This is an old revision of the document!
Table of Contents
Best Practices
General
Tips & tricks
Always initialise primitive variables (other types do not require this)
// Bad int myVariable; std::string myString; // Good int myVariable{}; std::string myString;
Always set parameters as const reference if they are non-primitives
// Bad void myFunction(int firstParameter, std::string secondParameter) { //... } // Good void myFunction(int firstParameter, const std::string &secondParameter) { //... }
Do not use "using namespace" outside of a function => improves readability (and only use it when needed)
// Bad using namespace std; void myFunction() { string myString; //... } // Good void myFunction() { using namespace std; string myString; //... } // Better void myFunction() { std::string myString; //... }
Note that you can use namespace aliases to simplify a complex namespace hierarchy:
namespace myNamespace = some::complex::namespace::hierarchy;
Use references instead of pointers if possible => easier to use
(but note that Qt uses a lot of pointers for historical reasons)
// Bad void myFunction(MyObject *object) { object->function(); //... } // Good void myFunction(MyObject &object) { object.function(); //... }
Use smart pointers instead of raw/naked/dumb pointers => easier to use
(but note that Qt uses its own memory management system, so if using Qt classes or Qt-based classes you will have to use new, see this section)
// Bad MyObject *object = new MyObject; delete object; // Good #include <memory> std::unique_ptr<MyObject> object{std::make_unique<MyObject>()}; // Good #include <QObject> // parentObject is a pointer to a QObject MyObject *object = new MyObject(parentObject);
Never mix C and C++ code
C and C++ are very different languages. The fact that most of the C language can be compiled using a C++ compiler does not mean that you should mix both. For example, C-arrays are difficult to use correctly and should be avoided. Prefer using QList when writing Qt-based code and std::vector otherwise.
// Bad int *myArray = new int[42]; delete[] myArray ; // Good #include <QList> QList<int> myArray; // QList has no resize() function, so reserve() could be used to reserve memory and append() or push_back() could be used to add elements // Good #include <vector> std::vector<int> myArray(42);
Static arrays should use std::array. They offer the same performance while adding convenience functions like size().
// Bad int myArray[42]; // Good #include <array> std::array<int, 42> myArray;
In addition, note that C code relying on functions to free memory like free(), are not exception safe. This means that mixing exception-throwing code with these functions would trigger memory leaks in case an exception is thrown.
// Bad int* stuff = (int*)malloc(sizeof(int) * 42); QPushButton *button = new QPushButton(parent); // What happens if new triggers an exception? "stuff" is never freed and a memory leak occurs free(stuff); // Good #include <array> std::array<int, 42> stuff; QPushButton *button = new QPushButton(parent); // "stuff" never leaks memory
If you have to write a function that returns multiple values, prefer returning a std::tuple instead of using reference parameters (if possible)
// Bad void myFunction(int &outFirstVariable, std::string &outSecondVariable) { outFirstVariable = 42; outSecondVariable = "text"; } // Good #include <tuple> std::tuple<int, std::string> myFunction() { return std::make_tuple(42, "text"); } // auto result = myFunction(); // Use std::get<0>(result) to get the integer, std::get<1>(result) to get the std::string
When using events, prefer using lambdas (nameless functions) instead of static functions
#include <QPushButton> QPushButton *button = new QPushButton(parent); // Bad void MyObject::onClick() { //... } connect(button, SIGNAL(clicked()), this, SLOT(onClick())); // Good connect(button, &QPushButton::clicked(), [this]() { //... });
When incrementing a variable, prefer using the prefix operator rather than the postfix one
It is sometimes faster, but never slower: http://stackoverflow.com/questions/24901/is-there-a-performance-difference-between-i-and-i-in-c
// Bad i++; // Good ++i;
When performing operations on containers (arrays, vectors, etc.) prefer using range-based for (= "foreach") instead of index or iterators when possible
std::vector<int> myContainer = {42, 43, 44, 45}; // Bad for(std::vector<int>::iterator it = myContainer.begin(); it != myContainer.end(); ++it) { //... } // Not as bad for(int index = 0; index < myContainer.size(); ++index ) { //... } // Good for(int value: myContainer) { //... }
Never forward-declare variables
// Bad int i; int j; for(; i < 10; ++i) { for(; j < 10; ++j) { //... } } // Good for(int i{}; i < 10; ++i) { for(int j{}; j < 10; ++j) { //... } }
Never use typedef
It has been superseded by using since C++11.
// Bad typedef int MyInteger; // Good using MyInteger = int;
Note that using using you can also set template parameters now:
using Integer3DVector = Generic3DVector<int>;
Never use #define to create constants, use constexpr instead
// Bad #define MY_CONSTANT_VALUE 42 // Good constexpr int MyConstantValue = 42; // Note the naming change here, caps should only be used for preprocessor defines
Write small functions instead of huge ones
Never call virtual functions from a constructor
Qt specific
Containers
C++ Standard Library | Qt |
---|---|
std::vector | Contiguous memory bloc? Yes: QVector No: QList* |
std::list | QLinkedList |
std::set | QSet |
std::map | QMap |
std::unordered_map | QHash |
std::multimap | QMultiMap |
std::unordered_multimap | QMultiHash |
*If in doubt, use QList.
/!\ std::list is not equivalent to QList /!\
Prefer using QHash and QMultiHash over QMap and QMultiMap if you don't need the items to be sorted, their lookup time is smaller.
Inheritance
QObject-based:
- automatic memory management (no smart pointer required)
- constructor takes a parent QObject, defaulted to nullptr
- Q_OBJECT macro at the beginning of the class
- myobject.hpp
#pragma once #include <QObject> class MyObject: public QObject { Q_OBJECT public: MyObject(QObject *parent = nullptr); virtual ~MyObject(); }
- myobject.cpp
#include "myobject.hpp" MyObject::MyObject(QObject *parent): QObject(parent) { } MyObject::~MyObject() { }
QWidget-based:
- automatic memory management (no smart pointer required)
- constructor takes a parent QWidget, defaulted to nullptr
- Q_OBJECT macro at the beginning of the class
- mywidget.hpp
#pragma once #include <QWidget> class MyWidget: public QWidget { Q_OBJECT public: MyClass(QWidget *parent = nullptr); virtual ~MyClass(); }
- mywidget.cpp
#include "mywidget.hpp" MyWidget::MyWidget(QWidget *parent): QWidget(parent) { } MyWidget::~MyWidget() { }
Other classes:
- memory management through smart pointers
Exceptions
Exceptions allow the developer to use various features without having to constantly check for errors. Even if you are not using them explicitly they may be triggered by the standard library or even by new. Sadly, for historical reasons, Qt does not support them. This means that if you are using a feature coming from a third party library you have to catch exceptions to prevent issues with Qt code. Note that Qt containers are exception proof however.
If you are writing non-Qt code then you really should use exceptions and more importantly, write exception-safe code. Using smart pointers is a great and easy way to do this. For Qt-based code you will have to use C-style error checking based on booleans and “getErrorString” functions. This is, for me, Qt's main drawback.
Examples
#include <string> #include <memory> // An entity semantic class class MyExampleClass final { public: // This constructor is explicit to prevent something like this: MyExampleClass test = "some text"; explicit MyExampleClass(const std::string &myString): m_myVar{52}, m_myString{myString} { } // Entity semantic: always forbid copy & assignment MyExampleClass(const MyExampleClass &) = delete; MyExampleClass &operator=(const MyExampleClass &) = delete; private: // Variables are always at the end, because they represent an implementation detail int m_myVar{42}; std::string m_myString{"value"}; }; // A value semantic class class MyVector final // Always final: a value semantic class should *never* be inherited from { public: // We want to use the default implementation (wich is faster than anything we can do) MyVector() = default; MyVector(const MyVector &other): m_x(other.m_x), m_y(other.m_y) { } MyVector &operator=(MyVector other) { std::swap(m_x, other.m_x); std::swap(m_y, other.m_y); return *this; } private: // m_x and m_y are initialized with the default value for the type int: 0 // unless the initializer list in a constructor decides otherwise int m_x{}; int m_y{}; }; void test() { // I use scopes "{}" here to limit where my variables live and are accessible { // Allocation on the stack (fast) MyExampleClass myExampleClass{"some text"}; // myExampleClass is destroyed here } { // Allocation on the heap (slower, allows polymorphism) std::unique_ptr<MyExampleClass> myExampleClass{std::make_unique<MyExampleClass>("some text")}; // Or (with auto) auto myExampleClass{std::make_unique<MyExampleClass>("some text")}; // myExampleClass is destroyed here (thanks to the smart pointer) } }