Commande en C++
La Commande est un patron de conception comportemental qui convertit des demandes ou des traitements simples en objets.
Cette conversion permet de différer ou d’exécuter à distance des commandes, de gérer un historique de commandes, etc.
Complexité :
Popularité :
Exemples d’utilisation : La commande est très répandue en C++. Elle est souvent utilisée comme une alternative aux callbacks pour paramétrer les éléments d’une UI avec des actions. Elle est également utilisée pour mettre des tâches dans une file d’attente, suivre un historique de traitements, etc.
Identification : La commande peut être reconnue grâce à ses méthodes comportementales à l’intérieur d’un type abstrait ou d’une interface (demandeur). Elles appellent une méthode dans une implémentation d’un type abstrait différent ou d’une interface différente (récepteur) qui a été encapsulée par l’implémentation de la commande lors de sa création. Les classes Commande se limitent généralement à lancer des actions spécifiques.
Exemple conceptuel
Dans cet exemple, nous allons voir la structure du patron de conception Commande. Nous allons répondre aux questions suivantes :
- Que contiennent les classes ?
- Quels rôles jouent-elles ?
- Comment les éléments du patron sont-ils reliés ?
main.cc: Exemple conceptuel
/**
* The Command interface declares a method for executing a command.
*/
class Command {
public:
virtual ~Command() {
}
virtual void Execute() const = 0;
};
/**
* Some commands can implement simple operations on their own.
*/
class SimpleCommand : public Command {
private:
std::string pay_load_;
public:
explicit SimpleCommand(std::string pay_load) : pay_load_(pay_load) {
}
void Execute() const override {
std::cout << "SimpleCommand: See, I can do simple things like printing (" << this->pay_load_ << ")\n";
}
};
/**
* The Receiver classes contain some important business logic. They know how to
* perform all kinds of operations, associated with carrying out a request. In
* fact, any class may serve as a Receiver.
*/
class Receiver {
public:
void DoSomething(const std::string &a) {
std::cout << "Receiver: Working on (" << a << ".)\n";
}
void DoSomethingElse(const std::string &b) {
std::cout << "Receiver: Also working on (" << b << ".)\n";
}
};
/**
* However, some commands can delegate more complex operations to other objects,
* called "receivers."
*/
class ComplexCommand : public Command {
/**
* @var Receiver
*/
private:
Receiver *receiver_;
/**
* Context data, required for launching the receiver's methods.
*/
std::string a_;
std::string b_;
/**
* Complex commands can accept one or several receiver objects along with any
* context data via the constructor.
*/
public:
ComplexCommand(Receiver *receiver, std::string a, std::string b) : receiver_(receiver), a_(a), b_(b) {
}
/**
* Commands can delegate to any methods of a receiver.
*/
void Execute() const override {
std::cout << "ComplexCommand: Complex stuff should be done by a receiver object.\n";
this->receiver_->DoSomething(this->a_);
this->receiver_->DoSomethingElse(this->b_);
}
};
/**
* The Invoker is associated with one or several commands. It sends a request to
* the command.
*/
class Invoker {
/**
* @var Command
*/
private:
Command *on_start_;
/**
* @var Command
*/
Command *on_finish_;
/**
* Initialize commands.
*/
public:
~Invoker() {
delete on_start_;
delete on_finish_;
}
void SetOnStart(Command *command) {
this->on_start_ = command;
}
void SetOnFinish(Command *command) {
this->on_finish_ = command;
}
/**
* The Invoker does not depend on concrete command or receiver classes. The
* Invoker passes a request to a receiver indirectly, by executing a command.
*/
void DoSomethingImportant() {
std::cout << "Invoker: Does anybody want something done before I begin?\n";
if (this->on_start_) {
this->on_start_->Execute();
}
std::cout << "Invoker: ...doing something really important...\n";
std::cout << "Invoker: Does anybody want something done after I finish?\n";
if (this->on_finish_) {
this->on_finish_->Execute();
}
}
};
/**
* The client code can parameterize an invoker with any commands.
*/
int main() {
Invoker *invoker = new Invoker;
invoker->SetOnStart(new SimpleCommand("Say Hi!"));
Receiver *receiver = new Receiver;
invoker->SetOnFinish(new ComplexCommand(receiver, "Send email", "Save report"));
invoker->DoSomethingImportant();
delete invoker;
delete receiver;
return 0;
}
Output.txt: Résultat de l’exécution
Invoker: Does anybody want something done before I begin?
SimpleCommand: See, I can do simple things like printing (Say Hi!)
Invoker: ...doing something really important...
Invoker: Does anybody want something done after I finish?
ComplexCommand: Complex stuff should be done by a receiver object.
Receiver: Working on (Send email.)
Receiver: Also working on (Save report.)
