====== Binarna kompatibilita ======
Vlastnost, ktora trapi programatorov uz dlhe roky

==== Co je a co nie je nekompatibilna zmena ====
Skopirovane zo stranky http://techbase.kde.org/Policies/Binary_Compatibility_Issues_With_C++#The_Do.27s_and_Don.27ts

24.Okt.2011 - doplneny ziskany poznatok o enumoch v sekcii "You can"

== You can... (compatible changes) ==

    * add new non-virtual functions including signals and slots and constructors.
    * add a new enum to a class.
    * append new enumerators to an existing enum.
    * move enum from a class to different file and different namespace ( edit by JBE - I've been writing and testing it for more than 2 hours ;-) )
    * reimplement virtual functions defined in the primary base class hierarchy (that is, virtuals defined in the first non-virtual base class, or in that class's first non-virtual base class, and so forth) if it is safe that programs linked with the prior version of the library call the implementation in the base class rather than the new one. This is tricky and might be dangerous. Think twice before doing it. Alternatively see below for a workaround.
          * Exception: if the overriding function has a covariant return type, it's only a binary-compatible change if the more-derived type has always the same pointer address as the less-derived one. If in doubt, do not override with a covariant return type. 
    * change an inline function or make an inline function non-inline if it is safe that programs linked with the prior version of the library call the old implementation. This is tricky and might be dangerous. Think twice before doing it.
    * remove private non-virtual functions if they are not called by any inline functions (and have never been).
    * remove private static members if they are not called by any inline functions (and have never been).
    * add new static data members.
    * change the default arguments of a method. It requires recompilation to use the actual new default argument values, though.
    * add new classes.
    * export a class that was not previously exported.
    * add or remove friend declarations to classes.
    * rename reserved member types
    * extend reserved bit fields, provided this doesn't cause the bit field to cross the boundary of its underlying type (8 bits for char & bool, 16 bits for short, 32 bits for int, etc.)
    * add the Q_OBJECT macro to a class if the class already inherits from QObject
    * add a Q_PROPERTY, Q_ENUMS or Q_FLAGS macro as that only modifies the meta-object generated by moc and not the class itself 

== You cannot... (incompatible changes) ==

    * For existing classes:
          * unexport or remove an exported class
          * change the class hierachy in any way (add, remove, or reorder base classes). 
    * For template classes:
          o change the template arguments in any way (add, remove or reorder). 
    * For existing functions of any type:
          * unexport it.
          * remove it.
                * Remove the implementation of existing declared functions. The symbol comes from the implementation of the function, so this is effectively the function. 
          * inline it (this includes moving a member function's body to the class definition, even without the inline keyword).
          * add an overload (BC, but not SC: makes &func ambiguous), adding overloads to already overloaded functions is ok (any use of &func already needed a cast).
          * change its signature. This includes:
                * changing any of the types of the arguments in the parameter list, including changing the const/volatile qualifiers of the existing parameters (instead, add a new method)
                * changing the const/volatile qualifiers of the function
                * changing the access rights to some functions or data members, for example from private to public. With some compilers, this information may be part of the signature. If you need to make a private function protected or even public, you have to add a new function that calls the private one.
                * changing the CV-qualifiers of a member function: the const and/or volatile that apply to the function itself.
                * extending a function with another parameter, even if this parameter has a default argument. See below for a suggestion on how to avoid this issue
                * changing the return type in any way
                * Exception: non-member functions declared with extern "C" can change parameter types (be very careful). 
    * For virtual member functions:
          * add a virtual function to a class that doesn't have any virtual functions or virtual bases.
          * add new virtual functions to non-leaf classes as this will break subclasses. See below for some workarounds or ask on mailing lists.
          * change the order of virtual functions in the class declaration.
          * override an existing virtual function if that function is not in the primary base class (first non-virtual base class, or the primary base class's primary base class and upwards).
          * override an existing virtual function if the overriding function has a covariant return type for which the more-derived type has a pointer address different from the less-derived one (usually happens when, between the less-derived and the more-derived ones, there's multiple inheritance or virtual inheritance).
          * remove a virtual function, even if it is a reimplementation of a virtual function from the base class 
          * change pure virtual function to virtual function (sometimes this can work - when function is not called anywhere)
    * For static non-private members or for non-static non-member public data:
          * Remove or unexport it
          * Change its type
          * Change its CV-qualifiers 
    * For non-static members:
          * add new, data members to an existing class.
          * change the order of non-static data members in a class.
          * change the type of the member, except for signedness
          * remove existing non-static data members from an existing class. 

If you need to add extend/modify the parameter list of an existing function, you need to add a new function instead with the new parameters. In that case, you may want to add a short note that the two functions shall be merged with a default argument in later versions of the library:

<code>
void functionname( int a );
void functionname( int a, int b ); //BCI: merge with int b = 0
</code>

== You should... ==

In order to make a class to extend in the future you should follow these rules:

    * add d-pointer. See below.
    * add non-inline virtual destructor even if the body is empty.
    * reimplement event in widget classes, even if the body for the function is empty.
    * make all constructors non-inline.
    * write non-inline implementations of the copy constructor and assignment operator unless the class cannot be copied by value (e.g. classes inherited from QObject can't be)