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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// at least)

// 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);

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]()
{
    //...
});

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

Reference: https://stackoverflow.com/questions/496440/c-virtual-function-from-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)
    }
}

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