Difference between revisions of "Policies/Binary Compatibility Issues With C++"

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== Definition ==
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{{Moved To Community}}
 
 
A library is '''binary compatible''', if a program linked dynamically to a former version of the library continues running with newer versions of the library without the need to recompile.
 
 
 
If a program needs to be recompiled to run with a new version of library but doesn't require any further modifications, the library is '''source compatible'''.
 
 
 
Binary compatibility saves a lot of trouble. It makes it much easier to distribute software for a certain platform. Without ensuring binary compatibility between releases, people will be forced to provide statically linked binaries. Static binaries  are bad because they
 
* waste resources (especially memory)
 
* don't allow the program to benefit from bugfixes or extensions in the libraries
 
 
 
In the KDE project, we will provide binary compatibility within the life-span of a major release for the core libraries (kdelibs kdepimlibs).
 
 
 
== The Do's and Don'ts ==
 
 
 
You can...
 
 
 
* 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.
 
* reimplement virtual functions defined in the primary base class hierarchy '''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.''
 
* 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.'' Because of this, classes that are supposed to stay binary compatible should '''always''' have non-inline destructor, even if it's empty, otherwise the compiler will automatically generate an empty inlined one.
 
* Remove private non-virtual functions '''if''' they are not called by any inline functions.
 
* change the default arguments of a method. It requires recompilation to use the actual new default argument values, though.
 
* add new '''static''' data members.
 
* add new classes.
 
* export a class that was not previously exported.
 
* add or remove friend declarations to classes.
 
 
 
You cannot...
 
* add new virtual functions as this will change the layout of the virtual table and thus break subclasses. ''See below for some workarounds or ask on mailing lists.''
 
* change the order of virtual functions in the class declaration. This will just as well change the layout of the virtual table.
 
* change the signature of a function. This includes:
 
** changing any of the types of the arguments in the parameter list (instead, add a new method)
 
** changing the return type
 
** extending a function with another parameter, even if this parameter has a default argument
 
Suggestion: when adding new functions with the same name and different/extended argument lists, 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 cpp>
 
void functionname( int a );
 
void functionname( int a, int b ); //BCI: merge with int b = 0
 
</code>
 
 
 
* change the access rights to some functions or data members, for example from <tt>private</tt> to <tt>public</tt>. 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.
 
* add new data members to a class or change order of data members in a class (doesn't apply to static ones).
 
* change the class hierachy apart from adding new classes.
 
 
 
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 <tt>event</tt> 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)
 
 
 
== Techniques for Library Programmers ==
 
 
 
The biggest problem when writing libraries is, that one cannot safely add data members since this would change the size and layout of every class, struct, or array containing objects of the type, including subclasses.
 
 
 
=== Bitflags ===
 
One exception are bitflags. If you use bitflags for enums or bools, you can safely round up to at least the next byte minus 1. A class with members
 
 
 
<code cpp>
 
uint m1 : 1;
 
uint m2 : 3;
 
uint m3 : 1;
 
</code>
 
<code cpp>
 
uint m1 : 1;
 
uint m2 : 3;
 
uint m3 : 1;
 
uint m4 : 2; // new member
 
</code>
 
without breaking binary compatibility. Please round up to a maxmimum of 7 bits (or 15 if the bitfield was already larger than 8). Using the very last bit may cause problems on some compilers.
 
 
 
=== Using a d-Pointer===
 
Bitflags and predefined reserved variables are nice, but far from being sufficient.  This is where the d-pointer technique comes into play.  The name "d-pointer" stems from Trolltech's Arnt Gulbrandsen, who first introduced the technique into Qt, making it one of the first C++ GUI libraries to maintain binary compatibility even between bigger release. The technique was quickly adapted as general programming pattern for the KDE libraries by everyone who saw it. It's a great trick to be able to add new private data members to a class without breaking binary compatibility.
 
 
 
'''Remark:''' The d-pointer pattern has been described many
 
times in computer science history under various names, e.g. as pimpl,
 
as handle/body or as cheshire cat.  Google helps finding online papers
 
for any of these, just add C++ to the search terms.
 
 
 
In your class definition for class Foo, define a forward declaration
 
<code cpp>
 
class FooPrivate;
 
</code>
 
and the d-pointer in the private section:
 
<code cpp>
 
private:
 
    FooPrivate* d;
 
</code>
 
The FooPrivate class itself is purely defined in the class implementation file (usually *.cpp ), for example:
 
<code cpp>
 
class FooPrivate {
 
public:
 
    FooPrivate()
 
        : m1(0), m2(0)
 
    {}
 
    int m1;
 
    int m2;
 
    QString s;
 
};
 
</code>
 
 
 
All you have to do now is to create the private data in your constructors or your init function with
 
<code cpp>
 
  d = new FooPrivate;
 
</code>
 
and to delete it again in your destructor with
 
<code cpp>
 
delete d;
 
</code>
 
 
 
You may not want all member variables to live in the private data object, though. For very often used members, it's faster to put them directly in the class, since inline functions cannot access the d-pointer data. Also note that all data covered by the d-pointer is obviously private. For public or protected access, provide both a set and a get function. Example
 
<code cppqt>
 
QString Foo::string() const
 
{
 
    return d->s;
 
}
 
 
 
void Foo::setString( const QString& s )
 
{
 
    d->s = s;
 
}
 
</code>
 
 
 
<h2>Trouble shooting</h2>
 
 
 
=== Adding new data members to classes without d-pointer ===
 
 
 
If you don't have free bitflags, reserved variables and no d-pointer either, but you absolutely have to add a new private member variable, there are still some possibilities left. If your class inherits {{qt|QObject}}, you can for example place the additional data in a special child and find it by traversing over the list of children. You can access the list of children with QObject::children(). However, a fancier and usually faster approach is to use a hashtable to store a
 
mapping between your object and the extra data. For this purpose, Qt provides a pointer-based dictionary called {{qt|QHash}} (or {{qt3|QPtrDict}} in Qt3).
 
 
 
The basic trick in your class implementation of class Foo is:
 
* Create a private data class FooPrivate.
 
* Create a static QHash&lt;Foo *, FooPrivate&gt;.
 
*Note that some compilers/linkers (almost all, unfortunately) do not manage to create static objects in shared libraries. They simply forget to call the constructor. Therefore you should use the  <tt>Q_GLOBAL_STATIC</tt> macro to create and access the object:
 
 
 
<code cppqt>
 
// BCI: Add a real d-pointer
 
Q_GLOBAL_STATIC(QHash<Foo *,FooPrivate>, d_func);
 
static FooPrivate* d( const Foo* foo )
 
{
 
    FooPrivate* ret = d_func()->value( foo, 0 );
 
    if ( ! ret ) {
 
        ret = new FooPrivate;
 
        d_func()->insert( foo, ret );
 
    }
 
    return ret;
 
}
 
static void delete_d( const Foo* foo )
 
{
 
    FooPrivate* ret = d_func()->value( foo, 0 );
 
    delete ret;
 
    d_func()->remove( foo );
 
}
 
</code>
 
 
 
* Now you can use the d-pointer in your class almost as simple as in the code before, just with a function call to d(this). For example:
 
 
 
<code cppqt>
 
d(this)->m1 = 5;
 
</code>
 
 
 
* Add a line to your destructor:
 
<code cppqt>
 
delete_d(this);
 
</code>
 
* Do not forget to add a BCI remark, so that the hack can be removed in the next version of the library.
 
* Do not forget to add a d-pointer to your next class.
 
 
 
=== Adding a reimplemented virtual function ===
 
 
 
As already explained, you can safely reimplement a virtual function defined in one of the base classes only if it is safe that the programs linked with the prior version call the implementation in the base class rather than the new one. This is because the compiler sometimes calls virtual functions directly if it can determine which one to call. For example, if you have
 
<code cpp>
 
void C::foo()
 
{
 
    B::foo();
 
}
 
</code>
 
 
 
then B::foo() is called directly. If class B inherits from class A which implements foo() and B itself doesn't reimplement it, then C::foo() will in fact call A::foo(). If a newer version of the library adds B::foo(), C::foo() will call it only after a recompilation.
 
 
 
Another more common example is:
 
<code cpp>
 
B b; // B derives from A
 
b.foo();
 
</code>
 
then the call to foo() will not use the virtual table. That means that
 
if B::foo() didn't exist in the library but now does, code that was
 
compiled with the earlier version will still call A::foo().
 
 
 
If you can't guarantee things will continue to work without a recompilation, move functionality from A::foo() to a new protected function A::foo2() and use this code:
 
<code cpp>
 
void A::foo()
 
{
 
    if( B* b = dynamic_cast< B* >( this ))
 
        b->B::foo(); // B:: is important
 
    else
 
        foo2();
 
}
 
void B::foo()
 
{
 
    // added functionality
 
    A::foo2(); // call base function with real functionality
 
}
 
</code>
 
All calls to A::foo() for objects of type B (or inherited) will result in  calling B::foo(). The only case that will not work as expected are calls to A::foo() that explicitly specify A::foo(), but B::foo() calls A::foo2() instead and there should not be other places doing so.
 
 
 
=== Using a new class ===
 
 
 
A relatively simple method of "extending" a class can be writing a replacement class that will include also the new functionality (and that may inherit from the old class to reuse the code). This of course requires adapting and recompiling applications using the library, so it is not possible this way to fix or extend functionality of classes that are used by applications compiled against an older version of the library. However, especially with small and/or performance-critical classes it may be simpler to write them without making sure they'll be simple to extend in the future and if the need arises later write a new replacement class that will provide new features or better performance.
 
 
 
=== Adding new virtual functions to leaf classes ===
 
This technique is one of cases of using a new class that can help if there's a need to add new virtual functions to a class that should stay binary compatible and there is no class inheriting from it that should also stay binary compatible (i.e. all classes inheriting from it are in applications). In such case it's possible to add a new class inheriting from the original one that will add them. Applications using the new functionality will of course have to be modified to use the new class.
 
<code cpp>
 
class A {
 
public:
 
    virtual void foo();
 
};
 
class B : public A { // newly added class
 
public:
 
    virtual void bar(); // newly added virtual function
 
};
 
void A::foo()
 
{
 
    // here it's needed to call a new virtual function
 
    if( B* this2 = dynamic_cast< B* >( this ))
 
        this2->bar();
 
}
 
</code>
 
It is not possible to use this technique when there are other inherited classes that should also stay binary compatible because they'd have to inherit from the new class.
 
 
 
=== Using signals instead of virtual functions ===
 
Qt's signals and slots are invoked using a special virtual method created by the Q_OBJECT macro and it exists in every class inherited from {{qt|QObject}}. Therefore adding new signals and slots doesn't affect binary compatibility and the signals/slots mechanism can be used to emulate virtual functions.
 
 
 
<code cppqt>
 
class A : public QObject {
 
Q_OBJECT
 
public:
 
    A();
 
    virtual void foo();
 
signals:
 
    void bar( int* ); // added new "virtual" function
 
protected slots:
 
    // implementation of the virtual function in A
 
    void barslot( int* );
 
};
 
 
 
A::A()
 
{
 
    connect(this, SIGNAL( bar(int*)), this, SLOT( barslot(int*)));
 
}
 
 
 
void A::foo()
 
{
 
    int ret;
 
    emit bar( &ret );
 
}
 
 
 
void A::barslot( int* ret )
 
{
 
    *ret = 10;
 
}
 
</code>
 
 
 
Function bar() will act like a virtual function, barslot() implements the actual functionality of it. Since signals have void return value, data must be returned using arguments. As there will be only one slot connected to the signal returning data from the slot this way will work without problems. Note that with Qt4 for this to work the connection type will have to be Qt::DirectConnection.
 
 
 
If an inherited class will want to re-implement the functionality of bar() it will have to provide its own slot:
 
<code cppqt>
 
class B : public A {
 
Q_OBJECT
 
public:
 
    B();
 
protected slots: // necessary to specify as a slot again
 
    void barslot( int* ); // reimplemented functionality of bar()
 
};
 
 
 
B::B()
 
{
 
    disconnect(this, SIGNAL(bar(int*)), this, SLOT(barslot(int*)));
 
    connect(this, SIGNAL(bar(int*)), this, SLOT(barslot(int*)));
 
}
 
 
 
void B::barslot( int* ret )
 
{
 
    *ret = 20;
 
}
 
</code>
 
 
 
Now B::barslot() will act like virtual reimplementation of A::bar(). Note that it is necessary to specify barslot() again as a slot in B and that in the constructor it is necessary to first disconnect and then connect again, that will disconnect A::barslot() and connect B::barslot() instead.
 
 
 
Note: the same can be accomplished by implementing a virtual slot.
 
 
 
[[Category:Policies]] [[Category:C++]]
 

Latest revision as of 18:24, 10 March 2016

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