Dr. Hartmut Schorrig, www.vishia.org, 2021-01-24

1. Approach

There are at least two arguments to store data in a struct instead a class and wrap it with a C++ class:

  • Legacy or C-backward compatibility: Data are already defined in a struct.

  • Transfer data as memory image 'serialized' .

The last argument can be become important in a later phase of programming. If all data are stored in a class, it have to be refactored.

2. Memory image of a class is not useable

A class may contain additional information for virtual operations, RunTimeTypeInformation or such. A memory image of a class is not obviosly defined, it depends on several conditions.

A struct memory image is well defined. All elements are in the order of defintion in the struct. The only one problem is a automatic compiler-done alignment.

For the alignment problem it is recommended to store all element at a position which is able to divide by the size of the element, if it is lesser than 8. It means, for example:

typedef MyStruct_T {
  char a;
  int16 i1;    //Memory address 1, or aligned to 2, or to 4, or to 8?
  float f;     //Memory address 3, or aligend ...  NEGATIVE PATTERN
} MyStruct_s;

The adequate positiv pattern may be:

typedef MyStruct_T {
  char a;
  char dummy;
  int16 i1;    //Memory address 2, it should be able to align with 16-bit memory access
  float f;     //Memory address 4, it is aligned
} MyStruct_s;

But that should be checked in the individual case. Always align to 8 (for run on a 64-bit-system) should run any time.

You can copy the content of a struct for data exchange for example in the payload of a datagram:

MemUnit* payload = ...;
int sizeData = sizeof(myDataFromStruct)
memcpy(payload + currPos, myDataFromStruct, sizeData);
payload += sizeof(myDataFromStruct);

You can evaluate the data on the other side as equal defined struct (using the same header).

You should not do that with class data. You may use a complex serialize algorithm for class data, it may be the proper solution (if you have time and calculation time).

3. Rule to build a class with struct data

struct MyData_T {
  int16 d1;
  int16 d2;
  float f1;
  double d1;    //regard alignment problem
} MyData_s;     //<<<=== use suffix _s
class MyData : protected MyData_s
{               //should not have extra data
  public: void myOperation(float x);
                //should define the operation.
}

If the class has own data, it should be private. For reflection generation it are not visible, reflection are not generated.

3.1. Using ObjectJc as base information

See ../Base/ObjectJc_en.html

especially Base/ObjectJc_en.html, Chapter Initializing for C++

3.2. Using this pattern for reflection generation

The reflection, see ../Base/ClassJc_en.html or ../../../Inspc//html/InspcComm.html is possible for symbolic access inside an application or for symbolic access from outside.

All struct definitions ending with _s are stored with the 'basename' for the reflection class without this _s suffix. If a class with exact this name exists, then reflection information from this class are not generated. It is assumed that this class contains the _s struct data. Of course only if the class instance is really based on the _s struct the reflection can be used for this instance (see ../Base/ObjectJc_en.html#CppObjectJc). But extra variable in the class are unknown for the reflection access. This may be an advantage, of this as side effect: specific private variable can be hidden.

If a class bases on a struct but has own obviously variable for reflection, it should have another name. Then it has its own reflection. Reflections are generated generally for struct and class. Only the special case, using a struct with the same name and suffix _s as base class (struct), prevents reflection generation.

…​todo some more 2021-01-24