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Development/Tutorials/Common Programming Mistakes
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{{TutorialBrowser|
series=Getting Started|
name=Common Programming Mistakes|
reading=[[Policies/API_to_Avoid|APIs to avoid]]
}}
Translations:Development/Tutorials/Common Programming Mistakes/1/kjh
{{TutorialBrowser|
series=Getting Started|
name=Common Programming Mistakes|
reading=[[Policies/API_to_Avoid|APIs to avoid]]
}}
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This tutorial aims to combine the experience of KDE developers regarding Qt and KDE frameworks dos and don'ts. Besides actual mistakes, it also covers things that are not necessarily "bugs" but which make the code either slower or less readable.
This section guides you through some of the more dusty corners of C++ which either tend to be misused or which people often simply get wrong.
If you have a method in a class that does not access any members and therefore does not need an object to operate, make it static. If additionally, it is a private helper function that is not needed outside of the file, make it a file-static function. That hides the symbol completely.
Symbols defined in a C++ anonymous namespace do not have internal linkage. Anonymous namespaces only give a unique name for that translation unit and that is it; they do not change the linkage of the symbol at all. Linkage is not changed on those because the second phase of two-phase name lookup ignores functions with internal linkages. Also, entities with internal linkage cannot be used as template arguments.
So for now instead of using anonymous namespaces use static if you do not want a symbol to be exported.
First and foremost: it is fine to delete a null pointer. So constructs like this that check for null before deleting are simply redundant:
Note however, that '''a null check ''is'' required when you delete an array''' - that's because a relatively recent compiler on Solaris does not handle it properly otherwise.
When you delete a pointer, make sure you also set it to 0 so that future attempts to delete that object will not fail in a double delete. So the complete and proper idiom is:
You may notice that null pointers are marked variously in one of four ways: 0, 0L, NULL and nullptr. In C, NULL is defined as a null void pointer. In C++, more type safety is possible due to stricter type checking. Modern C++11 implementations (and all C++14 implementations) define NULL to equal the special value nullptr. Nullptr can be automatically cast to boolean false, but a cast to an integer type will fail. This is useful to avoid accidentally. Older C++ implementations before c++11 simply defined NULL to 0L or 0, which provides no additional type safety - one could assign it to an integer variable, which is obviously wrong. For code which does not need to support outdated compilers the best choice is nullptr.
In pointer context, the integer constant zero means "null pointer" - irrespective of the actual binary representation of a null pointer. Note, however, that if you want to pass a null pointer constant to a function in a variable argument list, you *must* explicitly cast it to a pointer - the compiler assumes integer context by default, which might or might not match the binary representation of a pointer.
You will encounter four major styles of marking class member variables in KDE, besides unmarked members:
* '''m_variable''' lowercase m, underscore and the name of the variable starting with a lowercase letter. This is the most common style and one preferred for code in kdelibs.
* '''_variable''' underscore and the name of variable starting with a lowercase letter. This style is actually usually frowned upon as this notation is also used in some code for function parameters instead.
Unmarked members are more common in the case of classes that use [[Policies/Library Code Policy#D-Pointers|d-pointers]].
As it often happens there is no one correct way of doing it, so remember to always follow the syntax used by the application/library to which you are committing. If you're creating a new file, you may want to follow the coding style of the library or module you're adding the file to.
Note that symbols starting with undercores are reserved to the C library (underscore followed by capital or double underscore are reserved to the compiler), so if you can, avoid using the last type.
Try to limit the number of static variables used in your code, especially when committing to a library. Construction and initialization of a large number of static variables really hurts the startup times.
Do not use class-static variables, especially not in libraries and loadable modules though it is even discouraged in applications. Static objects lead to lots of problems such as hard to debug crashes due to undefined order of construction/destruction.
Instead, use a static pointer, together with <tt>K_GLOBAL_STATIC</tt> which is defined in <tt>kglobal.h</tt> and is used like this:
See the [http://api.kde.org/4.x-api/kdelibs-apidocs/kdecore/html/group__KDEMacros.html#ga75ca0c60b03dc5e4f9427263bf4043c7 API documentation] for <tt>K_GLOBAL_STATIC</tt> for more information.
If you need some constant data of simple data types in several places, you do good by defining it once at a central place, to avoid a mistype in one of the instances. If the data changes there is also only one place you need to edit.
Even if there is only one instance you do good by defining it elsewhere, to avoid so-called "magic numbers" in the code which are unexplained (cmp. 42). Usually this is done at the top of a file to avoid searching for it.
Define the constant data using the language constructs of C++, not the preprocessor instructions, like you may be used to from plain C. This way the compiler can help you to find mistakes by doing type checking.
<syntaxhighlight lang="cpp-qt">
// Correct!
static const int AnswerToAllQuestions = 42;
// Wrong!
#define AnswerToAllQuestions 42
</syntaxhighlight>
If defining a constant array do not use a pointer as data type. Instead use the data type and append the array symbol with undefined length, <tt>[]</tt>, behind the name. Otherwise you also define a variable to some const data. That variable could mistakenly be assigned a new pointer to, without the compiler complaining about. And accessing the array would have one indirection, because first the value of the variable needs to be read.
<syntaxhighlight lang="cpp-qt">
// Correct!
static const char SomeString[] = "Example";
// Wrong!
static const char* SomeString = "Example";
// Wrong!
#define SomeString "Example"
</syntaxhighlight>
You will reduce compile times by forward declaring classes when possible instead of including their respective headers. The rules for when a type can be used without being defined are a bit subtle, but intuitively, if the only important aspect is the name of the class, not the details of its implementation, a forward declaration is permissible. Two examples are when declaring pointers to the class or using the class as a function argument.
Prefer to use <tt>const_iterators</tt> over normal iterators when possible. Containers, which are being implicitly shared often detach when a call to a non-const <tt>begin()</tt> or <tt>end()</tt> methods is made ({{qt|QList}} is an example of such a container). When using a const_iterator also watch out that you are really calling the const version of <tt>begin()</tt> and <tt>end()</tt>. Unless your container is actually const itself this probably will not be the case, possibly causing an unnecessary detach of your container. So basically whenever you use const_iterator initialize them using <tt>constBegin()</tt>/<tt>constEnd()</tt> instead, to be on the safe side.
Cache the return of the <tt>end()</tt> (or <tt>constEnd()</tt>) method call before doing iteration over large containers. For example:
This avoids the unnecessary creation of the temporary <tt>end()</tt> (or <tt>constEnd()</tt>) return object on each loop iteration, largely speeding it up.
When using iterators, always use pre-increment and pre-decrement operators (i.e., <tt>++itr</tt>) unless you have a specific reason not to. The use of post-increment and post-decrement operators (i.e., <tt>itr++</tt>) cause the creation of a temporary object.
This code will potentially crash because it is a dangling iterator after the call to erase().
You have to rewrite the code this way:
This problem is also discussed in the [https://doc.qt.io/qt-5/qmap-iterator.html#details Qt documentation for QMap::iterator] but applies to '''all''' Qt iterators
A very "popular" programming mistake is to do a <tt>new</tt> without a <tt>delete</tt> like in this program:
You see, ''pollute()'' instantiates a new object ''polluter'' of the class ''t''. Then, the variable ''polluter'' is lost because it is local, but the content (the object) stays on the heap. I could use this program to render my computer unusable within 10 seconds.
* stop the polluter in an [http://en.cppreference.com/w/cpp/memory/unique_ptr] (which will automatically delete the polluter when returning from the method)
There's also std::shared_ptr and QSharedPointer. This is the generally preferred way to do it in modern C++; explicit memory management should be avoided when possible.
Qt code involving QObject generally uses parent/child relations to free allocated memory; when constructing a QObject (e.g. a widget) it can be given a parent, and when the parent is deleted it deletes all its children. The parent is also set when you add a widget to a layout, for example.
* T is defined in a library you link to (you'd get a linker error if this isn't the case, since it won't find the vtable or RTTI info)
* T is "well-anchored" in that library. By "well-anchored" I mean that the vtable is not a COMMON symbol subject to merging at run-time by the dynamic linker. In other words, the first virtual member in the class definition must exist and not be inlined: it must be in a .cpp file.
For instance, we've seen some hard-to-track problems in non-KDE C++ code we're linking with (I think NMM) because of that. It happened that:
Some classes in the NMM library did not have well-anchored vtables, so dynamic_casting failed inside the Phonon NMM plugin for objects created in the NMM's own plugins.
In this section we will go over some common problems related to the design of Qt/KDE applications.
Although the design of modern C++ applications can be very complex, application windows can be loaded and displayed to the user very quickly through the technique of [http://www.kdedevelopers.org/node/509 delayed initialization]. This technique is relatively straightforward and useful at all stages of an interactive program.
Notice that <tt>window</tt> is created before the <tt>a.exec()</tt> call that starts the event loop. This implies that we want to avoid doing anything non-trivial in the top-level constructor, since it runs before we can even show the window.
The solution is simple: we need to delay the construction of anything besides the GUI until after the event loop has started. Here is how the example class MainWindow's constructor could look to achieve this:
<syntaxhighlight lang="cpp-qt">
MainWindow::MainWindow()
{
initGUI();
QTimer::singleShot( 0, this, SLOT(initObject()) );
}
void MainWindow::initGUI()
{
/* Construct your widgets here. Note that the widgets you
* construct here shouldn't require complex initialization
* either, or you've defeated the purpose.
* All you want to do is create your GUI objects and
* QObject::connect
* the appropriate signals to their slots.
*/
}
void MainWindow::initObject()
{
/* This slot will be called as soon as the event loop starts.
* Put everything else that needs to be done, including
* restoring values, reading files, session restoring, etc here.
* It will still take time, but at least your window will be
* on the screen, making your app look active.
*/
}
</syntaxhighlight>
Using this technique may not buy you any overall time, but it makes your app ''seem'' quicker to the user who is starting it. This increased perceived responsiveness is reassuring for the user as they get quick feedback that the action of launching the app has succeeded.
When (and only when) the start up can not be made reasonably fast enough, consider using a {{class|KSplashScreen}}.
In this section we will go over some of our most common pet-peeves which affect data structures very commonly seen in Qt/KDE applications.
Non-POD ("plain old data") types should be passed by const reference if at all possible. This includes anything other than the basic types such as <tt>char</tt> and <tt>int</tt>.
Take, for instance, {{qt|QString}}. They should always be passed into methods as <tt>const {{qt|QString}}&</tt>. Even though {{qt|QString}} is implicitly shared it is still more efficient (and safer) to pass const references as opposed to objects by value.
If you ever need to delete a QObject derived class from within one of its own methods, do not ever delete it this way:
This will sooner or later cause a crash because a method on that object might be invoked from the Qt event loop via slots/signals after you deleted it.
Instead always use <tt>{{QtMethod|QObject|deleteLater}}</tt> which tries to do the same thing as <tt>delete this</tt> but in a safer way.
It is common to want to see if a {{qt|QString}} is empty. Here are three ways of doing it, the first two of which are correct:
While there is a distinction between "null" {{qt|QString}}s and empty ones, this is a purely historical artifact and new code is discouraged from making use of it.
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