happy with this code, and I’d like to keep srcinfo as it is now (on the stack, just like upstream expect it.)

Is there a way to rewrite the above constructor using a member initialization list ? I’d like to pass the check of gcc -Weffc++.

Current warning is:

jpeg.cxx:92:3: error: ‘acme::Decompress::srcinfo’ should be initialized in the member initialization list [-Werror=effc++]
   92 |   Decompress() { jpeg_create_decompress(&srcinfo); }
      |   ^~~~~~~~~~

>Solution :

If (which we can probably safely assume here) jpeg_create_decompress() can operate on a jpeg_decompress_struct containing undefined values, then the code you posted is fine as is . I wouldn’t personally touch it.

However, the question can still be answered as aksed:

Most question formulated as "How to do __blank__ in an initialization list" have the same answer: Do it in a function.

jpeg_decompress_struct make_initialized_jds() {
  jpeg_decompress_struct result;
  jpeg_create_decompress(&result);
  return result;
}

class Decompress {
  typedef jpeg_decompress_struct *ptr;
  jpeg_decompress_struct srcinfo;

 public:
  Decompress() : srcinfo(make_initialized_jds()) {}
  ~Decompress() { jpeg_destroy_decompress(&srcinfo); }
  operator ptr() { return &srcinfo; }
  jpeg_decompress_struct *operator->() { return &srcinfo; }
};

In modern compilers making use of NRVO, this won’t even result in any additional overhead.

Edit: From the updated question: If you just want to make the warning go away, you can explicitely initialize the struct using a member initialization:

class Decompress {
  typedef jpeg_decompress_struct *ptr = nullptr; // <---- might as well. 
  jpeg_decompress_struct srcinfo = {0}; // <----- here

 public:
  Decompress() { jpeg_create_decompress(&srcinfo); }
  ~Decompress() { jpeg_destroy_decompress(&srcinfo); }
  operator ptr() { return &srcinfo; }
  jpeg_decompress_struct *operator->() { return &srcinfo; }
};

This will give srcinfo a defined value before invoking jpeg_create_decompress().